Calpain inhibition prevents pacing-induced cellular remodeling in a HL-1 myocyte model for atrial fibrillation

Objective: Atrial fibrillation (AF) is a progressive disease. Previously, clinical and animal experimental studies in AF revealed a variety of myocyte remodeling processes including L-type Ca2+ channel reduction and structural changes, which finally result in electrical remodeling and contractile dysfunction. There are indications that myocyte remodeling is mediated by Ca2+ overload induced calpain activation. To study in more detail the mechanisms underlying myocyte remodeling and to develop strategies for drug-interference, we utilised a paced cell model for AF. Methods and results: HL-1 atrial myocytes were subjected to a 10 times increase in rate over basal values by electrical field stimulation at 5 Hz. It was found that 24-h pacing reduced plasmalemmal levels of L-type Ca2+ channel alpha1C subunit by - 72% compared to controls. No changes in amount of the potassium channel Subunits Kv4.3 and Kv1.5 were found. Pacing also induced marked structural changes; myolysis and nuclear condensation, paralleled by a 14-fold increase in calpain activity. The pacing-induced reduction of L-type Ca2+ channel protein was fully prevented by treatment with verapamil, the active stereoisomer of methoxyverapamil D600, the calpain inhibitors PD150606 and E64d, and LaCl3. Interestingly, PD150606, E64d and LaCl3, but not (methoxy)verapamil, prevented structural changes. Conclusions: Paced HL-1 atrial myocytes undergo myocyte remodeling similar to that found in myocytes from patients with AF Calcium influx independent of the L-type Ca2+ channel and subsequent activation of calpain represent key features in the progression towards overt structural changes. Calpain inhibition may therefore represent a useful lead for therapeutic intervention in AF. (C) 2004 European Society of Cardiology. published by Elsevier B.V. All rights reserved.