The Mechanism of Muscarinic Agonist-Stimulated Inositol Phosphate Formation in Permeabilized Ileal Smooth Muscle

Abstract

Muscarinic agonists and guanylyl-5''-imidodiphosphate (Gpp(NH)p) stimulated formation of inositol phosphates in permeabilized longitudinal smooth muscle of guinea pig ileum. Gpp(NH)p markedly potentiated the formation of inositol bisphosphate (IP₂) and inositol trisphosphate (IP₃) stimulated by carbachol, but increased inositol monophosphate formation (IP₁) only slightly. Gpp(NH)p enhanced the formation of IP₂ + IP₃ induced by either acetylcholine or carbachol about fourfold in a synergistic manner, but enhanced the effects of oxotremorine and pilocarpine less than twofold in an additive manner. Elevation of Ca₂₊ concentration resulted in increases of the inositol phosphate levels stimulated by both carbachol and Gpp(NH)p. The optimal concentration of Ca₂₊ for carbachol-stimulated formations of IP₂ + IP₃ was shifted to a lower Ca₂₊ concentration in the presence of Gpp(NH)p. These findings suggest that muscarinic receptor-stimulated polyphosphoinositide hydrolysis in ileal smooth muscle results in inositol polyphosphate formation via GTP binding protein (G-protein). The muscarinic receptor-activated G-protein decreases the Ca₂₊ requirement of polyphosphoinositide hydrolysis. Muscarinic agonists stimulate inositol polyphosphate formation by interaction of the G-protein activation of a phosphoinositide specific phospholipase C with Ca₂₊ influx.