M-2 MUSCARINIC RECEPTORS INHIBIT FORSKOLIN - BUT NOT ISOPROTERENOL-MEDIATED RELAXATION IN BOVINE TRACHEAL SMOOTH-MUSCLE

The ability of the M-2 muscarinic receptor to inhibit the relaxant eff ects of forskolin and isoproterenol was investigated in bovine trachea . In most experiments, we measured contractile responses to oxotremori ne-M in smooth muscle isolated from bovine trachea in which a majority of M-3 receptors were inactivated by treatment with N-(2-chloroethyl) -4-piperidinyl diphenylacetate. In the presence of histamine (20 mu M) , the histamine H-2 antagonist cimetidine (10 mu M) and forskolin (4 m u M), responses to oxotremorine-M were antagonized by 1-dihydro-6H-pyr ido[2,3b][1,4]benzodiazepine-6-one (1 mu M) in a manner consistent wit h contractions mediated predominantly by M-2 receptors. When similar e xperiments were conducted in the presence of isoproterenol (0.1 mu M) instead of forskolin, contractions were antagonized in a manner consis tent with an M-3 receptor-mediated response. In similar experiments, w e measured the relaxant potency of isoproterenol and forskolin against histamine-induced contractions in N-(2-chloroethyl)-4-piperidinyl dip henylacetate-treated trachea. By itself, oxotremorine-M (7.5 nM) had n o contractile effect; however, it caused a substantial reduction in th e relaxant potency of forskolin although having little effect on that of isoproterenol. These experiments establish that M-2 receptors inhib it the relaxant effects of forskolin, but not isoproterenol. In untrea ted tissues, the relaxant responses to isoproterenol and forskolin wer e 10.8- and 14.2-fold more potent, respectively, against histamine tha n against oxotremorine-M-induced contractions of equal magnitude. Simi larly, the maximal stimulation of cAMP accumulation elicited by isopro terenol and forskolin was inhibited 58 and 62%, respectively, in the p resence of oxotremorine-M (80 nM) compared to that measured in the pre sence of histamine (20 mu M). Analysis of the data indicated that isop roterenol elicited relaxation at concentrations well beyond those that stimulated maximal levels of cAMP accumulation. Our results indicate that part of the relaxant response to isoproterenol is mediated throug h a non-cAMP-dependent mechanism, and that this mechanism is largely u nopposed by the M-2 receptor.