From pharmacomechanical coupling to G-proteins and myosin phosphatase

A brief summary of recent studies of pharmacomechanical coupling is present ed, with emphasis on the role of GTP-binding proteins and Ca2+-independent regulation of contraction (Ca2+-sensitization/desensitization) through regu latory myosin light chain (MLC20) phosphorylation and dephosphorylation. Ph armacomechanical regulation of cytosolic [Ca2+] is largely, though not sole ly, controlled by the phosphatidylinositol cascade and Ca2+-pumps of the pl asma membrane and the sarcoplasmic reticulum. The monomeric GTPase, RhoA, i sa major upstream component of Ca2+-sensitization. Its crystal structure an d apparently obligatory translocation to the plasma membrane for activation of its downstream effecters are described. Inhibition of RhoA activity by a membrane-permeant ADP-ribosylating bacterial exoenzyme, DC3B, causes seve re depression of the tonic component of agonist-induced contraction, sugges ting that this component is largely due to Ca2+-sensitization. A relatively specific inhibitor (Y27632) of Rho-kinase, a downstream effector of Ca2+-s ensitization (Uehata et al 1997). also inhibits oxytocin-induced Ca2+-sensi tization of myometrium. The major mechanism of physiological, G-protein-cou pled Ca2+-sensitization is through inhibition of smooth muscle myosin phosp hatase (SMPP-1M), whereas conventional or novel protein kinase Cs play very little or no role in this process. Mechanisms of Ca2+-desensitization incl ude inhibition of myosin light chain kinase and activation of SMPP-1M. Acti vation of SMPP-1M in phasic smooth muscle can be attributed, at least in pa rt, to the synergistic phosphatase activating activities of a cyclic nucleo tide-dependent kinase and its major substrate, telokin.