The Na+, which enters the matrix in exchange of Ca2+ via the NCE, is extruded from the matrix in exchange of H+ via the NHE. The function of the NHE is to maintain a low [Na+] (2–5 mM) in respiring mitochondria to favor the influx of Na+ and efflux of Ca2+ via the NCE.
Dash, RK and Beard, DA. Analysis of Cardiac Mitochondrial Na+/Ca2+ Exchanger Kinetics with a Biophysical Model of Mitochondrial Ca2+ Handing Suggests a 3:1 Stoichiometry. J Physiol, 2008.