title: G protein-mediated inhibition of myosin light-chain phosphatase in vascular smooth muscle.
abstract: The mechanism of G protein-mediated sensitization of the contractile apparatus of smooth muscle to Ca2+ was studied in receptor-coupled alpha-toxin-permeabilized rabbit portal vein smooth muscle. To test the hypothesis that Ca2+ sensitization is due to inhibition of myosin light-chain (MLC) phosphatase activity, we measured the effect of guanosine 5'-[gamma-thio]triphosphate and phenylephrine on the rate of MLC dephosphorylation in muscles preactivated with Ca2+ and incubated in Ca(2+)- and ATP-free solution containing 1-(5-chloronaphthalene-1-sulfonyl)-1H-hexahydro-1,4-diazepine (ML-9) to block MLC kinase activity. Guanosine 5'-[gamma-thio]triphosphate alone (300 microM) or in combination (3 microM) with phenylephrine decreased the rates of relaxation and dephosphorylation of MLC to about half of control values; this inhibition is sufficient to account for maximal G protein-mediated Ca2+ sensitization of MLC phosphorylation. The rate of thiophosphorylation of MLC with adenosine 5'-[gamma-thio]-triphosphate was not affected by guanosine 5'-[gamma-thio]triphosphate. We suggest that inhibition of protein phosphatase(s) by G protein(s) may have important regulatory functions.
authors.affiliation: Department of Physiology, University of Virginia School of Medicine, Charlottesville 22908.
authors.forename: T
authors.initials: T
authors.lastname: Kitazawa
authors.forename: M
authors.initials: M
authors.lastname: Masuo
authors.forename: A P
authors.initials: AP
authors.lastname: Somlyo
mesh_terms: D000255:Adenosine Triphosphate / Q000031:analogs & derivatives / Q000494:pharmacology; D000818:Animals; D001381:Azepines / Q000494:pharmacology; D000001:Calcimycin / Q000494:pharmacology; D002118:Calcium / Q000494:pharmacology; D019204:GTP-Binding Proteins / Q000502:physiology*; D016244:Guanosine 5'-O-(3-Thiotriphosphate) / Q000494:pharmacology*; D066298:In Vitro Techniques; D007700:Kinetics; D009119:Muscle Contraction / Q000187:drug effects; D009131:Muscle, Smooth, Vascular / Q000187:drug effects / Q000201:enzymology / Q000502:physiology*; D043263:Myosin-Light-Chain Phosphatase; D009218:Myosins / Q000378:metabolism; D010749:Phosphoprotein Phosphatases / Q000037:antagonists & inhibitors*; D010766:Phosphorylation; D011169:Portal Vein / Q000187:drug effects / Q000201:enzymology / Q000502:physiology*; D011817:Rabbits
chemical_list: D001381:Azepines; C056218:ML 9; C022571:adenosine 5'-O-(3-thiotriphosphate); D016244:Guanosine 5'-O-(3-Thiotriphosphate); D000001:Calcimycin; D000255:Adenosine Triphosphate; D010749:Phosphoprotein Phosphatases; D043263:Myosin-Light-Chain Phosphatase; D019204:GTP-Binding Proteins; D009218:Myosins; D002118:Calcium