Effects of calcium on stress and myosin phosphorylation in swine carotid media.

Phosphorylation of the 20,000 dalton myosin light chain is necessary for force generation, but not for force maintenance. The purpose of this study was to determine if the magnitude of the transient in myosin light chain phosphorylation affected force maintenance in swine carotid medial strips. Strips were either depolarized with K+ and then exposed to a Ca channel blocker (normal myosin light chain phosphorylation transient), or were exposed to the Ca channel blocker before K+ depolarization (blunted myosin light chain phosphorylation transient). The level of force maintained was the same regardless of the experimental protocol, indicating that the prior history of myosin light chain phosphorylation did not affect the maintained force. Whereas the phasic response to K+ depolarization of strips pretreated with Ca channel blocker was depressed in those treated with verapamil, only minimal effects were observed with nifedipine. These findings suggest an additional action for verapamil besides Ca channel blockade.