How vascular smooth muscle works.

Electrical recordings made from vascular smooth muscle during the last 15 years make it clear that the role of electrical activity in controlling contraction is often important but varies greatly both between different vessels and for different responses of a given vessel. Only a few mammalian vessels have widely conducted electrical activity, but others can develop this activity, with consequent rhythmical contraction, when made anoxic and stimulated. Action potentials play a part in smooth non-rhythmical responses of arteries to nerves and hormones, while passive conduction of depolarization round the vessel wall is responsible for ring contractions of arteries after local injury. Ca and K cause vasodilatation largely by hyperpolarizing the smooth muscle cells. Electrical activity plays no part in some responses. In particular noradrenaline can contract arteries by directly promoting entry of extracellular Ca, and also by releasing Ca stores by non-electrical means. These processes are particularly important in the inner muscle of arteries, which is not directly innervated.