Further characterization of the synergistic activation mechanism of cationic channels by M and M muscarinic receptors in mouse intestinal smooth muscle cells.

In mouse ileal myocytes, muscarinic receptor-mediated cationic current () occurs mainly through synergism of M and M subtypes involving G-type GTP-binding proteins and phospholipase C (PLC). We have further studied the M/M synergistic pathway. Carbachol-induced was markedly depressed by YM-254890, a G protein inhibitor. However, the was unaffected by heparin, calphostin C, or chelerythrine, suggesting that activation does not involve signaling molecules downstream of phosphatidylinositol 4,5-bisphosphate (PIP) breakdown. M-knockout (KO) mice displayed a reduced (10% of wild-type ) because of the lack of M-G signaling. The impaired was insensitive to neuropeptide Y possessing a G-stimulating activity. M-KO mice also displayed a reduced (6% of wild-type ) because of the lack of M-G signaling, and the was insensitive to prostaglandin F possessing a G-stimulating activity. These results suggest the importance of G/PLC-hydrolyzed PIP breakdown itself in activation and also support the idea that the M/M synergistic pathway represents a signaling complex consisting of M-G and M-G-PLC systems in which both G proteins are special for this pathway but not general in receptor coupling.