Ni2+ slows the activation kinetics of high-voltage-activated Ca2+ currents in cortical neurons: evidence for a mechanism of action independent of channel-pore block.

The effects of Ni2+ were evaluated on slowly-decaying, high-voltage-activated (HVA) Ca2+ currents expressed by pyramidal neurons acutely dissociated from guinea-pig piriform cortex. Whole-cell, patch-clamp recordings were performed with Ba2+ as the charge carrier. Ni2+ blocked HVA Ba2+ currents (IBas) with an EC50 of approximately 60 microM. Additionally, after application of nonsaturating Ni2+ concentrations, residual currents activated with substantially slower kinetics than both total and Ni2+-sensitive I(Ba)s. None of the pharmacological components of slowly decaying, HVA currents activated with kinetics significantly different from that of total currents, indicating that the effect of Ni2+ on I(Ba)s kinetics cannot be attributed to the preferential inhibition of a fast-activating component. The effect of Ni2+ on I(Ba) amplitude was voltage-independent over the potential range normally explored in our experiments (-60 to +20 mV), hence the Ni2+-dependent decrease of I(Ba) activation rate is not due to a voltage- and time-dependent relief from block. Moreover, Ni2+ significantly reduced I(Ba) deactivation speed upon repolarization, which also is not compatible with a depolarization-dependent unblocking mechanism. The dependence on Ni2+ concentration of the I(Ba) activation-rate reduction was remarkably different from that found for I(Ba) block, with an EC50 of approximately 20 microM and a Hill coefficient of approximately 1.73 vs. approximately 1.10. These results demonstrate that Ni2+, besides inhibiting the I(Ba)s under study probably by exerting a blocking action on the pore of the underlying Ca2+ channels, also interferes with Ca2+-channel gating kinetics, and strongly suggest that the two effects depend on Ni2+ occupancy of binding sites at least partly distinct.