This review discusses the mechanisms by which second messengers and calcium regulate smooth muscle contraction beyond myosin light chain phosphorylation.
In smooth muscle cells, development of force results from MgA TP-dependent cyclic interactions of myosin in thick filaments with actin in thin filaments. The force of contraction, in turn, is regulated by the concentration of free Ca2+ surrounding these myofilaments. The purposes of the present chapter are to discuss (a) Ca2+ -dependent mechanisms other than myosin light chain phosphorylation by which contractile force in smooth muscle may be reg ulated, and (b) other second messenger mechanisms that regulate contractile elements or the myosin kinase/phosphatase system . Since the discovery that phosphorylation of the 20-kDa light chains of myosin by Ca2+ /calmodulin-dependent myosin light chain kinase allows smooth muscle myosin MgATPase to be activated by actin, many in vestigators have studied the relationship between light chain phosphorylation and active force in smooth muscle (reviewed in 42; Hai & Murphy, this volume). It is generally accepted that myosin phosphorylation and de phosphorylation are sufficient to regulate contraction. While myosin phos phorylatiOn/dephosphorylation may be the dominant regulatory pathway, the coexistence of thin-filament linked regulation is not ruled out (51).
Kamm et al. (1989) studied this question.