Differential effects of Zn2+ on activation, deactivation, and inactivation kinetics in neuronal voltage-gated Na+...

Whole-cell, patch-clamp recordings were carried out in acutely dissociated neurons from entorhinal cortex (EC) layer II to study the effects of Zn(2+) on Na(+) current kinetics and voltage dependence. In the presence of 200 μM extracellular Cd(2+) to abolish voltage-dependent Ca(2+) currents, and 100 mM extracellular Na(+), 1 mM Zn(2+) inhibited the transient Na(+) current, I (NaT), only to a modest degree (~17% on average). A more pronounced inhibition (~36%) was induced by Zn(2+) when extracellular Na(+) was lowered to 40 mM. Zn(2+) also proved to modify I (NaT) voltage-dependent and kinetic properties in multiple ways. Zn(2+) (1 mM) shifted the voltage dependence of I (NaT) activation and that of I (NaT) onset speed in the positive direction by ~5 mV. The voltage dependence of I (NaT) steady-state inactivation and that of I (NaT) inactivation kinetics were markedly less affected by Zn(2+). By contrast, I (NaT) deactivation speed was prominently accelerated, and its voltage dependence was shifted by a significantly greater amount (~8 mV on average) than that of I (NaT) activation. In addition, the kinetics of I (NaT) recovery from inactivation were significantly slowed by Zn(2+). Zn(2+) inhibition of I (NaT) showed no signs of voltage dependence over the explored membrane-voltage window, indicating that the above effects cannot be explained by voltage dependence of Zn(2+)-induced channel-pore block. These findings suggest that the multiple, voltage-dependent state transitions that the Na(+) channel undergoes through its activation path are differentially sensitive to the gating-modifying effects of Zn(2+), thus resulting in differential modifications of the macroscopic current's activation, inactivation, and deactivation. Computer modeling provided support to this hypothesis.