Length-dependent modulation of myosin phosphorylation and contractile force in coronary arterial smooth muscle

Phosphorylation of the 20,000 dalton myosin light chain (LC) is a central regulatory mechanism of smooth muscle contraction. Our previous findings on airway smooth muscle have led us to hypothesize that length-dependent and length-independent modulation of myosin phosphorylation coexist in smooth muscle. In this study, we tested the general applicability of this hypothesis by investigating the length dependences of myosin phosphorylation and contractile force in bovine coronary arterial smooth muscle. Comparison of the time courses of myosin phosphorylation at optimal (Lo) and preshortened lengths indicated that the initial peak myosin phosphorylation induced by K(+)-depolarization, histamine, and endothelin were all length dependent. Additional experiments focusing on the length dependence of steady-state myosin phosphorylation revealed that the length dependence of K(+)-depolarization-induced steady-state myosin phosphorylation (0.13 mol P(i)/mol LC/Lo) was significant, but smaller than that found in airway smooth muscle. In contrast, the length dependence of endothelin-induced steady-state myosin phosphorylation (0.03 mol P(i)/mol LC/Lo) was insignificant. The different length dependences of depolarization- and endothelin-induced myosin phosphorylation were found to correlate with different length-force relations. The length-force relationships in K+-depolarized and endothelin-activated tissues remained different even when the force was normalized by the maximum value at Lo. Variable length-force relations have been reported but the biochemical basis is not understood. Results from this study suggest that length-dependent modulation of myosin phosphorylation may be an important determinant of length-force relationship in smooth muscle.