Both M(1) and M(3) receptors regulate exocrine secretion by mucous acini

We investigated the role of M(1) and M(3) receptors in regulating exocrine secretion from acini isolated from rat sublingual glands. In secretion experiments, we derived affinity values (K-B) from Schild regression analysis for the antagonists pirenzepine (61.0 nM) and 4-diphenylacetoxy-N-methylpiperidine (4-DAMP; 1.06 nM). The K-B for 4-DAMP is similar to its affinity value [equilibrium dissociation constant from competition studies (K-i); 1.81 nM] determined from radioligand competition experiments. In contrast, the K-B for pirenzepine is between its high-affinity (17.6 nM) and low-affinity (404 nM) K-i values. In separate secretion experiments, we found that the M(1) receptor antagonist, M(1)-toxin, induces a rightward shift in the concentration-response curve to muscarinic agonist and inhibits maximal secretion by 40%. The inhibitory effect of M(1)-toxin appears specific for M(1) receptor blockade, since the toxin abolishes acinar high-affinity pirenzepine-binding sites and does not inhibit secretion induced by nonmuscarinic agents. Additional pharmacological studies indicate muscarinic receptors do not function through putative neural elements within isolated acini. Our combined results are consistent with both M(1) and M(9) receptors directly regulating mucous acinar exocrine secretion and indicate M(3) receptors alone are insufficient to induce a maximal muscarinic response.