Cardiac action potential prolongation in the absence of NCX1 Na+-dependent inactivation: A mechanistic insight.

Abstract

The electrogenic Na+-Ca2+ exchanger (NCX1) transports three Na+ into the cell while extruding one Ca2+, thus regulating both cardiac excitability and contractility. In addition to being transported, intracellular Na+ inhibits NCX1 via a mechanism termed Na+-dependent inactivation. The physiological role of this regulation remains unknown. We created a novel mouse line in which the native NCX1 carries mutation K229Q. This modification removes NCX1 Na+-dependent inactivation without affecting other regulatory properties (Matsuoka et al., JPG 109:273-86, 1997; John et al., JGP 150:245-257, 2018). K229Q mice show a prolonged QT interval and action potential (Steccanella et al., Biophys. J. 118, 100a, 2020). To determine the mechanism underlying these altered electrical properties, we recorded NCX1 currents from patch clamped myocytes. Currents were elicited by a voltage ramp protocol and acquired in the absence and presence of intracellular Na+. In the absence of cytosolic Na+ both WT and K229Q exchangers do not undergo Na+-dependent inactivation and thus generated similar current amplitudes (WT = −0.13±0.04 pA/pF, n = 7/5 cells/animals; K229Q = −0.12±0.04, n = 7/3; p = 0.87, measured at −70 mV, mean±SEM). In contrast, in the presence of 25mM Na+, NCX1 currents recorded from K229Q cardiomyocytes were significantly larger than those measured from WT cells (WT = 0.72±0.07 pA/pF, n = 8/4; K229Q = 1.07±0.10, n = 10/6; p = 0.009, measured at 50 mV). This is consistent with increased activity of NCX1-K229Q which lacks Na+-dependent inactivation. L-type Ca2+ current amplitudes (WT = −12.26±1.32 pA/pF, n = 7/5; K229Q = −14.82±2.29, n = 6/3; p = 0.36, measured at 10 mV) and transcript levels of CaV1.2, KV2.1, KV4.2, and Kir2.1 channels were not significantly different between WT and K229Q mice. These findings suggest that action potential prolongation observed in K229Q cardiomyocytes is primarily mediated by NCX1 current enhanced by the lack of Na+-dependent inactivation and that NCX1 Na+-dependent inactivation is an important modulator of cardiac electrical activity.