Different Pathways for Ca2+Influx and Intracellular Release of Ca2+Mediated by Muscarinic Receptors in Ileal Longitudinal Smooth Muscle

Abstract

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 × 10-5M. On the other hand, in the presence of Ca2+, the ED50 value was 2.5 × 10-7M, indicating that a low concentration of CCh (< 10-7M) 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 × 10-5M), 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.