Rhythmic activity in smooth muscle from small subcutaneous arteries

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Abstract
Isolated strips of small subcutaneous arteries (200-500 [mu] o.d.) from the dog''s paw show 2 components in their contractile behavior: a tonic increase in tension and superimposed phasic contractions which are often grouped to form rhythmic waves. This pattern of activity occurs spontaneously in physiological salt solution and in response to various vasoactive agents (catecholamines, serotonin, angiotensin, dog plasma). The phasic activity signifies cell-to-cell propagation and single-unit organization in this vascular smooth muscle. The frequency of the phasic contractions is augmented with increasing doses of stimulating agents. Passive stretch also increases the frequency of the phasic contractions, a phenomenon that may have some bearing on the theory of autoregulation of blood flow in vivo. Supranormal concentrations of Ca++ have an inhibitory effect on the rhythmic activity, probably due to the membrane-stabilizing action of this ion. Increased [K +] (10-30 mM) stimulates an inactive strip and this leads to partial relaxation. It is suggested that reduction of pacemaker frequency in vascular smooth muscle with high myogenic activity may be the mechanism responsible for the K vasodilatation in vivo. Some implications of single-unit organization of vascular smooth muscle are discussed.

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