Zn2+ current is mediated by voltage-gated Ca2+ channels and enhanced by extracellular acidity in mouse cortical neurones
- 1Mammalian neuronal voltage-gated Ca2+ channels have been implicated as potential mediators of membrane permeability to Zn2+. We tested directly whether voltage-gated Ca2+ channels can flux Zn2+ in whole-cell voltage-clamp recordings from cultured murine cortical neurones.
- 2In the presence of extracellular Zn2+ and no Na+, K+, or other divalent cations, a small, non-inactivating, voltage-gated inward current was observed exhibiting a current-voltage relationship characteristic of high-voltage activated (HVA) Ca2+ channels. Inward current was detectable at Zn2+ levels as low as 50 μm, and both the amplitude and voltage sensitivity of the current depended upon Zn2+ concentration. This Zn2+ current was sensitive to blockade by Gd3+ and nimodipine and, to a lesser extent, by ω-conotoxin GVIA.
- 3Zn2+ could permeate Ca2+ channels in the presence of Ca2+ and other physiological cations. Inward currents recorded with 2 mm Ca2+ were attenuated by Zn2+ (IC50= 210 μm), and currents recorded with Zn2+ were unaffected by up to equimolar Ca2+ concentrations. Furthermore, the Zn2+-selective fluorescent dye Newport Green revealed a depolarisation-activated, nimodipine-sensitive Zn2+ influx into cortical neurones that were bathed in a physiological extracellular solution plus 300 μm ZnCl2.
- 4Surprisingly, while lowering extracellular pH suppressed HVA Ca2+ currents, Zn2+ current amplitude was affected oppositely, varying inversely with pH with an apparent pK of 7·4. The acidity-induced enhancement of Zn2+ current was associated with a positive shift in reversal potential but no change in the kinetics or voltage sensitivity of channel activation.
- 5These results provide evidence that L- and N-type voltage-gated Ca2+ channels can mediate Zn2+ entry into cortical neurones and that this entry may be enhanced by extracellular acidity.