Unloaded shortening velocity, a mechanical parameter associated with the rate of cross-bridge cycling, was investigated in chemically skinned guinea pig taenia coli and hog carotid artery. Shortening velocity was measured by the technique described by Edman, whereby large length steps are rapidly imposed on the muscle and the time under unloaded conditions is determined from the isometric myograms. Shortening velocity determined in this manner was similar to V^ from the Hill force-velocity

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... ions reported for both living and skinned taenia coli, and, in the case of carotid artery, was at least as large as that reported for living muscle. The behavior of shortening velocity was qualitatively similar for both preparations. Shortening velocity was strongly temperature dependent, with a Q ]o of approximately 3.6. Shortening velocity was found to be dependent on both the Ca ++ and calmodulin concentration. In contrast to the dependence of isometric force on Ca ++ -calmodulin, shortening velocity could be increased further by the addition of Ca ++ and/or calmodulin under conditions when isometric force was maximized. Incubation with ATP-7S, which presumably maximizes the phosphorylahon of myosin, did not increase shortening velocity beyond the maximum value obtained in the presence of Ca ++calmodulin alone. The development of shortening velocity after exposure to high Ca ++ solution was found to precede that of isometric force. The steady state value tended to be slightly lower than the maximum shortening velocity, the largest difference observed being less than 1.5-fold. Thus, whereas both isometric force and shortening velocity are dependent on the Ca ++ -calmodulin concentration in skinned smooth muscle, the dependencies are not identical, differing with respect to temporal development and concentration. These differences may underlie the decline in velocity with maintained isometric force observed in living smooth muscle. (Circ Res 53: 342-351, 1983)