Effects of Ca-antagonists on the stretch activation of vascular smooth muscles

Abstract

Helical or ring strips of cerebral, renal coronary arteries(A) of rabbits, dogs and guinea pigs and saphenous A of aged-SHR (50-60 weeks) developed an active tension in response to quick stretch at a rate of 10 cm/sec to 140 % of the initial muscle length. Cerebral and saphenous A were more responsive to quick stretch than other A. The contraction produced by quick stretch was myogenic and depended not only transmembrane supply of Ca, but also on release of Ca, possibly from the inner surface of the plasma membrane susceptible to chemical skinning. Stretch-induced contraction under control conditions was more resistant to Ca-antagonists(Ca-A) and Ca-withdrawal than the contractions produced by high K and muscle direct electrical stimulation, which were mainly dependent on extracellular Ca. In contrast, the contractile activation due to stretch once enhanced by potentiators of Ca-influx such as cardiac glycosides, hemolysate and vasoactive amines was readily suppressed by Ca-A. Ca-A acts by blocking the entry of Ca and inhibits the refilling of Ca in store sites. This fundamental action explains the cerebral, coronary and other peripheral vasodilating and antihypertensive effects of Ca-A. Contractile activation by quick stretch seems to provide a useful alternative technique to electrical and chemical stimuli studies on myogenic vascular tone.