MODULATION OF CORONARY SMOOTH-MUSCLE K(CA) CHANNELS BY G(S)ALPHA-INDEPENDENT OF PHOSPHORYLATION BY PROTEIN KINASE-A

The occupancy of beta-receptors in the smooth muscle membrane of the c oronary arteries produces vasodilation and a concomitant hyperpolariza tion. Large conductance calcium-activated K (K(Ca)) channels are likel y to be involved in such hyperpolarization, since they are densely dis tributed in coronary myocytes, and they are targets of beta-adrenergic stimulation in other smooth muscles. We sought to explore if coronary smooth muscle K(Ca) channels are modulated by beta-agonists and we st udied the mechanisms of their activation. We found that K(Ca) channels reconstituted into lipid bilayers were activated in the presence of G TP by the beta-adrenergic receptor agonist isoproterenol. K(Ca) channe ls were also stimulated on non-specific activation of an endogenous G protein(s) with guanosine 5'-O-(3-thiotriphosphate) (GTPgammaS), on ad dition of a purified activated stimulatory G protein (G(s)alpha), and when the catalytic subunit of protein kinase A (PKA) was added. Inhibi tion of PKA activity prevented K(Ca) channel stimulation by PKA, but n ot by endogenous G protein or by exogenous G(s)alpha. These results in dicate that beta-adrenoceptor activation of coronary smooth muscle K(C a) channels results from a dual control: 1) a membrane delimited, poss ibly direct action of G(s), independent of PKA-mediated phosphorylatio n; and 2) by PKA-dependent phosphorylation.