Intracellular [Na ⁺] modulates synergy between Na ⁺/Ca ²⁺ exchanger and L-type Ca ²⁺ current in cardiac excitation-contraction coupling during action potentials

Excitation-contraction coupling (ECC) in cardiac myocytes involves triggering of Ca ²⁺ release from the sarcoplasmic reticulum (SR) by L-type Ca channels, whose activity is strongly influenced by action potential (AP) profile. The contribution of Ca ²⁺ entry via the Na ⁺/Ca ²⁺ exchanger (NCX) to trigger SR Ca ²⁺ release during ECC in response to an AP remains uncertain. To isolate the contribution of NCX to SR Ca ²⁺ release, independent of effects on SR Ca ²⁺ load, Ca ²⁺ release was determined by recording Ca ²⁺ spikes using confocal microscopy on patch-clamped rat ventricular myocytes with [Ca ²⁺] _i fixed at 150 nmol/L. In response to AP clamps, normalized Ca ²⁺ spike amplitudes (ΔF/F ₀) increased sigmoidally and doubled as [Na ⁺] _i was elevated from 0 to 20 mmol/L with an EC ₅₀ of ∼10 mmol/L. This [Na ⁺] _i-dependence was independent of I _Na as well as SR Ca ²⁺ load, which was unchanged under our experimental conditions. However, NCX inhibition using either KB-R7943 or XIP reduced ΔF/F ₀ amplitude in myocytes with 20 mmol/L [Na ⁺] _i, but not with 5 mmol/L [Na ⁺] _i. SR Ca ²⁺ release was complete before the membrane repolarized to-15 mV, indicating Ca ²⁺ entry into the dyad (not reduced extrusion) underlies [Na ⁺] _i-dependent enhancement of ECC. Because I _Ca,L inhibition with 50 mmol/L Cd ²⁺ abolished Ca ²⁺ spikes, our results demonstrate that during cardiac APs, NCX enhances SR Ca ²⁺ release by synergistically increasing the efficiency of I _Ca,L-mediated ECC.