Distinct roles of M1 and M3 muscarinic acetylcholine receptors controlling oscillatory and non-oscillatory [Ca²⁺]_i increase

We examined ACh-induced [Ca²⁺]_i dynamics in pancreatic acinar cells prepared from mAChR subtype-specific knockout (KO) mice. ACh did not induce any [Ca²⁺]_i increase in the cells isolated from M1/M3 double KO mice. In the cells from M3KO mice, ACh (0.3-3μM) caused a monotonic [Ca²⁺]_i increase. However, we found characteristic oscillatory [Ca²⁺]_i increases in cells from M1KO mice in lower concentrations of ACh (0.03-0.3μM).We investigated the receptor specific pattern of [Ca²⁺]_i increase in COS-7 cells transfected with M1 or M3 receptors. ACh induced the oscillatory [Ca²⁺]_i increase in M3 expressing cells, but not in cells expressing M1, which exhibited monotonic [Ca²⁺]_i increases. IP₃ production detected in fluorescent indicator co-transfected cells was higher in M1 than in M3 expressing cells. From the examination of four types of M1/M3 chimera receptors we found that the carboxyl-terminal region of M3 was responsible for the generation of Ca²⁺ oscillations.The present results suggest that the oscillatory Ca²⁺ increase in response to M3 stimulation is dependent upon a moderate IP₃ increase, which is suitable for causing Ca²⁺-dependent IP₃-induced Ca²⁺ release. The C-terminal domain of M3 may contribute as a regulator of the efficiency of Gq and PLC cooperation.