Intracellular Calcium Alters IKs Amplitude and Kinetics in Rabbit Myocytes

Abstract

The slowly activating delayed rectifier K+ current (IKs) contributes to repolarization of the cardiac action potential (AP). Intracellular Ca2+ ([Ca2+]i) can modulate IKs, but details of this potentially important regulation are largely unknown. Here, we aimed at assessing [Ca2+]i regulation of IKs and how it governs myocyte AP duration. IKs was recorded from freshly isolated rabbit ventricular myocytes using the patch clamp technique with intracellular pipette solutions that buffered [Ca2+]i to 0, 100, 300, and 500 nM. From a holding potential of −50 mV, IKs was recorded by implementing step-like pulses from −40 to 50 mV in 10 mV increments for 3 s, followed by a tail-pulse to −50 mV for 3 s. When the [Ca2+]i was increased to 300 nM, the maximally activated tail IKs (IMAX) was more than 2-fold greater compared to 0 nM [Ca2+]i (IMAX ∼0.8 pA/pF vs. 0.3 pA/pF). Importantly, when the pipette solution contained 500 nM [Ca2+]i (IMAX ∼1.0 pA/pF), IMAX was 3-fold greater than 0 nM and more than 2-fold greater than 100 nM [Ca2+]i (IMAX ∼0.4 pA/pF). The potential of half-maximal activation (V½) was not different for any situations (∼15 mV). However, deactivation kinetics of tail IKs were slower for cells recorded with 300 and 500 nM [Ca2+]i compared to 0 and 100 nM [Ca2+]i (τdeact ∼1200 ms vs. 800 ms). These results indicate that a rise in [Ca2+]i increases IKs amplitude in rabbits, without altering the voltage dependence of activation, and slows IKs deactivation. Computational modeling suggests these [Ca2+]i-dependent changes might contribute to ventricular AP duration alterations, especially in the presence of adrenergic activation.