Down-regulation of G-protein-mediated Ca2+ sensitization in smooth muscle

Prolonged treatment with guanosine 5'-[gamma-thio]triphosphate (GTP gamma S; 5-16 h, 50 mu M) of smooth muscle permeabilized with Staphylococcus aureus alpha-toxin down-regulated (abolished) the acute Ca2+ sensitization of force by GTP gamma S, AlF4-, phenylephrine, and endothelin, but not the response to phorbol dibutyrate or a phosphatase inhibitor, tautomycin. Down-regulation also abolished the GTP gamma S-induced increase in myosin light chain phosphorylation at constant [Ca2+] and was associated with extensive translocation of p21(rhoA) to the particulate fraction, prevented its immunoprecipitation, and inhibited its ADP ribosylation without affecting the immunodetectable content of G-proteins(p21(rhoA), p21(ras), G(alpha q/11), G(alpha i3), and G(beta)) or protein kinase C (types alpha, beta(1), beta(2), delta, epsilon, eta, theta, and zeta). We conclude that the loss of GTP gamma S- and agonist-induced Ca2+ sensitization through prolonged treatment with GTP gamma S is not due to a decrease in the total content of either trimeric (G(alpha q/11), G(alpha i3), and G Alpha(beta)) or monomeric (p21(rhoA) and p21(ras)) G-protein or protein kinase C but may be related to a structural change of p21(rhoA) and/or to down-regulation of its (yet to be identified) effector.