CYCLIC-GMP CAUSES CA2-MUSCLE BY ACTIVATING THE MYOSIN LIGHT-CHAIN PHOSPHATASE( DESENSITIZATION IN VASCULAR SMOOTH)

Using permeabilized, arterial smooth muscle strips where membrane-asso ciated pathways remain intact but intracellular Ca2+ stores are deplet ed, we investigated mechanism(s) for the Ca2+ desensitization of contr actile force by cGMP. The nonhydrolyzable analog 8-bromo-cGMP, when ap plied to these strips with submaximal Ca2+ levels clamped, dramaticall y and reversibly reduced the steady state levels of phosphorylation at 20-kDa myosin light chain and contractile force, with a nanomolar con centration required to obtain 50% reduction, Supramaximal concentratio ns of 8-bromo-cGMP (10 mu M), however, did not change the steady state relationship between phosphorylation and force, When light chain phos phatase activity was blocked at pCa 6.7, 10 mu M 8-bromo-cGMP did not affect the rates of rise of light chain phosphorylation and contractil e force, When light chain kinase activity was blocked, 10 mu M 8-bromo -cGMP significantly accelerated light chain dephosphorylation and forc e relaxation from the maximal contraction steady state, The light chai n phosphorylation time course of a pCa 6.0-induced contraction in the presence of 8-bromo-cGMP exhibited kinetics that are predictable from a mathematical model in which only light chain phosphatase activity is increased, The results of this study strongly suggest that cGMP indir ectly activates light chain phosphatase, the first proposed mechanism for cGMP-induced Ca2+ desensitization in vasodilatation.