Phosphatase inhibition with okadaic acid does not alter the relationship between force and myosin light chain phosphorylation in permeabilized smooth muscle.

The phosphatase inhibitor, okadaic acid, has been used to test the hypothesis that myosin light chain phosphatase activity plays a central role in latchbridge formation in smooth muscle. In the permeabilized rabbit portal vein there is a non-linear relationship between myosin light chain phosphorylation and force production such that maximum force output occurs with about 50% phosphorylation. Treatment of the muscle with okadaic acid does not change this relationship even though there is a profound inhibition of phosphatase activity. The data suggest that dephosphorylation of the myosin light chain while the myosin is in the force producing state does not account for the high force output with low levels of light chain phosphorylation in smooth muscle.