Acetylcholine activates nonselective cation channels in guinea pig ileum through a G protein

Acetylcholine (ACh) depolarizes the membrane of mammalian intestinal myocytes by activating a nonselective cation channel (G. D. Benham, T. B. Bolton, and R. J. Lang. Nature Lond. 316: 345-347, 1985; R. Inoue, K. Kitamura, and H. Kuriyama. Pfluegers Arch. 410: 69-74, 1987). Here, we present evidence that occupation of the muscarinic receptor by ACh couples to channel activation via a G protein; the coupling can be blocked by pertussis toxin or by intracellular guanosine 5'-O-(2-thio-diphosphate) (GDP beta S), whereas intracellular guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) activates the channel in the absence of ACh. The currents, activated by either ACh or GTP gamma S, are nonadditive, conduct sodium ions, and are similar in their voltage dependence and facilitation by submicromolar calcium ions in the cytosol.