Calcium in the Vascular Smooth Muscle Cell

Abstract

These experiments were designed to show whether the sheep carotid artery loses its response to electrical discharges in Ca-free solution at a rate that can be accounted for by diffusion delays, or so slowly as to indicate that depolarization can utilize cell Ca stores in the absence of extracellular Ca. Electrical activity plays an important role in this vessel’s activity. The artery usually fires one or two action potentials when depolarized (8), and its outermost smooth muscle cells can fire longer trains (13). These discharges are not usually conducted far but steady depolarization induced at one point, for example by injury, is conducted passively round the wall to cause a ring contraction (5). Much of the depolarizing current of the action potentials is carried by Na, but Ca influx probably contributes since spikes can occasionally be obtained in Na-free solution with Ca (9). Ca influx is therefore probably the minor component of a mixed Ca-Na spike rather than the major component as in intestinal muscle (3, 4, 7). Ca⁴⁵ influx increases during K depolarization of rabbit aorta (2) though largely as a direct result of the ionic change rather than of the depolarization produced by it.