Evidence for the coupling of muscarinic M3 receptor to cyclic AMP formation and poly-phosphatidylinositol turnover in rat salivary glands.

The biochemical events which are coupled to the subtypes of muscarinic cholinergic receptors in rat salivary glands were studied. The subtype property of this receptor system was characterized by the use of 3H-QNB binding to glandular membrane homogenates. The Kd and Bmax values of this binding were found to be 0.2 nM and 210 fmole/mg protein respectively. In the drug-displacement study on the 3H-QNB binding the following potency order was obtained (based on the IC50 values calculated from each individual dose-response curve): Atropine greater than 4-DAMP greater than HHSiD greater than Pirenzepine greater than AF-DX 116. This order is a typical M3 muscarinic subtype, according to the recent consensus. Carbachol (0.1 mM) caused a 4.4-fold increased in IP3 production over the basal level, when tissue fragments were prelabeled with 3H-inositol. The above mentioned cholinergic antagonists could block this event in a similar potency order. Carbachol (0.1 mM) did not have a significant effect on the basal level of cAMP formation of these tissue homogenates. On the other hand, it had a 33% reduction of isoproterenol (10 microM)-enhancing cAMP formation. The reduction effect of carbachol on cAMP formation could also be blocked by the above mentioned cholinergic antagonists in a similar order. Our results indicated that rat salivary glands have M3 muscarinic receptors and the activation of these receptors causes changes in both phosphatidylinositol turnover and cAMP formation.