Comparison of functional antagonism between isoproterenol and M-2 muscarinic receptors in guinea pig ileum and trachea

The ability of the M-2 muscarinic receptor to mediate an inhibition of the relaxant effects of forskolin and isoproterenol was investigated in guinea pig ileum and trachea. In some experiments, trachea was first treated with 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) mustard to inactivate M-3 rec eptors. The contractile response to oxotremorine-M was measured subsequentl y in the presence of both histamine (10 mu M) and isoproterenol (10 nM). Un der these conditions, [[2-[(diethylamino)methyl]-1-piperidinyl]acetyl]-5,11 -dihydro-6H-pyrido[2,3-b]- [1,4]benzodiazepine-6-one (AF-DX 116) antagonize d the contractile response to oxotremorine-M in a manner consistent with an M-3 mechanism. However, when the same experiment was repeated using forsko lin (4 mu M) instead of isoproterenol, the response to oxotremorine-hn exhi bited greater potency and was antagonized by AF-DX 116 in a manner consiste nt with an M-2 mechanism. We also measured the effects of pertussis toxin t reatment on the ability of isoproterenol to inhibit the contraction elicite d by a single concentration of either histamine (0.3 mu M) or oxotremorine- M (40 nM) in both the ileum and trachea. Pertussis toxin treatment had no s ignificant effect on the potency of isoproterenol for inhibiting histamine- induced contractions in the ileum and trachea. In contrast, pertussis toxin treatment enhanced the relaxant potency of isoproterenol not in the trache a. Also, pertussis toxin treatment enhanced the relaxant potency of forskol in against oxotremorine-M-induced contractions in the ileum and trachea. We investigated the relaxant potency of isoproterenol when very low, equi-eff ective (i.e., 20-34% of maximal response) concentrations of either histamin e or oxotremorine-hn were used to elicit contraction. Under these condition s, isoproterenol exhibited greater relaxant potency against histamine in th e ileum but exhibited similar relaxant potencies against histamine and oxot remorine-M in the trachea. Following 4-DAMP mustard treatment, a low concen tration of oxotremorine-M (10 nM) had no contractile effect in either the i leum or trachea. Nevertheless, in 4-DAMP mustard-treated tissue, oxotremori ne-M (10 nM) reduced the relaxant potency of isoproterenol against histamin e-induced contractions in the ileum, but not in the trachea. We conclude th at in the trachea the M-2 receptor mediates an inhibition of the relaxant e ffects of forskolin, but not isoproterenol, and the decreased relaxant pote ncy of isoproterenol against contractions elicited by a muscarinic agonist relative to histamine is not due to activation of M-2 receptors but rather to the greater contractile stimulus mediated by the M-3 receptor compared w ith the H-1 histamine receptor.