Revisit of the Cardiac Inward Rectifier Potassium Current IK1

Inward rectifier potassium currents are present in different types of cells. In the heart, the inward rectifier potassium current I K1 plays a crucial role in maintaining cardiac resting membrane potential and excitability. It is generally believed that the strong inward rectification of cardiac I K1 channels makes it conduct substantial current near the resting potential but carry little or no current at depolarized potentials. However, recent studies in native cardiac myocytes and HEK 293 cell line stably expressing human Kir2.1 gene have demonstrated that a significant transient outward current carried by I K1 channels is activated by the upstroke of action potential. This review will revisit cardiac I K1 channels, especially the previously-ignored transient outward component of I K1 carried by Kir2.1 channels.