G-protein coupling of muscarinic receptors in adult and neonatal rat submandibular cells.

The submandibular glands of neonatal and adult rats express muscarinic cholinergic receptors. Receptor occupancy initiates signaling through activation of phospholipase C, hydrolysis of inositol phospholipids, and calcium mobilization. The increased cytoplasmic [Ca2+] activates ion transport pathways, resulting in secretion of primary saliva. We have previously shown that muscarinic receptors are present in the gland of neonates and that they couple effectively to inositol trisphosphate production and Ca2+ mobilization but that the monovalent ion transport paths are poorly activated. To characterize age-related differences in signal transduction further, we examined the coupling of muscarinic receptors to G-proteins by determining the effect of GTP on the IC50 for competition by the muscarinic agonist carbachol with the radiolabeled antagonist, 3H-quinuclidinyl benzylate. Data were fit to one-site and two-site models, and in all cases the two-site model provided the better fit. Using the two-site model, a substantial GTP-induced shift from high affinity to low affinity binding was observed in membranes from adults, whereas more of the receptors were already in the low affinity form in the membranes from neonates, and little additional shift was induced by GTP. These results suggest differences in the G-protein coupling of muscarinic receptors in submandibular cells of adult and early postnatal rats that may be associated with differences in the content, affinity, or properties (i.e., posttranslational modifications) of G-proteins as the cells mature.