Ca²⁺-activated Cl^- current can be triggered by Na⁺ current-induced SR Ca²⁺ release in rabbit ventricle

The Ca²⁺-activated Cl^- current [I(Cl(Ca))] contributes to the repolarization of the cardiac action potential under physiological conditions. I(Cl(Ca)) is known to be primarily activated by Ca²⁺ release from the sarcoplasmic reticulum (SR). L-type Ca²⁺ current [I(Ca(L))] represents the major trigger for Ca²⁺ release in the heart. Recent evidence, however, suggests that Ca²⁺ entry via reverse-mode Na⁺/Ca²⁺ exchange promoted by voltage and/or Na⁺ current (I(Na)) may also play a role. The purpose of this study was to test the hypothesis that I(Cl(Ca)) can be induced by I(Na) in the absence of I(Ca(L)). Macroscopic currents and Ca²⁺ transients were measured using the whole cell patch-clamp technique in rabbit ventricular myocytes loaded with Indo-1. Nicardipine (10 μM) abolished I(Ca(L)) at a holding potential of -75 mV as tested in Na⁺-free external solution. In the presence of 131 mM external Na⁺ and in the absence of I(Ca(L)), a 4-aminopyridine-resistant transient outward current was recorded in 64 of 81 cells accompanying a phasic Ca²⁺ transient. The current reversed at -42.0 ± 1.3 mV (n = 6) and at +0.3 ± 1.4 mV (n = 6) with 21 and 141 mM of internal Cl^-, respectively, similar to the predicted reversal potential with low intracellular Cl^- concentration ([Cl^-](i)) (- 47.8 mV) and high [Cl^-](i) (-1.2 mV). Niflumic acid (100 μM) inhibited the current without affecting the Ca²⁺ signal (n = 8). Both the current and Ca²⁺ transient were abolished by 10 mM caffeine (n = 6), 10 μM ryanodine (n = 3), 30 μM tetrodotoxin (n = 9), or removal of extracellular Ca²⁺ (n = 6). These properties are consistent with those of I(Cl(Ca)) previously described in mammalian cardiac myocytes. We conclude that 1) I(Cl(Ca)) can be recorded in the absence of I(Ca(L)), and 2) I(Na)-induced SR Ca²⁺ release mechanism is also present in the rabbit heart and may play a physiological role in activating the Ca²⁺-sensitive membrane Cl^- conductance.