Quantitative analysis of the Ca -dependent regulation of delayed rectifier K current I in rabbit ventricular myocytes.

Key Points: [Ca ] enhanced rabbit ventricular slowly activating delayed rectifier K current (I ) by negatively shifting the voltage dependence of activation and slowing deactivation, similar to perfusion of isoproterenol. Rabbit ventricular rapidly activating delayed rectifier K current (I ) amplitude and voltage dependence were unaffected by high [Ca ] . When measuring or simulating I during an action potential, I was not different during a physiological Ca transient or when [Ca ] was buffered to 500 nm.

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