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 neuroblastoma (SH-SY5Y) cells with the whole-cell patch-clamp technique, using 10 mM Ba2+ as charge carrier. Currents were only evoked by depolarizations to -30 mV or more positive (holding potential -80 mV), inactivated partially during 200 ms depolarizing steps, and were abolished by 150 microM Cd2+. Currents could be enhanced by Bay K-8644 and partially inhibited by nifedipine, suggesting that they arose in part due to activation of L-type Ca2+ channels. Currents were also inhibited by the marine snail peptide omega-conotoxin GVIA (omega-CgTx). At a concentration of 10 nM inhibition by omega-CgTx was reversible, but at higher concentrations blockade was always irreversible. Although current inhibition by nifedipine was maximal at 1 microM, supramaximal concentrations reduced the inhibitory actions of omega-CgTx in a concentration-dependent manner. Ca2+ channel currents evoked from a holding potential of -50 mV showed no inactivation during 200 ms depolarizations but declined in amplitude with successive depolarizing steps (0.2 Hz). Current amplitudes could be restored by returning the holding potential to -80 mV. Currents evoked from -50 mV were inhibited by nifedipine and omega-CgTx to a similar degree as those evoked from -80 mV. Our results indicate that undifferentiated SH-SY5Y cells possess L- and N-type Ca2+ 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 of N-type channels by omega-CgTx.