Action Potential Prolongation in Cardiac Myocytes of Old Rats is an Adaptation to Sustain Youthful Intracellular Ca2+ Regulation

A. M. Janczewski, H. A. Spurgeon and E. G. Lakatta. Action Potential Prolongation in Cardiac Myocytes of Old Rats is an Adaptation to Sustain Youthful Intracellular Ca2+ Regulation. Journal of Molecular and Cellular Cardiology (2002) 34, 641–648. Advanced age in rats is accompanied by reduced expression of the sarcoplasmic reticulum (SR) Ca2+ pump (SERCA-2). The amplitudes of intracellular Ca2+ (Ca2+i) transients and contractions in ventricular myocytes isolated from old (23–24-months) rats (OR), however, are similar to those of young (4–6-months) rat myocytes (YR). OR myocytes also manifest slowed inactivation of L-type Ca2+ current (ICaL) and marked prolongation of action potential (AP) duration. To determine whether and how age-associated AP prolongation preserves the Ca2+i transient amplitude in OR myocytes, we employed an AP-clamp technique with simultaneous measurements of ICaL (with Na+ current, K+ currents and Ca2+ influx via sarcolemmal Na+-Ca2+ exchanger blocked) and Ca2+i transients in OR rat ventricular myocytes dialyzed with the fluorescent Ca2+ probe, indo-1. Myocytes were stimulated with AP-shaped voltage clamp waveforms approximating the configuration of prolonged, i.e. the native, AP of OR cells (AP-L), or with short AP waveforms (AP-S), typical of YR myocytes. Changes in SR Ca2+ load were assessed by rapid, complete SR Ca2+ depletions with caffeine. As expected, during stimulation with AP-S vs AP-L, peak ICaL increased, by 21±4%, while the ICaL integral decreased, by 19±3% (P<0.01 for each). Compared to AP-L, stimulation of OR myocytes with AP-S reduced the amplitudes of the Ca2+i transient by 31±6%, its maximal rate of rise (+dCa2+i/dtmax; a sensitive index of SR Ca2+ release flux) by 37±4%, and decreased the SR Ca2+ load by 29±4% (P<0.01 for each). Intriguingly, AP-S also reduced the maximal rate of the Ca2+i transient relaxation and prolonged its time to 50% decline, by 35±5% and 33±7%, respectively (P<0.01 for each). During stimulation with AP-S, the gain of Ca2+-induced Ca2+ release (CICR), indexed by +dCa2+i/dtmax/ICaL, was reduced by 46±4% vs AP-L (P<0.01). We conclude that the effects of an application of a shorter AP to OR myocytes to reduce +dCa2+i/dtmax and the Ca2+ transient amplitude are attributable to a reduction in SR Ca2+ load, presumably due to a reduced ICaL integral and likely also to an increased Ca2+ extrusion via sarcolemmal Na+-Ca2+ exchanger. The decrease in the Ca2+i transient relaxation rate in OR cells stimulated with shorter APs may reflect a reduction of Ca2+/calmodulin-kinase II-regulated modulation of Ca2+ uptake via SERCA-2, consequent to a reduced local Ca2+ release in the vicinity of SERCA-2, also attributable to reduced SR Ca2+ load. Thus, the reduction of CICR gain during stimulation with AP-S is the net result of both a diminished SR Ca2+ release and an increased peak ICaL. These results suggest that ventricular myocytes of old rats utilize AP prolongation to preserve an optimal SR Ca2+ loading, CICR gain and relaxation of Ca2+i transients.