Regional Heterogeniety of the Inwardly Rectifying Potassium Current in the Canine Heart

Background: The inward rectifier potassium current, IK1, regulates the terminal phase of repolarization of the action potential, as well as the resting membrane potential. Regional variation in IK1 has been noted in the canine heart, but the biophysical properties have not been directly compared. We examined the properties and functional contribution of IK1 in isolated myocytes from ventricular, atrial and Purkinje tissue.Methods and Results: Action potentials (AP) were recorded from canine left ventricular midmyocardium, left atrial and Purkinje tissue. The terminal rate of repolarization of the AP (as assessed by the minimum dV/dt) in ventricle, but not in Purkinje, depended on changes in external K+ ([K+]o). Isolated ventricular myocytes had the greatest density of IK1 while atrial myocytes had the lowest. Furthermore, the outward component of IK1 showed that 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 IK1, 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.Conclusions: IK1 and Kir2 subunit expression vary dramatically in regions of the canine heart, these variations in IK1 properties could potentially explain the differences in the AP rate of repolarization between heart regions in response to [K+]o changes.