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 present in several tissues: Its conductance increases under hyperpolarization and decreases with depolarization. In this work we studied the effects of Zn2+ and Ni2+ (5-30 mM) on the macroscopic K+ current through the inward rectifier system. The experiments were performed in the short muscle fibers of the lumbricalis muscle of toads with a two-microelectrode voltage clamp technique. The fibers were equilibrated in a control 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 slightly dependent on the membrane potential and time independent. Two mechanisms may be involved in the blocking action of these cations: Zn2+ and Ni2+ may either be blocking the pore of the channels or acting at a regulatory binding site on the extracellular surface in an unspecified manner.