Ni2+ slows the activation kinetics of high-voltage-activated Ca2+ currentsin cortical neurons: Evidence for a mechanism of action independent of channel-pore block

The effects of Ni2+ were evaluated on slowly-decaying, high-voltage-activat ed (HVA) Ca2+ currents expressed by pyramidal neurons acutely dissociated f rom guinea-pig piriform cortex. Whole-cell, patch-clamp recordings were per formed with Ba2+ as the charge carrier. Ni2+ blocked HVA Ba2+ currents (I(B a)s) with an EC50 of approximately 60 muM. Additionally, after application of nonsaturating Ni2+ concentrations, residual currents activated with subs tantially slower kinetics than both total and Ni2+-sensitive I(Ba)s. None o f the pharmacological components of slowly decaying, HVA currents activated with kinetics significantly different from that of total currents, indicat ing 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 Ni2on I-Ba amplitude was voltage-independent over the potential range normally explored in our experiments (-60 to +20 mV), hence the Ni2+-dependent decr ease of I-Ba activation rate is not due to a voltage- and time-dependent re lief from block. Moreover: Ni2+ significantly reduced I-Ba deactivation spe ed 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 fo r I,, block, with an EC50 of similar to 20 muM and a Hill coefficient of si milar to1.73 vs. similar to1.10. These results demonstrate that Ni2+, besid es 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+-cha nnel gating kinetics, and strongly suggest that the two effects depend on N i2+ occupancy of binding sites at least partly distinct.