SK & F 96365 inhibits carbachol-induced phosphoinositide turnover in human neuroblastoma SH-SY5Y and rat cerebellar granule cells.

SK & F 96365, a receptor-mediated Ca2+ entry inhibitor, has been reported to inhibit Ca2+ responses to various agonists without affecting internal Ca2+ release and phosphoinositide turnover. Since muscarinic acetylcholine receptor-mediated phosphoinositide turnover shows a marked dependence on factors affecting cytosolic Ca2+ concentration, the effects of SK & F 96365 on the coupling of muscarinic receptors to the phosphoinositide hydrolysis were examined in human neuroblastoma SH-SY5Y and rat cerebellar granule cells. SK & F 96365 concentration-dependently (3-30 microM) inhibited the inositol phosphate formation elicited by carbachol in both cellular systems. Moreover, SK & F 96365 inhibited the carbachol-induced inositol phosphate formation in the absence of extracellular Ca2+, and similar extent of inhibition was achieved in the presence or absence of extracellular Ca2+. In ligand binding studies, we found that the binding affinities for [3H] N-methyl-scopolamine in both cells were attenuated by SK & F 96365 (3-30 microM), while Bmax values for the ligand were not changed. The competition curves of SK & F 96365 showed a Ki value of 28.4 uM in SH-SY5Y cells. The results indicated that the decrease of carbachol-stimulated phosphoinositide hydrolysis by SK & F 96365 is due to the competitive inhibition of agonist binding to the M3 muscarinic receptors.