The Putative M1 Muscarinic Receptor Does Not Regulate

Muscarinic receptor activation stimulates phosphoinositide hydrolysis and inhibits cyclic AMP formation in dissociated embryonic chick heart cells. We used this preparation to examine the hypothesis that the putative M, and M2 receptor subtypes are selectively coupled to these two responses. Atropine blocks the effects of carbachol on cyclic AMP formation and phosphoinositide breakdown with nearly identical K1 values (1.9 and 0.8 nM); these values are close to the apparent KD (1.8 nM) of atropine competition for [3H] N-methylscopolamine binding. Pirenzepine blocks the effect of carbachol on cyclic AMP formation with a K1 of 48 nM, a value similar to the apparent KD (23 nM) determined in radioligand-binding studies. In contrast, a higher concentration of pirenzepine is needed to inhibit carbachol-stimulated phosphoinositide hydrolysis (K1 = 255 nM). Two selective agonists, McN-A343 and AHR 602, inhibit cyclic AMP formation but do not stimulate phosphoinositide hydrolysis in chick heart cells. Muscarinic receptor-mediated phos-phoinositide hydrolysis in 1321N1 astrocytoma cells is also insensitive to McN-A343 or AHR 602 and is antagonized only by relatively high concentrations of pirenzepine. The M, receptor, as previously defined, has high affinity for pirenzepine and is activated by McN-A343. We find that these ligands have greater activity at muscarinic receptors that inhibit cyclic AMP formation than at those that stimulate phosphoinositide hydrolysis. Thus, if different receptor subtypes are associated with these two responses, the M, receptor regulates cyclic AMP rather than phosphoinositide metabolism. Our data also demonstrate that the chick heart has muscarinic receptors with high affinity for piren-zepine, and thus, in contrast to rat heart, appears to have predominantly M1 receptors