Shortening velocity, active force and homogeneity of contraction during electrically evoked twitches in smooth muscle from rabbit urinary bladder

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Abstract
Force-velocity relations obtained by the quick release technique were studied at different times during the twitch at 25.degree. C in electrically stimulated longitudinal smooth muscle from rabbit urinary bladder. Maximal shortening velocity (Vmax) and maximal tension (P0) reach their highest values during the rising phase of the twitch. Vmax was calculated to be 0.30 .+-. 0.04 l/s (SE, n = 9). Vmax declines rapidly with time and at the peak of the twitch it is only about 70% of the maximum value. P0 at the peak does not differ significantly from its maximum value during the rising phase. In the relaxation phase of the twitch, both Vmax and P0 decrease progressively. Photographic studies of preparations marked with charcoal grains did not show any considerable inhomogeneity of contraction during isometric twitches, maximum movement of grains never exceeding 4% of total muscle length. No yield at the ends of the preparations were seen. The different time course of Vmax and Po can not be explained by longitudinal inhomogeneity of activation or deactivation of the preparations.