Brief rapid pacing depresses contractile function via Ca(2+)/PKC-dependent signaling in cat ventricular myocytes

The purpose of this study is to determine the effects of brief rapid pacing (RP; approximately 200-240 beats/min for approximately 5 min) on contractile function in ventricular myocytes. RP was followed by a sustained inhibition of peak systolic cell shortening (-44 +/- 4%) that was not due to changes in diastolic cell length, membrane voltage, or L-type Ca(2+) current (I(Ca,L)). During RP, baseline and peak intracellular Ca(2+) concentration ([Ca(2+)](i)) increased markedly. After RP, Ca(2+) transients were similar to control. The effects of RP on cell shortening were not prevented by 1 microM calpain inhibitor I, 25 microM L-N(5)-(1-iminoethyl)-orthinthine, or 100 microM N(G)-monomethyl-L-arginine. However, RP-induced inhibition of cell shortening was prevented by lowering extracellular [Ca(2+)] (0.5 mM) during RP or exposure to chelerythrine (2-4 microM), a protein kinase C (PKC) inhibitor, or LY379196 (30 nM), a selective inhibitor of PKC-beta. Exposure to phorbol ester (200 nM phorbol 12-myristate 13-acetate) inhibited cell shortening (-46 +/- 7%). Western blots indicated that cat myocytes express PKC-alpha, -delta, and -epsilon as well as PKC-beta. These findings suggest that brief RP of ventricular myocytes depresses contractility at the myofilament level via Ca(2+)/PKC-dependent signaling. These findings may provide insight into the mechanisms of contractile dysfunction that follow paroxysmal tachyarrhythmias.