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Vol. 50, Issue 4, 683-722, December 1998
Pharmacology Department, Menarini Ricerche, Florence, Italy
I. Introduction
II. The Myogenic Theory of Ureteral Peristalsis
A. The Pacemaker Function of the Renal Pelvis
B. Propagation of the Pacemaker Signal
C. Latent Pacemakers and Antiperistalsis
III. Myogenic Factors Regulating Motility of Renal Pelvis and Ureter
A. Action Potentials Recorded from the Smooth Muscle of the Pyeloureteral Tract
1. Action potentials in the renal pelvis.
2. Action potentials in the ureter.
B. Characterization of Ionic Currents/Channels
1. The calcium current.
2. Potassium (K) currents.
3. Sodium currents.
C. Excitation-Contraction Coupling in the Renal Pelvis and Ureter
1. Role of extracellular calcium.
2. Role of the internal calcium store.
3. Sodium/calcium exchange.
4. Role of intracellular pH.
D. The Refractory Period of the Ureter
E. Modulatory Influence of Prostanoids
F. An Integrated View of Myogenic Factors Regulating Excitation-Contraction Coupling in the Renal Pelvis and Ureter
IV. Innervation of the Renal Pelvis and Ureter
A. Sources of Innervation to the Pyeloureteral Tract
B. Distribution of Nerves
C. Ureteric Ganglia
D. Cholinergic Nerves
E. Noradrenergic Nerves
F. Tachykinins and Calcitonin Gene-Related Peptide
G. Neuropeptide Y
H. Vasoactive Intestinal Polypeptide
I. Nitroxergic Nerves
J. Efferent Roles of Nerves in the Pyeloureteral Tract
V. Sensory Neuropeptides in the Pyeloureteral Complex: Release, Actions, and Receptors
A. Adequate Stimuli and Mechanisms Regulating the Release of Sensory Neuropeptides
B. Actions and Neurotransmitter Role of Sensory Neuropeptides in the Pyeloureteral Tract
C. Mechanisms of the Inhibitory Action of CGRP
1. CGRP, hyperpolarization and blockade of phasic contractions.
2. CGRP and cAMP accumulation.
3. Role of intracellular calcium in the action of calcitonin gene-related peptide.
4. Influence of "exercise" and glucose metabolism on the action of calcitonin gene-related peptide.
D. Sensory Nerves and Inflammation
VI. Pyeloureteral Reflexes
A. Afferent Nerves and Reflexes Arising from the Renal Pelvis
B. Afferent Nerves and Reflexes Arising from the Ureter
VII. Ureteral Pain
VIII. Pathophysiological Significance of the Sensory Innervation of the Pyeloureteral Complex
Acknowledgments
References
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I. Introduction |
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The main, and possibly only, function of the pyeloureteral complex is to ensure the unidirectional transport of urine from the kidney to the urinary bladder. An extremely efficient set of mechanisms operates to achieve this target: the basic process regulating ureteral peristalsis is myogenic, being initiated by spontaneously active pacemaker cells in the renal pelvis. In recent years, the mechanisms of excitability and electromechanical coupling in ureter cells have been extensively investigated; several pharmacological tools have been instrumental for a further understanding of the myogenic regulation of pyeloureteral motility.
A role of innervation in regulating pyeloureteral motility has been
advocated from time to time (See Boyarsky and Labay, 1969
; Weiss, 1992;
Amann, 1993) but its definition has remained elusive until recently. In
particular, a convincing proof for a local control of ureteral motility
by cholinergic or noradrenergic nerves has not been obtained. The
existence of a prominent sensory innervation of the renal pelvis and
ureter is self-evident when considering that renal colic arising from
the presence/passage of a stone along the ureter is one of the most
intense and vivid forms of visceral pain. In the past few years,
important advancements have been made through the recognition of the
role of sensory innervation in determining local changes of
pyeloureteral motility and in producing neurogenic inflammation.
In both areas of investigation (myogenic and neurogenic regulation of pyeloureteral function), the use of novel drugs and pharmacological tools has been instrumental to unravel principles of physiological or pathophysiological relevance. Our purpose is to bring together the new information obtained regarding the myogenic and neurogenic factors regulating pyeloureteral motility, ureteral pain, and inflammation and to discuss this recently obtained information on the background of the previous literature.
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II. The Myogenic Theory of Ureteral Peristalsis |
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The roots of the myogenic theory of ureteral peristalsis can be
traced back to the pioneering work of Engelmann (1869) who localized
the origin of peristaltic pressure waves in the renal pelvis and
proposed that the impulse for ureteral contraction passes from one
ureter cell to another, the whole ureter working as a syncitium. The
basic organization of the pyeloureteral complex has a loose resemblance
to that regulating cardiac excitation and contractility (fig.
1). In both organs, the electrical and mechanical activities are initiated by spontaneously active cells (in
the atrium and renal pelvis, respectively) and are then conducted to
regions that are not normally active unless driven by a pacemaker (the
ventricles and ureter, respectively); in addition, both the ureter and
cardiac tissues have a plateau-type action potential (Bozler, 1942a,b;
Burnstock and Prosser, 1960). However, as evidenced at several places
in this review, the analogy holds true only at a very superficial
level.
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The main pieces of evidence indicating that ureteral peristalsis is
essentially a myogenic process are as follows: (a)
peristalsis occurs in the isolated pyeloureteral complex with
characteristics similar as those observed in vivo, providing that the
renal pelvis is attached to the ureter (Macht, 1916; Finberg and Peart,
1970; Malin et al., 1970); (b) an efficient
peristalsis occurs, with characteristics similar to those observed
before surgery, after denervation of the ureter (Wharton, 1932), in
autotransplanted ureters (O'Conor and Dawson-Edwards, 1959), and even
in a reversed ureter (Melick et al., 1961); (c)
peristalsis of the isolated pyeloureteral tract is unaffected by
application of tetrodotoxin or other neuron blocking agents (Golenhofen
and Hannappel, 1973; Teele and Lang, 1998).
A. The Pacemaker Function of the Renal Pelvis
Some debate has developed in the literature about the exact
localization of a putatively unique pacemaker responsible for initiation of peristaltic waves. Most authors have localized
the pacemaker in supraureteral structures (calyces and
proximal renal pelvis) (Benjamin, 1971; Constantinou, 1978; Kiil, 1957;
Hannappel and Golenhofen, 1974a; Morita et al., 1981;
Tsuchida et al., 1981). Kobayashi (1964, 1965) found the
pacemaker at the border between the renal pelvis and calyces.
Golenhofen and Hannappel (1973) presented evidence indicating that the
utmost renal ends of the renal pelvis are involved in the pacemaker
process. The other prevailing opinion indicated the pyeloureteral
junction as the main pacemaker (Barr, 1971; Weiss et al.,
1967; Weiss, 1971). The issue about the location of the pacemaker is
complicated by the existence of remarkable species-related differences
in the anatomical organization of the renal pelvis and calyces (Kiil and Setekleiv, 1973).
Because a frequency gradient exists in the renal pelvis in terms of
autorhythmicity, the most proximal portions of the renal pelvis are
thought to generate the physiological rhythm of peristalsis. The
existence of specialized cells capable of generating a spontaneous rhythm in the renal pelvis had been suggested in early anatomical studies. Gosling and Dixon (1972, 1974) described special smooth muscle
cells with pale-staining cytoplasm present in the renal calyx and
pelvis but not in the ureter and discussed the possibility that these
cells may have a special "pacemaker" quality. Lang et
al. (1998) recently described the presence of fibroblasts-like cells, resembling the interstitial cells of Cajal that are responsible for pacemaking activity in the smooth muscle of the intestine, to be
present in the proximal part of the guinea pig renal pelvis. Electrophysiological evidence also indicates an enrichment in the
density of pacemaker cells at the level of the proximal renal pelvis,
as reviewed in Section III.B.
According to the current data, it seems that the whole renal pelvis
acts as a pacemaker to generate all-or-none propulsion of urine and
that a proximal-to-distal gradient exists for setting the frequency of
spontaneous activity. When the proximal, middle, or distal renal pelvis
was dissected, spontaneous activity was detected from each subregion
with similar frequencies as those observed before dissection
(Constantinou et al., 1978; Constantinou and Yamaguchi,
1981), i.e., every subregion of the renal pelvis behaves as an oscillator.
A "multiple coupled oscillators" model has been developed to
explain the ability of the whole renal pelvis to act as the pacemaker and to respond to an increase in diuresis with an increased frequency of urine transport. According to this model, several pacemaker subunits
exist in the renal pelvis, the oscillation of each subunit being
insufficient to trigger peristalsis; coupling processes (mainly
electrical) between the subunits facilitate the synchronization and
summation of activity to increase the chance of triggering a conducted
peristaltic wave (Golenhofen and Hannappel, 1973; Constantinou and
Yamaguchi, 1981). According to this model, the force of the total
pacemaker will depend on (a) the force of each subunit and
(b) the degree of synchronization between subunits. Physical
stimuli, such as the distention, modulate the peristaltic frequency by
changing the degree of synchronization between subunits, possibly
through the generation of prostanoids (Section III.E). This model
can be applied independently from the speciesdependent anatomical complexities of the calyces and renal pelvis.
Lammers et al. (1996) have analyzed the spatial and temporal
variations in the pacemaker process by performing simultaneous extracellular electrophysiological recordings from a large number of
sites in the whole isolated sheep renal pelvis. By reconstructing the
initiation and spread of activity, they showed that the pacemaker was
located at the pyelocalyceal border and never in the body of the pelvis
or in the area of the pelviureteric junction. One single pacemaker was
invariably responsible for a particular spread of activation and fusion
of activity from two or more pacemakers did not take place. Spontaneous
shifts of the pacemaker could occur from one site to another along the
pyelocalyceal border. To what extent the observations of Lammers
et al. (1996) in the sheep renal pelvis can be applied to
other species is not known; the results contradict the hypothesis
(Gosling and Dixon, 1974) that in species with multicalyceal kidneys,
several pacemakers from distinct calyces could discharge simultaneously
and their impulses could merge together into a single wavefront.
Lammers et al. (1996) proposed that, in the intact renal
pelvis, factors such as the degree of filling of a particular calyx or
local stretch may determine the initiation of the pacemaker activity at
a given site.
B. Propagation of the Pacemaker Signal
Excitation waves spread from the renal pelvis to the ureter,
determining the propulsion of urine through an intermittent phasic-type contractility. Considering the ureter as a syncitium (Bozler, 1942a,b),
the propagation of impulses occurs as a purely myogenic process (via
electrotonic spread) at points of intimate contact between ureter
muscle cells or "gap junctions" (Notley, 1968; Uehara and
Burnstock, 1970; Tahara, 1990). This model predicts that the
suppression of action potentials at any site of the ureter will
suppress the propagation of contraction and peristalsis.
Lammers et al. (1996) analyzed the conduction of impulses in
the sheep isolated renal pelvis. They observed that conduction from the
site of initiation of the pacemaker current to the pyeloureteral junction is slow, inhomogeneous and contorted: multiple instances of
partial or total conduction block were observed in the renal pelvis. In
several cases, conduction block was related to the refractory period
but in other instances no apparent relationship was evident between the
occurrence of a conduction block and the length of preceding intervals.
Lammers et al. (1996) speculated that a poor coupling
between cells or stretch could be involved in conduction block within
the renal pelvis.
A conduction block also exists between the renal pelvis and the ureter
(Constantinou, 1974, 1978; Hrynczuk and Schwartz, 1975; Zawalinski
et al., 1975; Constantinou and Hrynczuk, 1976); under the
conditions of a normal diuresis, every pacemaker contraction of the
renal pelvis does not always propagate to the ureter, suggesting that a
urine flow-dependent mechanism triggers ureteral peristalsis at the
pelviureteric junction. Stretching forces exerted on the pyeloureteral region by accumulating urine increase the coupling strength until they enable an incoming "pacemaker" wave of
excitation to pass to the ureter (Constantinou and Yamaguchi, 1981).
With increasing urine flow rates, the frequency of peristaltic
contractions reaches that of the pacemaker; at this stage, further
increases in urine production are accommodated by bolus volume
increases, until the ureter assumes the form of an open duct
(Constantinou et al., 1974).
Regional variations in the excitability and in the rate of propagation
of the peristaltic wave have been described in the guinea pig, cat, and
rat ureter (e.g., see Kobayashi, 1965; Tindall, 1972; Tsuchiya and
Takei, 1990; Weiss, 1992). By using a three-chamber partitioned organ
bath (Meini et al., 1995), we demonstrated that: (a) depolarizing stimuli applied at either end of the guinea
pig ureter produce a propagated wave of excitation which travels to the
other end of the organ; (b) the rate of propagation of
contractions is independent of the site of application of the stimulus;
(c) latent pacemakers capable of generating a propagated
wave of excitation are present in all regions of the ureter; and
(d) the amplitude of propagated responses is independent
from the site and the nature of the depolarizing stimulus.
The findings of Meini et al. (1995) indicate also that, in
principle, mechanical events are not essential for the propagation of
impulses along the ureter. The main findings supporting this conclusion
are as follows: (a) Bay K 8644 and glibenclamide,
which produced a marked and a slight prolongation of action potential duration and enhancement of contraction, respectively (Maggi et al., 1994a,b), had no effect on the propagation of impulses in the
ureter; (b) 1 µM nifedipine produced
a large (>80%) inhibition of the mechanical response at the site
where depolarizing stimuli were applied without affecting the intensity
of the propagated response: the suppression of the propagated response
by nifedipine occurred in an all-or-none manner only when the response
to directly applied stimuli was also suppressed; (c) drugs
that suppress the action potential by producing hyperpolarization of
the membrane [cromakalim, calcitonin generelated peptide
(CGRP)b] suppressed the propagated
response: both the suppression of the propagated response and its
recovery upon washout of cromakalim or CGRP occur in an
all-or-none manner (Meini et al., 1995).
Altogether, these findings indicated that even marked "local" changes in action potential shape/duration and contractility do not affect or impair the propagation of impulses. In other words, action potentials that are profoundly altered in their characteristics are capable of sustaining the propagation of impulses through the functional syncitium of the ureter. This conclusion does not exclude the possibility that, in physiological conditions, the stretch of the ureteral wall produced by the advancing bolus of urine may affect or modulate excitability and peristalsis.
C. Latent Pacemakers and Antiperistalsis
Every single cell of the guinea pig ureter can fire an action
potential in response to depolarization (Imaizumi et al.,
1989a). Therefore, when applying stimuli of threshold strength, all
ureter smooth muscle cells can, in principle, act as pacemakers. Action potentials produced at any site along the ureter will propagate ortho-
and antidromically to determine propagated waves of
excitation/contraction, i.e., peristalsis or antiperistalsis (Meini
et al., 1995) (fig. 2).
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The activation of latent pacemakers in the ureter by chemical agents
may be an important event in pathophysiological conditions because
antiperistaltic waves of excitation can be generated in this way
(Weiss, 1992). Latent pacemakers in the ureteral smooth muscle can be
excited to fire action potentials and generate propagated contractions
by mediators released from intramural nerves (neurokinin A,
noradrenaline) or mast cells (histamine, serotonin) (Shuba, 1977a,b;
Maggi et al., 1986; 1988a; Dodel et al., 1996;
Iselin et al., 1996; Patacchini et al., 1998) and
by chemical mediators normally present in the urine (bradykinin,
endothelin-1) or by bacterial peptides produced in the urine during
infections (Maggi et al., 1992a).
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III. Myogenic Factors Regulating Motility of Renal Pelvis and Ureter |
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A. Action Potentials Recorded from the Smooth Muscle of the
Pyeloureteral Tract
1. Action potentials in the renal pelvis.
Zawalinski et
al. (1975) reported that action potentials recorded from the
renal pelvis exhibit a markedly different waveform as compared with
those recorded from the ureter smooth muscle. In particular,
"spontaneous" or pacemaker action potentials are recorded from the
renal pelvis (fig. 1): these display a simple waveform with a slowly
developing depolarization or "prepotential" preceding a slow spike.
Seki and Suzuki (1990) reported that slow waves recorded from the
smooth muscle of the rabbit renal pelvis are resistant to cholinergic,
noradrenergic, and neuronal blockers to suggest a purely myogenic
origin. Reduction of the extracellular concentration of sodium
([Na]o) or of calcium ions ([Ca]o)
inhibited the generation of slow waves.; Lang et al., 1995;
Lang and Zhang, 1996) recently reported single cell
electrophysiological recordings from the guinea pig renal pelvis. They
described three distinct types of cells on the basis of the different
action potentials that were recorded at this level, as follows:
(a) approximately 10% of cells ("pacemaker" cells)
that display spontaneous activity characterized by a slow rising and
repolarizing phases triggered on top of a slowly developing
prepotential; (b) the majority of cells displayed a more
complex, "ureter-like" action potential characterized by a fast
spike and a slow long-lasting plateau; (c) a third type
of action potential was also observed, "intermediate" between those
recorded from pacemaker and "driven" cells. Lang and coworkers also
reported that cells firing pacemaker potentials have lower resting
membrane potential than "driven" cells (
42 and 56 mV,
respectively). They observed that nifedipine and cadmium ions blocked
all spontaneous activity and depolarized the cell membrane indicating
the absolute requirement of L type Ca channels for action
potential generation in the renal pelvis. The action potential of both
"pacemaker" and "driven" cells was also modulated by potassium
(K) conductances, because blockade of Ca-dependent potassium (K)
channels by charybdotoxin or apamin increased the duration of action
potentials of "driven" cells, whereas other K channel blockers,
tetraethylammonium (TEA), or 4-aminopyridine (4-AP), increased the
frequency of action potential discharge in both "pacemaker" and
"driven" cells (Zhang and Lang, 1994; Lang et al.,
1995; Lang and Zhang, 1996).
proposed that the pacemaker action potentials are
recorded from the atypical smooth muscle cells identified by Gosling
and Dixon (1972, 1974) as the putative pacemaker elements and that the
action potentials recorded from "intermediate and "driven" cells
originate from smooth muscle cells that are in close and far electrical
contact with a pacemaker region, respectively.
In principle, the observed enrichment of pacemaker cells in the
proximal renal pelvis may depend on a sampling bias if, for some
reason, efficient impalements of pacemaker cells were easier to perform
in the proximal versus distal renal pelvis. This interpretation seems
unlikely because action potentials exhibiting differing waveforms and
closely resembling those recorded from single cells were also obtained
by multicellular electrophysiological recordings (sucrose gap
technique) from the proximal versus distal guinea pig renal pelvis
(Maggi et al., 1995a; Santicioli et al.,
1995a; Santicioli and Maggi, 1997). Spontaneous action potentials were discharged from both regions of the renal pelvis, but the frequency of
discharge was markedly higher in the proximal renal pelvis where it
approached the same frequency values recorded from the intact renal
pelvis in organ bath experiments (4.6 versus 1.2 cycles/min in the
proximal and distal renal pelvis) (Santicioli and Maggi, 1997). Action
potentials recorded from the proximal renal pelvis were bell-shaped,
did not show a plateau phase, and had a small after-hyperpolarization
although action potentials from the distal renal pelvis had a
"ureter-like" morphology (initial spike, long lasting plateau with
oscillations) and displayed a sizable after-hyperpolarization.
Moreover, the rate of rise of the action potential was markedly faster
in the distal than proximal renal pelvis (5.5 ± 1.3 and 50.9 ± 15 mV/s, respectively, n = 65 and 69, P < 0.01, Santicioli P and Maggi CA, unpublished
data). Nifedipine (1 µM) suppressed action potentials at
both levels and a submaximally effective concentration of nifedipine
(50 nM) shortened the action potential duration with
similar intensity at both levels. Bay K 8644 prolonged the action
potential duration in both regions of the renal pelvis (Santicioli and
Maggi, 1997). With regard to K channel blockers, charybdotoxin (30 nM) prolonged the action potential duration at both levels,
although 4-aminopyridine exerted a pure positive chronotropic effect in
the distal renal pelvis without affecting the resting membrane
potential or other parameters of the action potential and was also
without effect in the proximal renal pelvis (Santicioli and Maggi,
1997). The KATP channel opener, cromakalim, determines a
glibenclamide-sensitive suppression of spontaneous action potentials in
the guinea pig renal pelvis, suggesting the presence of
KATP channels at this level (Maggi et al.,
1995a).
2. Action potentials in the ureter.
The action potential of
the ureter is unusually long lasting and is characterized by an initial
spike followed by a plateau (fig. 3). In
the guinea pig, the action potential is further characterized by the
presence of multiple oscillations on the plateau (Kuriyama et
al., 1967; Kuriyama and Tomita, 1970; Shuba, 1977a,b), whereas this phase is not observed in other species (fig. 3). Tetrodotoxin has
no effect on the action potential of the guinea-pig ureter (Kuriyama
et al., 1967). Washizu (1966) did not report any change of the action potential when extracellular Na was replaced by lithium
(Li). Others reported that removal of Na abolished the plateau phase of
the action potential (Kuriyama and Tomita, 1970; Shuba, 1977a; Brading
et al., 1983). The removal of extracellular Ca totally
eliminated the action potential (Kuriyama and Tomita, 1970;
Kochemasova, 1971; Brading et al., 1983). Manganese ions (Mn) abolished the initial spike and almost totally eliminated the
accompanying contraction. Verapamil markedly depresses the action
potential duration and contraction and high concentration of nifedipine
almost completely abolished all electrical and mechanical responses
(Shuba, 1977a; Brading et al., 1983). The results of these early studies were interpreted to mean that two types of channels
are involved in determining depolarization of the smooth muscle of the
ureter, a "fast" Ca channel, and a "slow" channel, permeable to
both Ca and Na, which would be responsible for the initial spike and
for the plateau phase of the action potential, respectively.
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; Brading et al., 1983). The results of
pharmacological experiments indicate that in addition to TEA- and
charybdotoxin-sensitive K channels, cromakalim- and
glibenclamide-sensitive K channels (presumably a class of
KATP channels) also are present in the ureter and that
their activation determines hyperpolarization (Maggi et
al., 1994b; De Moura and Lemos Neto, 1994; Hernàndez et al., 1997). In the guinea pig ureter, glibenclamide
causes a slight but significant prolongation of action potential
duration; because several K conductances can be affected by
glibenclamide, these findings do not necessarily imply that
KATP channels modulate the action potential of the
guinea-pig ureter (Maggi et al., 1994b).
Several studies on the action potential of the ureter were performed
before the discovery that application of depolarizing stimuli
determines the release of mediators from sensory nerves (see Section
V.). Therefore, an indirect influence of neurotransmitters, especially
an inhibitory influence of calcitonin gene-related peptide (CGRP) on
the action potential, cannot be ruled out totally. This is especially
relevant when considering that endogenous CGRP modulates the resting
membrane potential, the refractory period, the excitability, and
availability of Ca channels in the smooth muscle of the ureter (see
Section V.B.). A comparison of parameters used to excite the smooth
muscle of the guinea pig ureter in different studies suggests that in
vitro capsaicin pretreatment, which blocks CGRP release from sensory
nerves, enables the study of the effect of drugs on electromechanical
coupling without the confounding factor represented by the concomitant
activation of the inhibitory sensory innervation. In particular, a very
long width of applied electrical pulses (range 50 msec to 3 sec) has
been required to obtain reproducible responses in the intact guinea-pig
ureter (Kuriyama and Tomita, 1970; Shuba, 1977a,b; Brading et
al., 1983; Bullock and Wray, 1998); this parameter of
stimulation can be considerably reduced (1-5 msec) in
capsaicin-pretreated preparations (Maggi et al.,
1994a,b, 1996a). In vitro capsaicin pretreatment or application of a
CGRP receptor antagonist augments the excitability of the smooth muscle
of the ureter to electrical stimulation: under these conditions, (fig.
4) the improved stability of electrical recordings makes it easier to perform a thorough quantitative pharmacological analysis of the effects of drugs on the action potentials of the renal pelvis and ureter (Maggi et al.,
1994a; 1995a,b; 1996a,b; Santicioli and Maggi, 1997). Further
improvement in excitability and stability of recordings can be obtained
by omitting magnesium ions from the bathing solution (Patacchini et al., 1998).
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for review), alone and in combination, on the refractory period, and
action potential of the guinea pig ureter. We found that ryanodine exerts various effects on the action potential that include an increased excitability of ureter smooth muscle to depolarizing stimuli
(fig. 4), a reduction in the interval between stimulus application and
the onset of the action potential, a slight decrease in the action
potential duration, and an increase in the after-hyperpolarization (Maggi et al., 1994a). These effects are consistent with
a model whereby Ca release from the sarcoplasmic reticulum participates in the setting of the resting membrane potential, probably by activating Ca-dependent K channels. The removal of this mechanism would
explain the depolarization observed upon application of ryanodine and
also as a consequence, faster achievement of the threshold for
firing of the action potential during a depolarization step. A somewhat
opposite effect would occur during the Ca load produced by the action
potential itself: in this case, a blockade of Ca uptake into the
sarcoplasmic reticulum may cause a faster or stronger activation of
Ca-dependent K channels, leading to a substantial reduction of action
potential duration and an increased after-hyperpolarization (Maggi
et al., 1994a).
Importantly, a dissociation between action potential duration and
contractility was evidenced in the presence of ryanodine: in fact, the
length of the contractionrelaxation cycle was not reduced by
ryanodine, despite the shortening of action potential duration.
Moreover, a marked additive effect of ryanodine and Bay K 8644 on
the duration of action potential and contractionrelaxation cycle
was evidenced (fig. 5), further
indicating that Ca reuptake into the sarcoplasmic reticulum has a role
in terminating the contractile cycle of the guinea pig ureter.
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; Lang, 1989): the threshold for initiating an action potential lies between 50 and 40 mV, there is a slow upstroke velocity of 1 V/sec and an overshoot to approximately +30 mv
with an action potential duration of approximately 1 sec. In cells with
large inward currents oscillations of potential during the plateau
phase were recorded (Lang, 1989). Overall, the action potential
recorded from single ureteral cells had a remarkable resemblance to
that recorded from the intact ureter, although the spikes recorded from
the intact tissue are more regular and regenerative than the potential
oscillations seen in isolated cells (Lang, 1989).
B. Characterization of Ionic Currents/Channels
1. The calcium current.
Lang (1989, 1990) and Imaizumi
et al. (1989a,b; 1990) showed that voltage-dependent Ca
channels provide the main inward current detectable in smooth muscle
cells of the guinea pig ureter. The inward Ca current (ICa)
of ureter smooth muscle exhibits a biexponential decay with a duration
intermediate between that observed in rapidly spiking smooth muscles
(which is remarkably shorter) and that observed in certain weakly
spiking vascular smooth muscles (which is remarkably longer) (Lang,
1989). noted several important aspects
of ICa in guinea pig ureter: (a) Ca-induced
inactivation of ICa occurs with a time course much slower
than that observed in other smooth muscles; (b) the
inactivation does not increase significantly in response to a train of
depolarizing pulses; and (c) a very slowly inactivating
or noninactivating " window" current can flow through the Ca
channels at the plateau potential. Altogether, these observations
suggest that the very slowly inactivating ICa is
responsible for the plateau of action potential and that further activation of the current is involved in the generation of the spikes
(Imaizumi et al., 1989a). Imaizumi et al.
(1989b) measured a non-inactivating ICa, peaking at
approximately 10 mV, and calculated that it may be large enough to
explain the sustained tonic contractions observed in response to
prolonged depolarization of the guinea pig ureter.
in conditions enabling the exclusion of the interference of time-dependent K currents. Lang (1990) found that the activation of
ICa occurs at potential positive to 40 mV and maximal
activation was at +20 mV. He also noted, in contrast to other smooth
muscles, that the extracellular Ca concentration [Ca]o
does not determine any substantial acceleration of ICa
inactivation in ureteral cells, indicating some peculiarity in the
mechanisms of current-dependent inactivation of ICa in the
ureter. Sui and Kao (1997a,b), using Ca rather than Ba as the
physiological charge carrier, provided a detailed estimate of the
ICa voltage-current relationship. They concluded that the
very slow inactivation of ICa and the feeble Ca-induced
inactivation of ICa are causally related and contribute significantly to determining the plateau of the action potential of the
ureter. According to their estimates, a significant window current
allows Ca influx at membrane potentials of 20 to 0 mV, which can help
to sustain the plateau phase of the action potential (Sui and Kao,
1997a,b).
2. Potassium (K) currents.
Two main outward currents have been
characterized in the guinea pig ureter (Lang, 1989; Imaizumi, 1989a,
1990; Sui and Kao, 1997c), a TEA- and charybdotoxin-sensitive
Ca-dependent K current or IK(Ca) and a voltage-dependent
Ca-insensitive transient K current (also termed A-current or
ITO) which is TEA-insensitive but blocked by
4-aminopyridine (4-AP).
noted that the average outward IK(Ca) in
the ureter is very small as compared with that measured in other smooth
muscle cells, and that IK(Ca) in ureter cells has a quite
low probability of opening in the range of voltage which is achieved
during the action potential. According to these authors, the meagerness
of IK(Ca) in the presence of a persistent ICa
is central to explain the long plateau of the action potential in
ureteral smooth muscle cells.
Sui and Kao (1997c) also proposed that the fluctuations observed on the
plateau of the action potential are caused by slight changes in the net
membrane current originating from the balance between ICa
and IK(Ca). This interpretation contrasts with that forwarded by Imaizumi et al. (1989a): they proposed that
a release of Ca from the sarcoplasmic reticulum would activate the
IK(Ca) responsible for transient hyperpolarizations causing
the oscillations superimposed on the plateau of the action potential.
Imaizumi et al. (1989a) reported that spontaneous
transient hyperpolarizations (< 10 mV in amplitude) occur in
approximately 60% of ureteral cells. Under voltage clamp conditions,
spontaneous transient outward currents (STOCs) were measured, similar
to those described in other types of smooth muscle cells (Benham and
Bolton, 1986). In ureteral cells, STOCs were first enhanced and then
abolished by the application of caffeine, suggesting that they
originate from the activation of a Ca-dependent outward current linked
to the spontaneous release of Ca from the sarcoplasmic reticulum. According to Imaizumi et al. (1989a), episodic releases
of quantal packets of Ca from the sarcoplasmic reticulum would
determine STOCs, in turn responsible for the fluctuations observed on
the plateau phase of the action potential. In line with this
interpretation, Imaizumi et al. (1989a) reported that
the multiple oscillations observed on the plateau of the action
potential of guinea pig ureter muscle cells were blocked by TEA or
caffeine. On the other hand, Sui and Kao (1997c), by using either
caffeine or 8-(diethylamino)octyl-3,4,5-trimethoxybenzoate HCl, failed
to get evidence for a role of Ca mobilization from the internal store
in regulating the fluctuations of the action potential.
We observed that ryanodine, cyclopiazonic acid, and thapsigargin, three
drugs which, via different mechanisms, produce a depletion of
the sarcoplasmic reticulum Ca store (Maggi et al.,
1994a; 1995b; 1996a; Maggi CA and Santicioli P, unpublished data) all
produce depolarization of the guinea pig ureter smooth muscle. These
observations are consistent with a role of Ca released from the
sarcoplasmic reticulum in the setting of membrane potential via
activation of Ca-dependent K channels. However, the same three drugs
did not abolish the multiple oscillations present on the plateau phase of the action potential of the guinea pig ureter (Maggi et
al., 1994a; 1995b), an observation that is against the idea
that Ca mobilization from the internal store would be responsible for this phase of the action potential.
The role of the transient K current (ITO) is unclear
because it would be rapidly inactivated by depolarization and does not seem to be suited for modulating the shape of the action potential. Because a similar current was shown to regulate interspike intervals in
neurons, it was speculated (Lang, 1989; Imaizumi et al.,
1990) that, in ureteral cells, ITO may regulate the
generation of spontaneous action potentials in response to stretch or
that produced in response to excitatory transmitters. ITO
may regulate membrane excitability by opposing the Ca current activated
around the threshold of the action potential. The selective positive
chronotropic effect exerted by 4-AP in the guinea pig proximal renal
pelvis may suggest a role of ITO in regulating the
frequency of pacemaker potentials (Santicioli and Maggi, 1997).
A remarkable and distinguishing feature of ionic currents recorded from
the guinea pig ureter is the absence of a 4-AP-sensitive voltage-dependent and Ca-insensitive K current (IK), which
has been recorded in most other smooth muscle cells. As pointed out by
Lang (1989), the action potential in most readily spiking smooth muscle
cells is kept brief by the sum of the rapid inactivation of the Ca
current and a rapidly activating K current which, in part, is
Ca-insensitive. IK is by far the most important outward current in weakly spiking smooth muscle cells: therefore, in the absence of an equivalent IK, the duration of the action
potential is chiefly determined by the inactivation of the Ca current.
IK(Ca) seems to be the major target for the excitatory
action of noradrenaline and histamine in the guinea pig ureter (Muraki et al., 1994). Noradrenaline causes a marked inhibition
of IK(Ca) which is stronger than its concomitant inhibition
of ICa; by contrast, noradrenaline does not affect
ITO (Muraki et al., 1994). By suppressing IK(Ca) noradrenaline causes a marked prolongation of the
action potential duration which may account for its excitatory effect on electromechanical coupling of the ureter.
Despite pharmacological evidence for the existence of cromakalim- and
glibenclamide-sensitive K channels in the guinea pig renal pelvis and
ureter (see Section III.A.), no KATP current has been
characterized thus far in these smooth muscles.
3. Sodium currents.
It has been shown that a reduction of
extracellular Na ([Na]o) decreases the plateau phase of
the action potential of the guinea pig ureter, a finding which was
initially interpreted to mean that a "slow" channel, admitting both
Na and Ca, could be involved in this phase, whereas a " fast" Ca
channel would be responsible mainly for the initial spike (unaffected
by Na replacement) (Kuriyama and Tomita, 1970; Shuba, 1977a).
showed that, in single ureter
cells, the replacement of extracellular Na with Li results in a marked acceleration in the decay of ICa. They excluded the
possibility that an increase in outward currents or a faster
inactivation of ICa could be responsible for this effect
and speculated that a late component of the inward current may be
dependent upon [Na]o. Because the removal of
extracellular Ca totally eliminates the inward current, it seems that
an elevation of [Ca]i is required for establishing the
Na-dependent contribution to ICa. This behavior may involve
the activation of an electrogenic Na/Ca exchange (Aickin et
al., 1984; 1987; Aaronson and Benham, 1989) or of a
Ca-dependent Na current (such as a nonselective cation channel)
(Imaizumi et al., 1989a).
In a subsequent study, Muraki et al. (1991) reported
that two distinct Na currents can be detected in guinea pig ureter
muscle cells, one that is fast inactivating (< 10 msec) and
sensitive to blockade by tetrodotoxin (EC50 11 nM) and a second one that is slowly inactivating (> 500 msec) and tetrodotoxin-resistant. The fast inactivating current was
only observed in approximately 10 to 20% of ureter cells. It was
speculated that this current may affect membrane excitability and
action potential conduction although stimuli of appropriate strength
produce tetrodotoxin-resistant action potentials in the guinea pig ureter.
Lang (1989), by using 7.5 mM Ba as charge carrier, found
that the inactivation kinetics of ICa are not consistently
changed by replacement of extracellular Na with TEA. Sui and Kao
(1997a) failed to detect any significant contribution of Na to the
inward current of ureteral myocytes nor did they observe an effect of Na on currents carried by other ions.
C. Excitation-Contraction Coupling in the Renal Pelvis and Ureter
1. Role of extracellular calcium.
Early studies on the
characteristics and ionic requirements of the action potential of the
guinea pig ureter had demonstrated that extracellular Ca is essential
for excitation-contraction coupling and that changes in duration of the
action potential are closely paralleled by changes in contractility
(see Section III.A.). Indeed, a suppression of action potentials, or a
pharmacological modulation (increase or decrease) of its duration by
nifedipine, Bay K 8644, or charybdotoxin, are closely paralleled by
equivalent changes in the amplitude and duration of spontaneous
contractions of the guinea pig renal pelvis (Santicioli and Maggi,
1997).; Maggi and Meli, 1984; Hertle and Nawrath,
1984a). However, when studying receptor-mediated contractility, such as
noradrenaline-induced contractions, Ca channel blockers preferentially
inhibit the phasic but not tonic-type responses (Golenhofen and
Hannappel, 1978; Hertle and Nawrath, 1984b). Pharmacologically, studies
with agonists and antagonists indicate that L-type but not
T-type or N-type Ca channels mediate electromechanical coupling in the
smooth muscle of the ureter (Hertle and Nawrath, 1989; Maggi et
al., 1994a; Maggi and Giuliani, 1995).
; Brading et al., 1983; Aickin
et al., 1984; Burdyga and Magura, 1986a). By shortening
the action potential duration, these drugs markedly depress the phasic
contraction accompanying the action potential of guinea pig ureter.
These observations support the idea that Ca influx via
voltage-dependent channels chiefly determines tension development in
the ureter, yet remarkably high concentrations of Ca channel blockers
are needed to suppress the initial spike.
The differential sensitivity of the initial spike and of the plateau
phase of the action potential to Ca channel blocking drugs is mirrored
by the effects of these drugs on changes in membrane potential and
tension produced by increasing concentrations of [K]o.
Mild elevations of [K]o determine the firing of action potentials and a concomitant phasic type contractility of the ureter,
whereas high [K]o determines a phasic and tonic
contraction sustained by the firing of action potentials and a
sustained depolarization (Washizu, 1967; Sunano, 1976; Johnishi and
Sunano, 1978). By analogy with their differential effects on the action
potential evoked by electrical depolarization, Ca channel blockers are
very potent in blocking the tonic component of K-induced contraction
whereas high concentrations of Ca channel blockers are needed to
suppress the phasic contractions (Sunano, 1976; Brading
et al., 1983; Burdyga and Magura, 1986a; Maggi and
Giuliani, 1995).
The different potency of nifedipine in inhibiting the phasic and tonic
components of the response to high K was found to be a time-dependent
phenomenon: in fact, nifedipine inhibited with similar potencies the
tonic contraction to 80 mM KCl in guinea pig isolated
ureter after 15 and 120 min contact times (EC50 0.67 and
0.29 nM, respectively), whereas its potency in inhibiting the phasic response was increased by approximately 77-fold by prolonging the incubation period (EC50 121 and 1.56 nM after 15 and 120 min contact time, respectively) (Maggi
and Giuliani, 1995). These results were interpreted to mean that the
L-type Ca channels responsible for generating the phasic
component of contraction to high K (presumably the same population of
Ca channels responsible for generating the initial spike of action
potential) are in a low affinity state for interaction with nifedipine.
In a subsequent study (Maggi et al., 1996b), we showed
that different inhibitors of protein kinase A (PKA) suppress the tonic response to high K (fig. 6) at
concentrations that have little or no effect on the phasic contractions
nor on the action potentials fired in response to electrical
stimulation. Contrary to nifedipine, the differential effect of PKA
inhibitors (H8 and H89) was not time-dependent; moreover, the effect of
H89 was not reproduced by protein kinase C or protein kinase G
inhibitors. These results indicated that PKA modulates the availability
of a subpopulation of L-type Ca channels, notably those
which are in a high affinity state for nifedipine. Because the
noninactivating component of Ica is held responsible for
the tonic contraction developing in response to prolonged
depolarization of the ureter (Imaizumi et al., 1989b),
it is possible that phosphorylation by PKA is particularly relevant for
the maintenance of the noninactivating state of the Ca channels.
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recently reported the simultaneous recording of
action potential, change in intracellular Ca ([Ca]i) and
tension development in the rat ureter. They found that approximately 30% of the elevation of [Ca]i occurred during the
upstroke of the action potential whereas approximately 70% of total
elevation of [Ca]i occurred was contributed by the
plateau phase of the action potential. The kinetics of tension
development and relaxation were significantly slower than those of Ca
elevation, the temporal gap likely being covered by
phosphorylation/dephosphorylation of myosin light chain.
2. Role of the internal calcium store. Because Ca channel blockers suppress the spontaneous activity of the renal pelvis and the evoked contractility of the ureter, it is evident that an influx of extracellular Ca is crucial for excitation-contraction coupling in the pyeloureteral tract. This knowledge has somewhat delayed the investigation of the role played by the internal Ca store in excitation-contraction coupling of the ureter, a topic that has recently readdressed.
Some investigators have failed to observe a sizable contractile response to caffeine in the guinea pig ureter and argued that the corresponding intracellular Ca store is insufficient to sustain a contraction (Burdyga and Magura, 1986a; Aaronson and Benham, 1989).
Burdyga and Magura (1986b) reported that contractile responses of the
guinea pig ureter to caffeine can be elicited by running the experiment
at a low temperature (21°C). The observation is consistent with a
model in which [Ca]i elevation produced by caffeine is
amplified/prolonged by lowering of temperature in such a way that the
[Ca]i elevation becomes sufficient to evoke a sizable and
reproducible increase in tension. However, a transient contraction to
caffeine also can be elicited at 37°C in Na-loaded preparations (by
exposure to ouabain, see Section III.C.3.) or in preparations depolarized by a high K medium. In these cases, it has been
hypothesized that Ca entering in the cell, either via the Na/Ca
exchanger operating in a reverse mode (in ouabain-pretreated tissues)
or via voltage-dependent Ca channels, had loaded the caffeine-sensitive
Ca store (Burdyga and Magura, 1986b).
Imaizumi et al. (1989a) reported that the application of
5 mM caffeine induces contraction of single cells from the
guinea pig ureter and transiently enhances spontaneous transient
outward currents. Burdyga et al. (1995) observed that
the rat and guinea pig ureter show a major difference with regard to
the source of intracellular Ca. They found that caffeine (20 mM at room temperature) determines a transient elevation of
[Ca]i and contraction of the guinea pig but not of the
rat ureter, whereas caffeine is ineffective in the rat ureter. In
contrast, carbachol produced a transient elevation of
[Ca]i and induced a contraction of the rat but not of the
guinea pig ureter. Burdyga et al. (1995) also found that ryanodine and cyclopiazonic acid block the responses to caffeine in the
guinea pig ureter, whereas only cyclopiazonic acid blocked the
mobilization of [Ca]i in the rat ureter.
It has to be mentioned that, in the above studies, no attempt had been
made to prevent the release of sensory neuropeptides (tachykinins and
CGRP) from intramural sensory nerves of the rat and guinea pig ureter,
therefore, the possible involvement of this factor in the observed
results remains unsettled. Moreover, the above studies focussed on the
possible role of sarcoplasmic reticulum Ca store in causing a
contraction of the ureter smooth muscle, without addressing possible
changes in membrane potential.
Working with capsaicin-pretreated guinea pig ureter at 37°C, we found
that caffeine (5 mM) causes a glibenclamide-sensitive transient hyperpolarization followed by a sustained depolarization (fig. 7). The hyperpolarization produced
by caffeine was blocked by glibenclamide, and was reduced when elicited
in a Ca-free medium indicating that a Ca-dependent
glibenclamide-sensitive K channel is involved in this effect (Maggi
et al., 1996a). The role of intracellular Ca stores in
regulating the membrane potential of the ureter smooth muscle is also
dealt with in Section III.B.2.
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; 1995b; 1996a; Maggi CA and Santicioli P, unpublished data). None of these drugs reduced the amplitude of depolarization-induced phasic
contractions even after a prolonged contact time: this observation
provides clear-cut pharmacological evidence to conclude that Ca release from the sarcoplasmic reticulum does not provide activator Ca for
contraction to any significant extent in the guinea pig ureter. However, these drugs produce a transient enhancement of the amplitude of contraction and especially prolonged the duration of the contractile cycle of the ureter (Maggi et al., 1994a, 1995b). These
observations imply that uptake of Ca into the sarcoplasmic reticulum is
an active mechanism of relaxation in ureteral smooth muscle.
The effect of ryanodine and cyclopiazonic acid also was examined on the
biphasic contractile response produced by increasing [K]o
in the guinea pig ureter (Maggi et al., 1994a, 1995b).
Both drugs enhanced the phasic contraction to KCl and prolonged the spiking phase on the ascending limb of the developing tonic
contractions. Both effects can be explained on the basis of the ability
of these drugs to increase excitability of the ureter, reduce the
refractory period for firing of action potentials, and potentiate
phasic contractility sustained by action potential generation.
Cyclopiazonic acid, ryanodine, and thapsigargin also produce a
transient enhancement of spontaneous contractions of the guinea pig
renal pelvis (Maggi et al., 1995a; Santicioli and Maggi,
1997) supporting the idea that Ca reuptake into the internal store
modulates the contractile cycle of the renal pelvis. When Ca-induced Ca release from sarcoplasmic reticulum was essential for
excitation-contraction, coupling a prolonged exposure to these drugs
should produce a suppression or inhibition of spontaneous phasic
contractility of the renal pelvis by depleting the Ca store. The
failure to observe a depression of contractility even after a prolonged
exposure (> 60 min) to these drugs indicates that, similar to the
ureter, the sarcoplasmic reticulum does not significantly contribute
activator Ca for contraction of the renal pelvis.
3. Sodium/calcium exchange.
The existence of a mechanism for
Na/Ca exchange in smooth muscle has been a matter of discussion (see
Van Breemen et al., 1979 for review). The molecular
cloning of different isoforms of the Na/Ca exchanger and analysis of
their presence/distribution in various tissues has positively solved
the question (see Matsuda et al., 1997 for review),
although the definition of the exact role of the exchanger in
excitation-contraction coupling of the ureter remains uncertain.
; Imaizumi
et al., 1989a). However, other workers have failed to
detect a significant contribution of [Na]o to the inward
current and/or modulation of ICa current in the ureter (Sui
and Kao, 1997a).
Some researchers have investigated the properties of the Na/Ca
exchanger by focussing on ion distribution measurements. Aickin et al. (1984) showed that after inhibition of the Na/K
pump by ouabain, a transient contraction develops in the guinea pig
ureter upon Na withdrawal. The contraction developing under these
circumstances is dependent upon the intracellular Na concentration
([Na]i) and the presence of extracellular Ca, but is
resistant to Ca channel blockers. Direct measurements of
[Na]i(Aickin, 1987; Aickin et al., 1987)
indicated that, in the presence of ouabain, [Na]i
stabilizes at a surprisingly low level and that Na could still be
extruded against the electrochemical gradient for Na by reducing
[Na]o or elevating [Ca]o. Lamont et
al. (1998) recently extended these observations by taking
advantage from the use of the Na-sensitive fluorophore,
sodiumbinding benzofuran isophtalate, to directly measure
[Na]i in guinea pig ureter. They observed that resting [Na]i is around 10 mM and rose to 25 mM in the presence of ouabain.
The contraction and the concomitant fall in [Na]i which
develop in ouabain-loaded preparations upon removal of
[Na]o were effectively blocked by manganese ions (Mn), a
finding consistent with their known blocking activity on the cardiac
Na/Ca exchanger (Aickin et al., 1984, 1987). By
simultaneously measuring [Na]i, [Ca]i and
tension, Lamont et al. (1998) established the existence of a linear relationship between [Na]i and developed
force in ouabain-pretreated ureters. They also noted that intracellular acidification and alkalinization produced a decrease and an
increase, respectively, of the Na withdrawal-induced contraction in
ouabain-pretreated ureters and that the changes in developed tension
were paralleled by equivalent changes in the [Ca]i response.
Burdyga and Magura (1988) studied the effect of temperature and Na
removal on the relaxation of phasic and tonic contractions in the
guinea pig ureter: they reported that the relaxation of the tonic
contraction produced by a high K medium was dependent on
[Na]o and speculated that Na/Ca exchange may be involved
in terminating tension development in response to this stimulus. Benham
and Aaronson (1989) showed that, in the presence of nifedipine, depolarization induced by long pulses of current causes a slow rise of
[Ca]i and activates an outward current that is dependent on the duration and strength of depolarization: the evidence suggests the existence of a Na/Ca exchanger that, in the conditions of recordings, is acting in the reverse mode (Ca entering the cell being
exchanged with extrusion of Na). Aaronson and Benham (1989) also showed
that the nifedipine-resistant Ca transients were potentiated when
lowering the [Na]o, demonstrating that Na/Ca exchange can modulate [Ca]i when [Na]i and membrane
potential are set or near their physiological levels in ureteral smooth
muscle cells. Accordingly, an electrogenic Ca extrusion mechanism
during the upstroke of the action potential would result in an inward
current that might explain the Na-dependency of the plateau phase of
the action potential. In fact, after a Ca load such as that occurring
during an action potential, an electrogenic Na/Ca exchange would be
expected to mediate a net inward current as though it contributed to
extrusion of the Ca load. Although being consistent with a contributory role of the Na/Ca exchanger to transmembrane Ca movements under normal
conditions in the guinea pig ureter, these observations also indicated
that any role of the exchanger in removing Ca from the cytoplasm is
affected by the presence of other Ca-removing pathways.
By using blockers of intracellular Ca handling from the sarcoplasmic
reticulum, ryanodine, and cyclopiazonic acid, we presented evidence
(Maggi et al., 1994a; 1995b) that Ca reuptake into the internal store modulates the contractile cycle of the guinea pig ureter. Cyclopiazonic acid potentiated the amplitude of contractions sustained by the firing of action potentials, however, the potentiating effect was transient and, during prolonged application of the drug, the
amplitude of evoked contractions returned toward the baseline. We
speculated that an increase in activity of the Na/Ca exchanger may
compensate the blockade of sarcoplasmic reticulum Ca pump function in
lowering [Ca]i (Maggi et al., 1995b). To
probe this hypothesis, we studied the effect of cyclopiazonic acid
after replacement of 60% [Na]o with either Li or
choline. In both media, the amplitude of the evoked contractions was
reduced and the potentiating effect of cyclopiazonic acid became
sustained (fig. 8). Both results are
consistent with the idea that Na/Ca exchanger could contribute to
action potential generation and removal of intracellular Ca in the
guinea pig ureter.
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4. Role of intracellular pH.
Intracellular pH
(pHi) is a fundamental parameter for cell physiology by
regulating, among others, enzyme activity and metabolic rate. The
mechanisms regulating pHi maintain this parameter at a
level considerably more alkaline than that which could be predicted on
the basis of the passive distribution of hydrogen ions (H) (Roos and
Boron, 1981).
,b) reported that an electroneutral Na/bicarbonate
cotransporter is the most important mechanism for the maintenance of
pHi in the guinea pig ureter. An amiloride-sensitive Na/H
exchanger also exists, but seems of minor importance in physiological
conditions. Both mechanisms extrude acid equivalents to protect the
inside of the cell from substantial intrinsic acid loading. A
chloride/bicarbonate exchanger is activated during intracellular
acidosis to transport acid equivalents inside the cell to restore a
more normal pH (Aickin et al., 1994a,b).
Because the pH of urine may vary between 4.5 and 8, other investigators
have addressed the question of changes in ureteral smooth muscle
excitability/contractility produced by the lowering of pH. Cole
et al. (1990) showed that intracellular acidification increases the force of contraction of human ureteral smooth muscle in
response to depolarizing stimuli. Burdyga et al. (1996)
analyzed the effects of changing intra- and extracellular pH on
[Ca]i, electrical activity, and tension in the guinea pig
ureter. They found that lowering pHi greatly augments the
phasic contractions of the ureter, whereas intracellular alkalin
