Human Cardiac Inwardly-Rectifying K ⁺ Channel Kir 2.1b Is Inhibited by Direct Protein Kinase C-Dependent Regulation in Human Isolated Cardiomyocytes and in an Expression System

Background— Protein kinases A (PKA) and C (PKC) are activated in ischemic preconditioning and heart failure, conditions in which patients develop arrhythmias. The native inward rectifier potassium current (IK 1 ) plays a central role in the stabilization of the resting membrane potential and the process of arrhythmogenesis. This study investigates the functional relationship between PKC and IK 1 . Methods and Results— In whole-cell patch-clamp experiments with isolated human atrial cardiomyocytes, the IK 1 was reduced by 41% when the nonspecific activator of PKC phorbol 12 myristate 13-acetate (PMA; 100 nmol/L) was applied. To investigate the effects of PKC on cloned channel underlying parts of the native IK 1 , we expressed Kir 2.1b heterologously in Xenopus oocytes and measured currents with the double-electrode voltage-clamp technique. PMA decreased the current by an average of 68%, with an IC 50 of 0.68 nmol/L. The inactive compound 4-α-PMA was ineffective. Thymeleatoxin and 1-oleolyl-2-acetyl-sn-glycerol, 2 specific activators of PKC, produced effects similar to those of PMA. Inhibitors of PKC, ie, staurosporine and chelerytrine, could inhibit the PMA effect (1 nmol/L) significantly. After mutation of the PKC phosphorylation sites (especially S64A and T353A), PMA became ineffective. Conclusions— The human IK 1 in atrial cardiomyocytes and one of its underlying ion channels, the Kir 2.1b channel, is inhibited by PKC-dependent signal transduction pathways, possibly contributing to arrhythmogenesis in patients with structural heart disease in which PKC is activated.