Reversible phosphorylation and dephosphorylation of the 20,000-dalton light chain of myosin during the contraction-relaxation-contraction cycle of arterial smooth muscle.

The myosin light chain of intact arterial smooth muscle displayed a cyclic phosphorylation-dephosphorylation and rephosphorylation in consort with contraction, followed by relaxation, followed by a second contraction of the muscle. Application of pharmacological contractile agonists and antagonists of vascular smooth muscle revealed that the level of light chain phosphorylation is related to the contractile state of the muscle. The central role of Ca2+ in regulation of light chain phosphorylation was shown by the effect of ethylene glycol bis(beta-aminoethyl ether)N,N,N',N'-tetraacetic acid (EGTA) in inhibiting contraction and light chain phosphorylation and in inducing relaxation and light chain dephosphorylation. Furthermore, studies with antipsychotic drugs, inhibitors of calmodulin, suggested that the Ca2+-dependent regulatory protein is intimately involved in both light chain phosphorylation and contraction of intact smooth muscle. In addition to the myosin light chain, several other proteins were also found to be phosphorylated in intact smooth muscle.