Inwardly rectifying Kir2 potassium channels: Differential properties and contributions to channels underlying cardiac IK1.

Abstract

The cardiac inwardly rectifying potassium current I_K1 is responsible for stabilizing the resting membrane potential and shaping the late phase of repolarization of the action potential. I_K1 channels are composed of subunits belonging to the Kir2 subfamily of inward rectifier potassium channels. Genetic and pharmacological manipulation of Kir2 channels in experimental animals and identification of mutations in the human <italic>Kir2.1</italic> gene have highlighted the role of I_K1 in the genesis of cardiac arrhythmias. Nevertheless, significant gaps remain in understanding of the essential properties of Kir2 channels and contributions of specific Kir2 subunits to I_K1. The results showed that different homomeric Kir2 channels exhibit distinct sensitivities to block by the polyamine spermine (a major intracellular factor underlying inward rectification in Kir2 channels) with Kir2 2 and Kir2.3 channels being the most and least sensitive, respectively. The rates of spermine unblock (channel activation) were significantly slower in Kir2.3 channels compared with that in Kir2.1 and Kir2.2 channels. Experiments using a concatemeric approach showed that the activation of heteromeric Kir2.1/Kir2.3 channels was significantly slower than that of homomeric Kir2.1 channels. In mouse, the kinetics of activation of I_K1 was fast in both ventricular and atrial myocytes, suggesting no significant contribution of the Kir2.3 subunit. The strength of rectification in atrial myocytes was weaker than that in ventricular myocytes, and it also varied greatly between individual myocytes isolated from both tissues. The data support the hypothesis that variation in concentration of free intracellular polyamines contributes to regional and cellular heterogeneity of I_K1. The contribution of Kir2.1 and Kir2.2 subunits to the function of heteromeric channels was studied using a concatemeric approach. Measurements of single-channel conductances, mean open times and Ba<super>2+</super> sensitivities showed that Kir2.1 or Kir2.2 subunits exert neither a 'dominant' nor an 'anomalous' effect on any of the properties of heteromeric channels and provided a set of unique parameters useful for deciphering the subunit composition of I_K1. The results have provided insights into the function of Kir2 channels and the contributions of distinct Kir2 subunits to I_K1, and will be important for the development of future treatments for arrhythmias.