THE PUTATIVE M-1 MUSCARINIC RECEPTOR DOES NOT REGULATE PHOSPHOINOSITIDE HYDROLYSIS - STUDIES WITH PIRENZEPINE AND MCN-A343 IN CHICK HEART AND ASTROCYTOMA-CELLS

Abstract

Muscarinic receptor activation stimulates phosphoinositide hydrolysis and inhibits cAMP formation in dissociated embryonic chick heart cells. This preparation was used to examine the hypothesis that the putative M1 and M2 receptor subtypes are selectively coupled to these 2 responses. Atropine blocks the effects of carbachol on cAMP formation and phosphoinositide breakdown with nearly identical KI 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 cAMP formation with a KI 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 (KI = 255 nM). Two selective agonists, McN-A343 and AHR 602, inhibit cAMP formation but do not stimulate phosphoinositide hydrolysis in chick heart cells. Muscarinic receptor-mediated phosphoinositide 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 M1 receptor, as previously defined, has high affinity for pirenzepine and is activated by McN-A343. These ligands have greater activity at muscarinic receptors that inhibit cAMP formation that at those that stimulate phosphoinositide hydrolysis. Thus, if different receptor subtypes are associated with these 2 responses, the M1 receptor regulates cAMP rather than phosphoinositide metabolism. Data also demonstrates that the chick heart has muscarinic receptors with high affinity for pirenzepine, and thus, in contrast to rat heart, appears to have predominantly M1 receptors.