COEXPRESSION OF LIGAND-GATED P-2X AND G-PROTEIN-COUPLED P-2Y RECEPTORS IN SMOOTH-MUSCLE - PREFERENTIAL ACTIVATION OF P-2Y RECEPTORS COUPLEDTO PHOSPHOLIPASE-C (PLC)-BETA-1 VIA AND THE PLC-BETA-3 VIA G-BETA-GAMMA(I3)

P-2 receptor subtypes and their signaling mechanisms were characterize d in dispersed smooth muscle cells. UTP and ATP stimulated inositol 1, 4,5-triphosphate formation, Ca2+ release, and contraction that were ab olished by U-73122 and guanosine 5'-O-(3-thio) diphosphate, and partly inhibited (50-60%) by pertussis toxin (PTX). ATP analogs (adenosine 5 '-(alpha,beta-methylene)triphosphate, adenosine 5'-(beta, gamma-methyl ene)triphosphate, and 2-methylthio-ATP) stimulated Ca2+ influx and con traction that were abolished by nifedipine and in Ca2+-free medium, Mi cromolar concentrations of ATP stimulated both Ca2+ influx and Ca2+ re lease. ATP and UTP activated G(q/11) and G(i3) in gastric and aortic s mooth muscle and heart membranes, G(q/11) and G(i1) acid/or G(i2) in l iver membranes, and G(o) and G(i1-3) in brain membranes. Phosphoinosit ide hydrolysis stimulated by ATP and UTP was mediated concurrently by G alpha(q/11)-dependent activation of phospholipase (PL) C-beta 1 and G beta gamma(i3)-dependent activation of PLC-beta 3. Phosphoinositide hydrolysis was partially inhibited by PTX or by antibodies to G alpha( q/11), G(beta), PLC-beta 1, or PLC-beta 3, and completely inhibited by the following combinations (PLC-beta 1 and PLC-beta 3 antibodies; G a lpha(q/11) and G(beta) antibodies; PLC-beta 1 and G(beta) antibodies; PTX with either PLC-beta 1 or G alpha(q/11) antibody). The pattern of responses implied that P-2Y2 receptors in visceral, and probably vascu lar, smooth muscle are coupled to PLC-beta 1 via G alpha(q/11) and to PLC-beta 3 via G beta gamma(i3). These receptors co-exist with ligand- gated P-2X1 receptors activated by ATP analogs and high levels of ATP.