AJP - Cell Physiology, Vol 255, Issue 6 C719-C723, Copyright © 1988 by American Physiological Society

ARTICLES

[Ca2+]-dependent myosin phosphorylation in phorbol diester stimulated smooth muscle contraction

C. M. Rembold and R. A. Murphy
Department of Physiology, University of Virginia, School of Medicine, Charlottesville 22908.

Phorbol diesters, potent activators of protein kinase C, can produce a slow contraction in arterial smooth muscle. Such observations have prompted proposals that protein kinase C may have direct regulatory functions in contraction. In this paper, we present evidence that [Ca2+]-dependent myosin light chain phosphorylation is responsible for the contraction induced by low-dose phorbol diester and during force development in response to high-dose phorbol diester stimulation. The relationships between myoplasmic [Ca2+], myosin phosphorylation, and steady-state stress induced by low-dose phorbol dibutyrate were similar to those observed with contractile agonists. However, prolonged exposure to high-dose phorbol dibutyrate induced high stress with elevated phosphorylation that was not associated with elevations in aequorin-estimated [Ca2+]. Our results suggest that phorbol diesters can increase myoplasmic [Ca2+], and the resulting increase in myosin phosphorylation quantitatively explains the contraction.

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