Human cardiac inwardly-rectifying K+ channel Kir2.1b is inhibited by direct protein kinase C-dependent regulation in human isolated cardiomyocytes and in an expression system

Back-ground-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 cut-rent (IK1) 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 IK1. Methods and Results-In whole-cell patch-clamp experiments with isolated human atrial cardiomyocytes, the IK1 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 IK1, 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 IC50 of 0.68 nmol/L. The inactive compound 4-alpha-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 (I nmol/L) significantly. After mutation of the PKC phosphorylation sites (especially S64A and T353A), PMA became ineffective. Conclusions-The human IK1 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.