Studies on maitotoxin-induced intracellular Ca(2+) elevation in chinese hamster ovary cells stably transfected with cDNAs encoding for L-type Ca(2+) channel subunits.

The aim of the present study was to characterize the role played by different L-type Ca2+ channel subunits in [Ca2+]i increase induced by maitotoxin (MTX). In the presence of 5 mM extracellular K+, MTX (0.01–0.5 ng/ml) induced a significant concentration-dependent increase in Fura-2-monitored [Ca2+]i in single Chinese hamster ovary (CHO) cells expressing the α1c (CHOCα9 cells) or the α1cβ3α2δ (CHOCα9β3α2/δ4 cells) subunits of voltage-gated Ca2+ channels (VGCCs), whereas the effect was much reduced in wild-type CHO cells lacking VGCCs. In addition, MTX effect on CHOCα9, CHOCα9β3α2/δ4, and GH3 cells (0.01–0.1 ng/ml) was inhibited by the selective L-type Ca2+ channel entry-blocker nimodipine (10 μM); a nimodipine-insensitive component was still present, particularly at high (>1 ng/ml) toxin concentrations. In CHOCα9β3α2/δ4 cells, depolarizing concentrations of extracellular K+ (55 mM) reinforced the [Ca2+]i increase induced by MTX (0.1 ng/ml), and this effect was prevented by nimodipine (10 μM). Finally, patch-clamp experiments in CHOCα9β3α2/δ4 cells showed that low MTX concentrations (0.03 ng/ml) induced the occurrence of an inward current at −60 mV, which was completely prevented by Cd2+ (100 μM) and by nimodipine (10 μM), whereas the same dihydropyridine concentration (10 μM) failed to prevent the electrophysiological effects of a higher toxin concentration (3 ng/ml). In conclusion, the results of the present study showed that MTX-induced [Ca2+]i elevation involves two components: 1) an action on L-type VGCCs at the pore-forming α1c subunit level, which is responsible for the greatest rise of [Ca2+]i; and 2) a VGCC-independent mechanism that is present both in excitable and in nonexcitable cells and is responsible for a lower elevation of [Ca2+]i.