Cyclic GMP-dependent stimulation reverses G-protein-coupled inhibition of smooth muscle myosin light chain phosphate

8-bromo-cyclic guanosine monophosphate (8-br-cGMP), at constant Ca2+, accelerated the dephosphorylation of the regulatory myosin regulatory light chain and relaxation of permeabilized rabbit ileum smooth muscle. These effects were independent of myosin light chain kinase activity. Similarly, 8-br-cGMP relaxed GTPgammaS-induced force and inhibited the GTPgammaS-induced increase in myosin light chain phosphorylation at constant [Ca2+]. We conclude that cyclic GMP-dependent kinase, activated by 8-br-cGMP, increases smooth muscle myosin light chain phosphatase activity and counteracts the inhibition of the latter enzyme by G-protein(s).