Attention to details concerning the individual components of the contractile process of smooth muscle creates the impression that there is a striking difference not only between this muscle and striated muscle, but also among smooth muscles from various sources. Examples of its dissimilarity are found in the great and variable permeability of the membrane to Na; the individuality of endoplasmic reticulum, relaxing factor, and response to Ca; and its relatively weak but very extensible contractile protein. Although the individualities may be only quantitative, they are responsible for large differences in the mechanical event which is the end-product of the overall contractile process. Emphasis in the past has been placed on the uniqueness, variabifity, and lability of the smooth muscle contractile system. However, when one looks within the basic mechanisms of the smooth muscle contractile process, its excitation, coupling, chemo-mechanical transducing, and energy metabolism, one sees processes that are generally similar to those of striated muscle.
Ca modulates membrane excitability, is essential for excitation-contraction coupling, and may have another more direct function in the relationship between the chemical energy source (ATP) and the contractile machine (actomyosin). Mg depresses membrane excitability or excitation-contraction coupling but, in some cases, has a positive influence on contraction. It does not seem to be essential for contraction and its usual effect reflects depressed excitation or coupling. The action of K is strikingly ambivalent; an increase in concentration of K in the environment of the cell results in depolarization, increased excitability, and increased contraction; whereas intracellular K is essential for the normal reversible contractile process. In its absence a rigid contracture develops and in excess it plasticizes the contractile machine so that tension development is impaired. Na is not essential for the contraction-relaxation cycle, but in the intact system it has an important controlling influence on membrane excitability and on excitation- contraction coupling. The action of Na is largely dependent on interrelationships between the concentrations of Na and Ca. The influence of anions on smooth muscle contraction is determined primarily by the ease with which they permeate the cell membrane. In accordance with this facility they influence membrane potential and may have a more direct effect on the rate of movement of Ca through the membrane.
The generalization most profitable to live with in dealing with the influence of electrolytes on the contractile process of smooth muscle is that each may affect several of the components of the overall contractile process. The net effect of a shift in electrolyte concentration on the size of the contraction will depend on the relative magnitude and direction of the influence of this change on each process. In a general way the basic components of the overall contractile process of smooth muscle resemble those of skeletal muscle.
- 1964 by The Williams & Wilkins Co.