Quantitative analysis of the Ca2+-dependent regulation of delayed rectifier K+ current IKs in rabbit ventricular myocytes

The slowly activating delayed rectifier K+ current (I-Ks) contributes to repolarization of the cardiac action potential (AP). Intracellular Ca2+ ([Ca2+](i)) and beta-adrenergic receptor (beta-AR) stimulation modulate I-Ks amplitude and kinetics, but details of these important I-Ks regulators and their interaction are limited. We assessed the [Ca2+](i) dependence of I-Ks in steady-state conditions and with dynamically changing membrane potential and [Ca2+](i) during an AP. I-Ks was recorded from freshly isolated rabbit ventricular myocytes using whole-cell patch clamp. With intracellular pipette solutions that controlled free [Ca2+](i), we found that raising [Ca2+](i) from 100 to 600nm produced similar increases in I-Ks as did beta-AR activation, and the effects appeared additive. Both beta-AR activation and high [Ca2+](i) increased maximally activated tail I-Ks, negatively shifted the voltage dependence of activation, and slowed deactivation kinetics. These data informed changes in our well-established mathematical model of the rabbit myocyte. In both AP-clamp experiments and simulations, I-Ks recorded during a normal physiological Ca2+ transient was similar to I-Ks measured with [Ca2+](i) clamped at 500-600nm. Thus, our study provides novel quantitative data as to how physiological [Ca2+](i) regulates I-Ks amplitude and kinetics during the normal rabbit AP. Our results suggest that micromolar [Ca2+](i), in the submembrane or junctional cleft space, is not required to maximize [Ca2+](i)-dependent I-Ks activation during normal Ca2+ transients.