Elsevier

Brain Research

Volume 768, Issues 1–2, 12 September 1997, Pages 177-184
Brain Research

Research report
Role of impaired cAMP and calcium-sensitive K+ channel function in altered cerebral hemodynamics following brain injury

https://doi.org/10.1016/S0006-8993(97)00641-0Get rights and content

Abstract

Previous studies have shown that pial arteries constricted and responses to dilator opioids were blunted after fluid percussion injury (FPI) in newborn pigs. Membrane potential of vascular muscle is a major determinant of vascular tone and activity of K+ channels is a major regulator of membrane potential. Recent data show that opioids elicit dilation via the sequential production of cAMP and subsequent activation of calcium-sensitive K+ (KCa2+) channels by this second messenger. The present study was designed to investigate the effect of FPI on cAMP and KCa2+ channel function. Chloralose-anesthetized piglets equipped with a closed cranial window were connected to a percussion device consisting of a saline-filled cylindrical reservoir and a metal pendulum. Brain injury of moderate severity (1.9–2.1 atm) was produced by allowing the pendulum to strike a piston on the cylinder. FPI blunted dilation to the cAMP analogs 8-Bromo cAMP and Sp 8-Bromo cAMPs (10−8, 10−6 M), (9±1 and 16±1 vs. 2±1 and 3±1% dilations to 8-Bromo cAMP before and after FPI, respectively, n=8). Similarly, FPI attenuated dilation to pituitary adenylate cyclase activating peptide (PACAP), an endogenous activator of adenylate cyclase, and NS 1619, a KCa2+ channel agonist (9±1 and 16±1 vs. 3±1 and 5±1% for NS 1619 10−8, 10−6 M before and after FPI, respectively, n=8). Moreover, FPI attenuated PACAP, methionine enkephalin, leucine enkephalin, and dynorphin induced elevations in CSF cAMP concentration (940±2, 1457±50, and 2191±53 vs. 810±17, 1033±36, and 1218±49 fmol/ml for control, PACAP 10−8, 10−6 M before and after FPI, respectively, n=8). These data show that cAMP and KCa2+ channel function is impaired after FPI. Further these data suggest that impaired cAMP and KCa2+ channel function contribute to altered cerebral hemodynamics following FPI.

Introduction

Traumatic injury is the leading cause of death for infants and children, and the presence of head injury greatly increases mortality [11]. Morbidity and mortality of head injured infants is particularly severe because they either die from this injury or become neurologically crippled [13]. Fluid percussion brain injury (FPI) is an experimental model for blunt head trauma [15]. Previous studies have shown that FPI resulted in pial arterial vasoconstriction and decreased cerebral blood flow within 10 min of injury in newborn pigs [6]. Additionally, responses to several nitric oxide (NO)-dependent dilator stimuli were blunted after FPI in piglets 4, 12, 27. However, little is currently known about the mechanism of control of the cerebral circulation in the newborn after traumatic brain injury.

The membrane potential of vascular smooth muscle is a major determinant of vascular tone and activity of potassium (K+) channels is a major regulator of membrane potential [21]. Activation or opening of these channels increases potassium efflux, thereby producing hyperpolarization of vascular muscle. Membrane hyperpolarization closes voltage-dependent calcium channels, causing relaxation of vascular muscle [20]. Several types of K+ channels, including ATP-sensitive (KATP), calcium-sensitive (KCa2+), delayed rectifier and inward rectifier K+ channels have been identified. Pharmacologic studies using activators and inhibitors have additionally provided functional evidence that K+ channels, especially KATP and KCa2+ channels, regulate tone of cerebral blood vessels in vitro and in vivo 10, 17, 20, 21.

Cyclic nucleotides are recognized as second messengers mediating the actions of peptides and hormones on vascular smooth muscle. Activation of KATP, but not KCa2+, channels has recently been observed to contribute to cGMP-induced pial artery dilation 2, 5. Alternatively, others have suggested that cAMP elicits dilation via activation of KCa2+ channels [25]. Although several cellular studies have indicated that opioids are negatively coupled to adenylate cyclase [24], recent in vivo studies show that opioids such as methionine enkephalin and leucine enkephalin elicit dilation, at least in part, via the sequential production of cAMP and subsequent activation of KCa2+ channels [3]. Blunted dilation to these two opioids is thought to contribute to pial artery vasoconstriction following FPI 26, 27. The prominence of NO in the control of vascular tone, in general, has resulted in the design of several studies focused on altered NO and cGMP function following FPI 14, 26, 27, 31. However, little is known about the potential contribution of impaired cAMP signal transduction mechanisms to altered cerebral hemodynamics following FPI.

Therefore, the present study was designed to investigate the effect of FPI on cAMP and KCa2+ channel function.

Section snippets

Materials and methods

Thirty-eight newborn pigs (1–5 days old) of either sex were used in these experiments. All protocols were approved by the Institutional Animal Care and Use Committee. Animals were anesthetized with ketamine hydrochloride (33 mg/kg) and acepromazine (3.3 mg) intramuscularly. Anesthesia was maintained with α-chloralose (30–50 mg/kg, supplemented with 5 mg/kg per hour i.v.). A catheter was inserted into a femoral artery to monitor blood pressure and to sample for blood gas tensions and pH. Drugs

Influence of FPI on cAMP function

The cAMP analogs, 8-Bromo cAMP and Sp 8-Bromo cAMPs (10−8, 10−6 M), elicited reproducible pial small artery (120–160 μm) and arteriole (50–70 μm) dilation (Table 1). FPI blunted dilation produced by these agents (Fig. 1). On an absolute value basis, the diameters for control and 10−8, 10−6 M Sp 8-Bromo cAMPs before and after FPI were 121±5, 133±5, and 142±6 vs. 101±6, 104±6, and 105±6 μm, respectively. Similar changes in absolute value were observed for other agents before and after FPI. PACAP

Discussion

The results of the present study show that FPI blunted pial artery dilation elicited by the cAMP analogs, 8-Bromo cAMP and Sp 8-Bromo cAMPs. Additionally, FPI attenuated pial dilation produced by the KCa2+ channel agonist, NS 1619. Since it has recently been observed that dilation to these cAMP analogs was reduced by blockers of KCa2+ channels and was, therefore, presumably due to opening of the KCa2+ channel [3], these data suggest that diminished dilation to this second messenger is due, at

Acknowledgements

The author thanks Joseph Quinn for excellent technical assistance in the performance of the experiments. This research was supported by grants from the National Institutes of Health and the American Heart Association (AHA). W.M.A. is an Established Investigator of the AHA.

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