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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. IKAChC 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 PKC[alpha]) inhibited, whereas novel isoforms (including PKC[epsilon]) enhanced, acetylcholine-regulated K current (IKACh) in inside-out patches. TP and ATR downregulated PKC[alpha] (by 33% and 37%, respectively) and caused membrane translocation of PKC[epsilon], 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 [Ca2 ]i during TP with the cell-permeable Ca2 chelator BAPTA-AM (1 [mu]mol/L) or inhibiting the Ca2 -dependent protease calpain with PD150606 (20 [mu]mol/L) prevented PKC[alpha] downregulation and TP enhancement of IKAChC. PKC[epsilon] 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 Ca2 -dependent isoforms inhibiting and novel isoforms enhancing activity. ATR causes a rate-dependent PKC isoform switch, with Ca2 /calpain-dependent downregulation of inhibitory PKC[alpha] and membrane translocation of stimulatory PKC[epsilon], enhancing IKAChC. These findings provide novel insights into mechanisms underlying IKAChC dysregulation in AF.

(C) 2011 American Heart Association, Inc.