CALCIUM-CHANNEL CURRENTS IN UNDIFFERENTIATED HUMAN NEUROBLASTOMA (SH-SY5Y) CELLS - ACTIONS AND POSSIBLE INTERACTIONS OF DIHYDROPYRIDINES AND OMEGA-CONOTOXIN

Ca2+ channel currents were recorded in undifferentiated human neurobla stoma (SH-SY5Y) cells with the whole-cell patch-clamp technique, using 10 mM Ba2+ as charge carrier. Currents were only evoked by depolariza tions to - 30 mV or more positive (holding potential - 80 mV), inactiv ated partially during 200 ms depolarizing steps, and were abolished by 150 mu M Cd2+. Currents could be enhanced by Bay K-8644 and partially inhibited by nifedipine, suggesting that they arose in part due to ac tivation of L-type Ca2+ channels. Currents were also inhibited by the marine snail peptide omega-conotoxin GVIA (omega-CgTx). At a concentra tion of 10 nM inhibition by omega-CgTx was reversible, but at higher c oncentrations blockade was always irreversible. Although current inhib ition by nifedipine was maximal at 1 mu M, supramaximal concentrations reduced the inhibitory actions of omega-CgTx in a concentration-depen dent manner. Ca2+ channel currents evoked from a holding potential of - 50 mV showed no inactivation during 200 ms depolarizations but decli ned in amplitude with successive depolarizing steps (0.2 Hz). Current amplitudes could be restored by returning the holding potential to - 8 0 mV. Currents evoked from - 50 mV were inhibited by nifedipine and om ega-CgTx to a similar degree as those evoked from - 80 mV. Our results indicate that undifferentiated SH-SY5Y cells possess L- and N-type Ca 2+ channels which can be distinguished pharmacologically but cannot be separated by using depolarized holding potentials. Furthermore, these data suggest that nifedipine has a novel action to inhibit blockade o f N-type channels by omega-CgTx.