Inhibition of N-type Ca2+ channel currents in human neuroblastoma (SH-SY5Y) cells by muscarine via stimulation of M3 receptors.

The effects of muscarine on whole-cell Ca2+ channel currents in SH-SY5Y cells were studied using conventional and perforated-patch-clamp techniques, with 10 mM Ba2+ as charge carrier. Muscarine (10-300 microM) caused concentration-dependent inhibitions of Ca2+ channel currents which were only reversible when perforated-patch recordings were used. Inhibition of currents was associated with slowing of activation kinetics in approximately 50% of cells. In the presence of 5 microM nifedipine, muscarine was still able to inhibit currents, but after pre-exposure of cells to 1 microM omega-conotoxin GVIA the inhibitory effects of muscarine were almost completely lost. In the presence of 100 microM muscarine, Bay K 8644 (5 microM) was still able to enhance current amplitudes. Pre-treatment of cells with pertussis toxin (250 ng/ml for 16-24 hr) or inclusion of 1 mM GDP-beta-S in the patch-pipette prevented the inhibitory actions of muscarine. Hexahydrosiladifenidol (0.1-1 microM) antagonized the actions of muscarine (calculated pA2 7.1) but the presence of 10 microM pirenzipine or 0.1 microM methoctramine in the bath solution did not alter the degree of current inhibition caused by 100 microM muscarine. In summary, these results indicate that muscarine in SH-SY5Y cells causes inhibition of N-type Ca2+ channels via a M3 receptor coupled to a pertussis toxin-sensitive G-protein.