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 through the activation of phospholipase C-mediated Ca2+ release from intracellular stores. In non-excitable cells, the initial Ca2+ release is typically followed by a prolonged Ca2+ influx phase that is important for the regulation of several Ca2+-sensitive responses, Here we describe an agonist concentration-dependent mechanism by which m3 muscarinic acetylcholine receptors (mAChRs) differentially regulate the magnitude of the release and influx components of a Ca2+ response. In transfected Chinese hamster ovary cells expressing m3 mAChRs, doses of the muscarinic agonist carbachol ranging from 100 nM to 1 mM evoked Ca2+ release responses of increasing magnitude; maximal Ca2+ release was elicited 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 concentrations. Manipulation of the membrane potential revealed that the carbachol-induced Ca2+ influx phase was diminished at depolarized potentials. Importantly, carbachol doses above 10 mu M were found to couple m3 mAChRs to the activation of an inward, monovalent cation current resulting in depolarization of the cell membrane and a selective decrease in the influx, but not release, component of the Ca2+ response, These studies demonstrate, in one experimental system, a mechanism by which a single subtype of G-protein-coupled receptor can utilize the information encoded in the concentration of an agonist to generate distinct intracellular Ca2+ signals.