Different pathways for Ca2+ influx and intracellular release of Ca2+ mediated by muscarinic receptors in ileal longitudinal smooth muscle.

Muscarinic receptor-mediated elevations in intracellular Ca2+ concentration (Ca2+]i) in the longitudinal smooth muscle of guinea pig ileum were studied by the use of fura-2 fluorescence. Dose-response analysis indicated a difference in the potencies of carbachol (CCh) to increase Ca2+]i in the presence and absence of extracellular Ca2+. For the increase in Ca2+]i due to Ca2+ release from intracellular stores in the absence of extracellular Ca2+, the ED50 value of CCh was 3 x 10(-5) M. On the other hand, in the presence of Ca2+, the ED50 value was 2.5 x 10(-7) M, indicating that a low concentration of CCh (less than 10(-7) M) caused influx of extracellular Ca2+ without Ca2+ release. Oxotremorine and pilocarpine induced Ca2+ influx, but were less potent inducers of Ca2+ release. CCh also stimulated the formation of inositol trisphosphates (IP3) with an ED50 value of (4.5 x 10(-5) M), which was similar to that for Ca2+ release from intracellular stores. Treatment of the smooth muscle with neomycin (1 mM), a phospholipase C inhibitor, abolished both CCh-induced IP3 formation and Ca2+ release from intracellular stores, but did not affect CCh-induced Ca2+ influx. These results suggest that the pathway for muscarinic stimulation of Ca2+ influx through plasma membranes is different from that for Ca2+ release from intracellular stores, which seems to be coupled with IP3 formation.