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