Gating mechanism of the cloned inward rectifier potassium channel from mouse heart

The complementary DNA encoding the inward rectifier potassium channel was cloned from the adult mouse heart by using the polymerase chain reaction. The clone had the nucleotide sequence identical to that of the IRK1 gene cloned from a mouse macrophage cell line. Northern blot analysis revealed that the transcript of this gene was mainly expressed in the ventricle, where the inward rectifier K+ channel plays a predominant role in maintaining the high negative value of the resting membrane potential. The current expressed by injection of the complementary RNA of the cloned gene into Xenopus oocytes showed a marked inward rectification that depends on the driving force of K+. A region of negative slope conductance was observed in the current-voltage relationship at potentials positive to the reversal potential. When the extracellular K+ concentration was raised, the increase in outward current amplitude resulted in the "crossover" of outward current-voltage relations. The fast time-dependent increase in current amplitude was recorded upon membrane repolarization from a potential positive to the reversal potential. The kinetics of the time-dependent current was very similar to that of the intrinsic gating mechanism of the native cardiac inward rectifier K+ channel. Our results suggest the existence of the intrinsic gating mechanism, accounting for the extent of rectification in the current-voltage relationship in the expressed channel.