Effects of initial length and active shortening on vascular smooth muscle contractility.

PETERSON, JOHN W., AND RICHARD J. PAUL. Effects of initial length and active shortening on vascular smooth muscle contractility. Am. J. Physiol. 227(5) : 1019-1024. 1974.-The active isometric tension-length relation has been studied in bovine mesenteric vein at 37°C under a variety of experimental conditions, including both passive and active changes in muscle length. The rest length Lo , defined as that length at which passive tension first appears, is shown to be of fundamental significance. Below I& , the forcelength relation is reversible and independent of the experimental methods by which decreases in length are imposed. Active stretch, however, is found to result in irreversible losses in isometric tension development. For lengths greater than L, , reversibility is seen only when all changes in length are made with the tissue passive (tinstimulated). Active shortening from lengths greater than L, results in irreversible losses in contractility. Such losses, when once induced by active contraction, are shown to persist at all lengths in the force-length relation, including lengths shorter than L, . These contraction-induced losses (which average 14 rt 5 y0 per contraction) are shown to be consistently present and cumulative with repeated contractions from lengths greater than L, . termination of active isometric tension, APO (total force minus passive force).