Regional variation of the inwardly rectifying potassium current in the canine heart and the contributions to differences in action potential repolarization

Abstract The inward rectifier potassium current, I K1 , contributes to the terminal phase of repolarization of the action potential (AP), as well as the value and stability of the resting membrane potential. Regional variation in I K1 has been noted in the canine heart, but the biophysical properties have not been directly compared. We examined the properties and functional contribution of I K1 in isolated myocytes from ventricular, atrial and Purkinje tissue. APs were recorded from canine left ventricular midmyocardium, left atrial and Purkinje tissue. The terminal rate of repolarization of the AP in ventricle, but not in Purkinje, depended on changes in external K + ([K + ] o ). Isolated ventricular myocytes had the greatest density of I K1 while atrial myocytes had the lowest. Furthermore, the outward component of I K1 in ventricular cells exhibited a prominent outward component and steep negative slope conductance, which was also enhanced in 10 mM [K + ] o . In contrast, both Purkinje and atrial cells exhibited little outward I K1 , even in the presence of 10 mM [K + ] o , and both cell types showed more persistent current at positive potentials. Expression of Kir2.1 in the ventricle was 76.9-fold higher than that of atria and 5.8-fold higher than that of Purkinje, whereas the expression of Kir2.2 and Kir2.3 subunits was more evenly distributed in Purkinje and atria. Finally, AP clamp data showed distinct contributions of I K1 for each cell type. I K1 and Kir2 subunit expression varies dramatically in regions of the canine heart and these regional differences in Kir2 expression likely underlie regional distinctions in I K1 characteristics, contributing to variations in repolarization in response to in [K + ] o changes.