Intracellular calcium modulation of voltage-gated sodium channels in ventricular myocytes.

Aims Cardiac voltage-gated sodium channels control action potential (AP) upstroke and cell excitability. Intracellular calcium (Cai2+) regulates AP properties by modulating various ion channels. Whether Cai2+ modulates sodium channels in ventricular myocytes is unresolved. We studied whether Cai2+ modulates sodium channels in ventricular myocytes at Cai2+ concentrations ([Cai2+]) present during the cardiac AP (0–500 nM), and how this modulation affects sodium channel properties in heart failure (HF), a condition in which Cai2+ homeostasis is disturbed. Methods and results Sodium current (INa) and maximal AP upstroke velocity (dV/dtmax), a measure of INa, were studied at 20 and 37°C, respectively, in freshly isolated left ventricular myocytes of control and HF rabbits, using whole-cell patch-clamp methodology. [Cai2+] was varied using different pipette solutions, the Cai2+ buffer BAPTA, and caffeine administration. Elevated [Cai2+] reduced INa density and dV/dtmax, but caused no INa gating changes. Reductions in INa density occurred simultaneously with increase in [Cai2+], suggesting that these effects were due to permeation block. Accordingly, unitary sodium current amplitudes were reduced at higher [Cai2+]. While INa density and gating at fixed [Cai2+] were not different between HF and control, reductions in dV/dtmax upon increases in stimulation rate were larger in HF than in control; these differences were abolished by BAPTA. Conclusion Cai2+ exerts acute modulation of INa density in ventricular myocytes, but does not modify INa gating. These effects, occurring rapidly and in the [Cai2+] range observed physiologically, may contribute to beat-to-beat regulation of cardiac excitability in health and disease. [ABSTRACT FROM AUTHOR]