Dynamic Late Na Current During Cardiac Action Potential Revealed by a Specific and Potent Inhibitor Gs967

Abstract

The late Na+ current (INa,L) is known to contribute to cardiac action potential (AP) plateau and upregulation of INaL leads to arrhythmogenic activities. We used an innovative self-action potential (sAP)-clamp technique to directly record the dynamic INa,L during the cell's own AP. We studied the dynamic INaL in rabbit ventricular myocytes under physiological conditions wherein [Na+]i and [Ca2+]i homeostasis were preserved. We then determined the effect of GS967, a newly developed selective and potent inhibitor of INa,L. Results: (1) During the various phases of AP waveform, the dynamic INa,L amplitude was low at phase-1, gradually increased during phase-2 to reach a peak, and declined at phase-3. This profile of INa,L magnitude and time course under AP-clamp is distinctively different from the monotonically declining INa,L current seen under rectangular pulse voltage-clamp due to non-equilibrium gating. (2) GS967 selectively inhibits INa,L during AP in a concentration-dependent manner with an IC50 of 57 nM. Unlike tetrodotoxin, GS967 did not significantly inhibit the fast Na+ current. Hence, GS967 shortened the AP duration without affecting the AP upstroke. (3) The INa,L peak amplitude was 0.78±0.07 A/F under physiological condition. Anemonia toxin II (ATX-II) at 5 nM increased the INaL amplitude to 1.25 ±0.14 A/F, and prolonged APD95 from 212.4±10.5 ms (control) to 305.6±19.1 ms (ATX-II). GS967 (1uM) effectively shortened APD and suppressed afterdepolarizations (EADs) induced by ATX-II. Conclusion: Our sAP-clamp data reveal a surprisingly large INa,L during AP plateau under physiological condition, which explains why INa,L significantly affect cardiac AP morphology and arrhythmogenesis. GS967 inhibits INa,L during AP plateau without blocking the fast Na+ current at AP upstroke, and therefore provides a promising therapeutic strategy to suppress cardiac arrhythmias without slowing conduction.