Differential protein kinase c isoform regulation and increased constitutive activity of acetylcholine-regulated potassium channels in atrial remodeling

Abstract

Rationale Atrial fibrillation (AF) causes atrial-tachycardia remodeling (ATR), with enhanced constitutive acetylcholine-regulated K+ current (IKAChC) contributing to action potential duration shortening and AF promotion. The underlying mechanisms are unknown. Objective: To evaluate the role of protein-kinase C (PKC) isoforms in ATR-induced IKAChC activation. Methods and Results: Cells from ATR-dogs (400-bpm atrial pacing for 1 week) were compared to control dog cells. In vitro tachypaced (TP; 3 Hz) canine atrial cardiomyocytes were compared to parallel 1-Hz paced cells. I KAChC single-channel activity was assessed in cell-attached and cell-free (inside-out) patches. Protein expression was assessed by immunoblot. In vitro TP activated IKAChC, mimicking effects of in vivo ATR. Discrepant effects of PKC activation and inhibition between control and ATR cells suggested isoform-selective effects and altered PKC isoform distribution. Conventional PKC isoforms (cPKC; including PKCα) inhibited, whereas novel isoforms (including PKCε) enhanced, acetylcholine-regulated K current (IKACh) in inside-out patches. TP and ATR downregulated PKCα (by 33% and 37%, respectively) and caused membrane translocation of PKCε, switching PKC predominance to the stimulatory novel isoform. TP increased [Ca2+]i at 2 hours by 30%, with return to baseline at 24 hours. Buffering [Ca 2+]i during TP with the cell-permeable Ca2+ chelator BAPTA-AM (1 μmol/L) or inhibiting the Ca-dependent protease calpain with PD150606 (20 μmol/L) prevented PKCα downregulation and TP enhancement of IKAChC. PKCε inhibition with a cell-permeable peptide inhibitor suppressed TP/ATR-induced IKAChC activation, whereas cPKC inhibition enhanced IKAChC activity in 1-Hz cells. Conclusions: PKC isoforms differentially modulate IKACh, with conventional Ca-dependent isoforms inhibiting and novel isoforms enhancing activity. ATR causes a rate-dependent PKC isoform switch, with Ca/calpain-dependent downregulation of inhibitory PKCα and membrane translocation of stimulatory PKCε, enhancing I KAChC. These findings provide novel insights into mechanisms underlying IKAChC dysregulation in AF. © 2011 American Heart Association. All rights reserved.