SIGNALING FOR CONTRACTION AND RELAXATION IN SMOOTH MUSCLE OF THE GUT

▪ Abstract Phosphorylation of Ser 19 on the 20-kDa regulatory light chain of myosin II (MLC 20 ) by Ca 2+ /calmodulin-dependent myosin light-chain kinase (MLCK) is essential for initiation of smooth muscle contraction. The initial [Ca 2+ ] i transient is rapidly dissipated and MLCK inactivated, whereas MLC 20 and muscle contraction are well maintained. Sustained contraction does not reflect Ca 2+ sensitization because complete inhibition of MLC phosphatase activity in the absence of Ca 2+ induces smooth muscle contraction. This contraction is suppressed by staurosporine, implying participation of a Ca 2+ -independent MLCK. Thus, sustained contraction, as with agonist-induced contraction at experimentally fixed Ca 2+ concentrations, involves (a) G protein activation, (b) regulated inhibition of MLC phosphatase, and (c) MLC 20 phosphorylation via a Ca 2+ -independent MLCK. The pathways that lead to inhibition of MLC phosphatase by G q/13 -coupled receptors are initiated by sequential activation of Gα q /α 13 , RhoGEF, and RhoA, and involve Rho kinase–mediated phosphorylation of the regulatory subunit of MLC phosphatase (MYPT1) and/or PKC-mediated phosphorylation of CPI-17, an endogenous inhibitor of MLC phosphatase. Sustained MLC 20 phosphorylation is probably induced by the Ca 2+ -independent MLCK, ZIP kinase. The pathways initiated by G i -coupled receptors involve sequential activation of Gβγ i , PI 3-kinase, and the Ca 2+ -independent MLCK, integrin-linked kinase. The last phosphorylates MLC 20 directly and inhibits MLC phosphatase by phosphorylating CPI-17. PKA and PKG, which mediate relaxation, act upstream to desensitize the receptors (VPAC 2 and NPR-C), inhibit adenylyl and guanylyl cyclase activities, and stimulate cAMP-specific PDE3 and PDE4 and cGMP-specific PDE5 activities. These kinases also act downstream to inhibit (a) initial contraction by inhibiting Ca 2+ mobilization and (b) sustained contraction by inhibiting RhoA and targets downstream of RhoA. This increases MLC phosphatase activity and induces MLC 20 dephosphorylation and muscle relaxation.