Calcium Channel Currents in Undifferentiated Human Neuroblastoma (SH‐SY5Y) Cells: Actions and Possible Interactions of Dihydropyridines and ω‐Conotoxin

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Abstract
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 μMCd2+. 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 ω‐conotoxin GVIA (ω‐CgTx). At a concentration of 10 nM inhibition by ω‐CgTx was reversible, but at higher concentrations blockade was always irreversible. Although current inhibition by nifedipine was maximal at 1μM, supramaximal concentrations reduced the inhibitory actions of ω‐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 ω‐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 ω‐CgTx.