Dual effect of Zn2+ on multiple types of voltage-dependent Ca2+ currents in rat palaeocortical neurons

The effects of Zn2+ were evaluated on high-voltage-activated Ca2+ currents expressed by pyramidal neurons acutely dissociated from rat piriform cortex. Whole-cell, patch-clamp experiments were carried out using Ba2+ (5 mM) as the charge carrier. Zn2+ blocked total high-voltage-activated Ba2+ currents with an IC50 of approximately 21 muM. In addition, after application of non-saturating Zn2+ concentrations, residual currents activated with substantially slower kinetics than control Ba2+ currents. Both of the above-mentioned effects of Zn2+ were also observed in high-voltage-activated currents recorded in the presence of nearly-physiological concentrations of extracellular Ca2+ (1 and 2 mM) rather than Ba2+. Under the latter conditions, 30 muM Zn2+ inhibited high-voltage-activated currents somewhat less than observed in extracellular Ba2+ (similar to47% and similar to41%, respectively, vs. similar to59%), but slowed Ca2+-current activation to very similar degrees. All of the pharmacological components in which Ba2+ currents could be dissected (L-, N-, P/Q-, and R-type) were inhibited by Zn2+, the percentage of current blocked by 30 muM Zn2+ ranging from 34 to 57%. Moreover, the activation kinetics of all pharmacological Ba2+ current components were slowed by Zn2+. Hence, the lower activation speed observed in residual Ba2+ currents after Zn2+ block is due to a true slowing of macroscopic Ca2+-current activation kinetics and not to the preferential inhibition of a fast-activating current component. The inhibitory effect of Zn2+ on Ba2+ current amplitude was voltage-independent over the whole voltage range explored (-60 to +30 mV), hence the Zn2+-dependent decrease of Ba2+ current activation speed is not the consequence of a voltage- and time-dependent relief from block. Zn2+ also caused a slight, but significant, reduction of Ba2+ current deactivation speed upon repolarization, which is further evidence against a depolarization-dependent unblocking mechanism. Finally, the slowing effect of Zn2+ on Ca2+-channel activation kinetics was found to result in a significant, extra reduction of Ba2+ current amplitude when action-potential-like waveforms, rather than step pulses, were used as depolarizing stimuli. We conclude that Zn2+ exerts a dual action on multiple types of voltage-gated Ca2+ channels, causing a blocking effect and altering the speed at which channels are delivered to conducting states, with mechanism(s) that could be distinct. (C) 2003 IBRO. Published by Elsevier Science Ltd. All rights reserved.