THE M3 MUSCARINIC ACETYLCHOLINE-RECEPTOR DIFFERENTIALLY REGULATES CALCIUM INFLUX AND RELEASE THROUGH MODULATION OF MONOVALENT CATION CHANNELS

Several types of transmembrane receptors regulate cellular responses t hrough the activation of phospholipase C-mediated Ca2+ release from in tracellular stores. In non-excitable cells, the initial Ca2+ release i s typically followed by a prolonged Ca2+ influx phase that is importan t for the regulation of several Ca2+-sensitive responses, Here we desc ribe an agonist concentration-dependent mechanism by which m3 muscarin ic acetylcholine receptors (mAChRs) differentially regulate the magnit ude of the release and influx components of a Ca2+ response. In transf ected Chinese hamster ovary cells expressing m3 mAChRs, doses of the m uscarinic agonist carbachol ranging from 100 nM to 1 mM evoked Ca2+ re lease responses of increasing magnitude; maximal Ca2+ release was elic ited by the highest carbachol concentration. In contrast, Ca2+ influx was maximal when m3 mAChRs were activated by moderate doses (1-10 mu M ) of carbachol, but substantially reduced at higher agonist concentrat ions. Manipulation of the membrane potential revealed that the carbach ol-induced Ca2+ influx phase was diminished at depolarized potentials. Importantly, carbachol doses above 10 mu M were found to couple m3 mA ChRs to the activation of an inward, monovalent cation current resulti ng in depolarization of the cell membrane and a selective decrease in the influx, but not release, component of the Ca2+ response, These stu dies demonstrate, in one experimental system, a mechanism by which a s ingle subtype of G-protein-coupled receptor can utilize the informatio n encoded in the concentration of an agonist to generate distinct intr acellular Ca2+ signals.