BLOCKADE OF THE INWARD RECTIFIER POTASSIUM CURRENTS BY ZINC AND NICKEL IONS IN VOLTAGE-CLAMPED TOAD MUSCLES

The inward rectifier is one of the voltage-sensitive K+ channels prese nt in several tissues: Its conductance increases under hyperpolarizati on and decreases with depolarization. In this work we studied the effe cts of Zn2+ and Ni2+ (5-30 mM) on the macroscopic K+ current through t he inward rectifier system. The experiments were performed in the shor t muscle fibers of the lumbricalis muscle of toads with a two-microele ctrode voltage clamp technique. The fibers were equilibrated in a cont rol solution containing 68 mM K2SO4 and then exposed to Zn2+ or Ni2+. We found that both cations reduced in a reversible manner the current carried by K+ ions, and this reduction was prevented by decreasing the external pH of the solution (pH 5). The blockade of current was sligh tly dependent on the membrane potential and time independent. Two mech anisms may be involved in the blocking action of these cations: Zn2+ a nd Ni2+ may either be blocking the pore of the channels or acting at a regulatory binding site on the extracellular surface in an unspecifie d manner.