ON THE MECHANISM OF MODULATION OF TRANSIENT OUTWARD CURRENT IN CULTURED RAT HIPPOCAMPAL-NEURONS BY DIVALENT AND TRIVALENT CATIONS

1. The mechanisms of Zn2+ modulation of transient outward current (TOC ) were studied in cultured rat hippocampal neurons, using the voltage- clamp technique. In the presence of micromolar concentrations of exter nal Zn2+, the voltage dependence of activation and inactivation was sh ifted to more positive membrane potentials. The gating of TOC was unal tered by internal application of Zn2+. The effects of Zn2+ were not mi micked by external Ca2+, except at very high concentrations (>10 mM). 2. The modulatory effects of external Zn2+ on TOC gating were not repr oduced, antagonized, nor enhanced by lowering external ionic strength, indicating that modulation by Zn2+ does not occur via screening of bu lk surface negative charge. 3. A range of other divalent and trivalent metal ions also was studied, and several were found to modulate the t ransient outward current when added to the extracellular medium. In pa rticular, Pb2+, La3+, and Gd3+ were potent modulators, showing activit y in the low micromolar range. Other metal ions were weaker modulators (e.g., Cd2+) or were without activity at the concentrations tested (F e3+, Cu2+, Ni2+). 4. The same range of ions also was tested on the del ayed rectifier K+ current in cultured rat hippocampal neurons. None of the ions studied had significant effects on delayed rectifier gating, although high (greater than or equal to 100 mu M) concentrations of P b2+ and La3+ reduced maximal current amplitude, suggesting the possibi lity of channel block. 5. These highly selective modulatory effects of metal ions suggest the existence of a distinct metal-ion binding site on the extracellular face of the K+ channels that generate the transi ent outward current.