Modulation of the cardiac Na-Ca exchanger by cytoplasmic protons: Molecular mechanisms and physiological implications.

A precise temporal and spatial control of intracellular Ca concentration is essential for a coordinated contraction of the heart. Following contraction, cardiac cells need to rapidly remove intracellular Ca to allow for relaxation. This task is performed by two transporters: the plasma membrane Na-Ca exchanger (NCX) and the sarcoplasmic reticulum (SR) Ca-ATPase (SERCA). NCX extrudes Ca from the cell, balancing the Caentering the cytoplasm during systole through L-type Ca channels. In parallel, following SR Ca release, SERCA activity replenishes the SR, reuptaking Ca from the cytoplasm. The activity of the mammalian exchanger is fine-tuned by numerous ionic allosteric regulatory mechanisms. Micromolar concentrations of cytoplasmic Ca potentiate NCX activity, while an increase in intracellular Na levels inhibits NCX via a mechanism known as Na-dependent inactivation. Protons are also powerful inhibitors of NCX activity. By regulating NCX activity, Ca, Na and H couple cell metabolism to Ca homeostasis and therefore cardiac contractility. This review summarizes the recent progress towards the understanding of the molecular mechanisms underlying the ionic regulation of the cardiac NCX with special emphasis on pH modulation and its physiological impact on the heart.