Muscarinic M3 receptors are coupled to two signal transduction pathways in rat submandibular cells.

The receptor subtypes involved in muscarinic-induced phosphoinositide hydrolysis and adenylate cyclase inhibition in rat submandibular acinar cells were characterized by comparing the inhibitory potencies of four muscarinic antagonists on the two signal transduction responses. Carbachol-induced phosphatidylinositol 4,5-bisphosphate (PIP2) hydrolysis was inhibited by all antagonists with a potency rank order of 4-diphenylacetoxy-N-methyl piperidine methobromide (4-DAMP) = atropine much greater than pirenzepine much greater than AF-DX 116 (P less than 0.01). The same rank order was observed in antagonist-reversal of the reduction of cAMP caused by carbachol in the model. These findings suggest that muscarinic effects are mediated by M3 receptors in both the phosphoinositide and adenylate cyclase pathways in the submandibular gland.