The effects of 12-o-tetradecanoylphorbol-13-acetate (TPA) on isometric tension, unloaded shortening velocity determined using the slack test (V^J, and phosphorylation of myosin light chain (MLC M ) were investigated in intact and skinned muscle tissues of the rabbit mesenteric artery. In intact muscles exposed to 128 mM K + , the V^ reached a maximum before the peak of the phasic component of contraction and reduced proportionally with changes in the amplitude of the subsequent

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... . TPA (0.1 f*M) enhanced to a greater extent the amplitude of the tonic than the phasic contraction. During the tonic phase, TPA enhanced the Vmu and amplitude of the contraction more than the phosphorylation of MLC M . In skinned muscles, Ca 2+ (0.3-1.0 /iM) increased the force, V^ and MLC^ phosphorylation, in a dose dependent manner. The relation between any two of the above three parameters was always linear. TPA (0.1 /iM) with phosphatidylserine (50 /ig/ml) time-dependently increased the force and V"", observed with 0.3-0.5 /itM Ca 2+ . The relation between the force and V^ shifted in the direction of that observed in the presence of high Ca 2+ , but in 0.5 /iM Ca 2+ , TPA with phosphatidylserine showed little change in the phosphorylation of MLC W . As estimated from the actions of TPA, protein kinase C may contribute to activation of contractile proteins through MLC W phosphorylation dependent and other regulatory processes. (Circulation Research 1988;63:893-902) W hen the cytosolic concentration of Ca 2+ is increased in smooth muscle, Ca 2+ binds to calmodulin, myosin light chain kinase is activated, and phosphorylation of the myosin light chain (20 kDa proteins of myosin light chain; MLC20) is triggered. These processes result in initiation of cross-bridge cycling and contraction. 1 There is a positive correlation between the phosphorylation of MLC20 and the isotonic shortening velocity in intact and skinned vascular smooth muscles. 1 However, factors other than phosphorylation of MLC20 or phosphorylation at an alternative site may be involved in regulation of the shortening velocity in smooth muscles. 23 Aksoy et aH showed that the maintenance of contraction is associated with a reduction in isotonic shortening velocity and in