Molecular diversity of the human Na⁺/H⁺ exchanger (SLC9) gene family. The mammalian NHE gene family has been classified as the Solute Carrier SLC9 family by the HUGO Gene Nomenclature Committee (HGNC). Evolutionary relationships were determined by multiple sequence alignments using the CLUSTAL W algorithm 140 and the tree was drawn using TreeView 106. The numbers represent the percent identity amongst the NHEs. The amino acid length, mass, and chromosomal location of each NHE isoform is also indicated.

Contributions of Na⁺/H⁺ exchangers (NHE) to cellular pH homeostasis. Plasma membrane Na⁺‐selective NHE operate in conjunction with Na⁺‐coupled bicarbonate transporters [i.e., Na⁺‐HCO₃⁻ cotransporters (NBC) and Na⁺‐dependent Cl⁻/HCO₃⁻ exchangers (NDCBE)] to protect the cytosol against excess acidification generated by cellular metabolism. These transporters use the electrochemical Na⁺ gradient established by Na⁺/K⁺‐ATPase pumps (NKA) to drive solute transport. These alkalinizing mechanisms are counterbalanced by the actions of plasma membrane Cl⁻/HCO₃⁻ or anion exchangers (AE), which acidify the cell. By contrast, nonselective (Na⁺, K⁺)/H⁺ exchangers modulate the luminal pH (and/or cation composition) of endomembrane compartments established by electrogenic vacuolar‐type H⁺‐translocating ATP‐hydrolases (V‐ATPase) and 2Cl⁻/1H⁺ exchangers belonging to the ClC family of Cl⁻ channels and transporters. Other H⁺ efflux pathways include a Zn²⁺‐inhibitable, voltage‐sensitive H⁺ conductance, though the precise molecular identity of this latter ion carrier has yet to be established. Cations such as K⁺ are also present in the lumen of secretory and endocytic compartments and might serve as counter‐ions by extrusion through ill‐defined carriers, thereby contributing to pH homeostasis.

Predicted molecular architecture of the mammalian Na⁺/H⁺ exchanger. The diagram simulates the predicted membrane topology (12 hydrophobic transmembrane helices in the N‐terminus and a hydrophilic C‐terminus oriented toward the cytoplasm) and higher‐ordered dimeric structure of the mammalian Na⁺/H⁺ exchanger based on the known three‐dimensional organization of bacterial Na⁺/H⁺ antiporters.

Molecular structures of Na⁺/H⁺ exchanger inhibitors. (A) Pyrazine‐based compounds; (B) benzoylguanidine‐based compounds; (C) indoloylguanidine‐based compounds; and (D) methacryloylguanidine‐based compounds.

Adenosine triphosphate (ATP) dependence and hormonal regulation of Na⁺/H⁺ exchanger activity. NHE activity is exquisitely sensitive to the energy status of the cell and to numerous stimuli (e.g., hormones, growth factors, and physical perturbations). In the case of NHE1, agents that decrease cellular ATP levels reduce transporter activity, whereas most other stimuli such as epidermal growth factor (EGF) activate the exchanger; in both cases, activity is largely modulated by changes in the H⁺ sensitivity of the transporter.

Roles of Na⁺/H⁺ exchangers in renal epithelial cells. Apical NHE such as NHE3 operate alongside carbonic anhydrases (tethered to the outer membrane leaflet), Cl⁻/HCO₃⁻ exchangers (AE) and water channels (aquaporins; AQP) to mediate (re)absorption of NaCl, HCO₃⁻, and fluid from the renal tubular lumen. NHE1 and analogous bicarbonate transporters [(AE) and Na⁺‐bicarbonate cotransporters (NBC)] located on the basolateral surface work in conjunction with their apical counterparts to drive net transepithelial (re)absorption of electrolytes and fluid.

Role of Na⁺/H⁺ exchangers (NHEs) in the pathophysiology of cardiac ischemia and reperfusion injuries. During episodes of low blood flow (ischemia), adenosine triphosphate (ATP) levels in cardiomyocytes are depleted due to depressed mitochondrial activity. The concentration of lactic acid is also increased due to anaerobic metabolism of glucose. This results in a rapid decrease in both intracellular and extracellular pH which leads to Na⁺_i and Ca²⁺_i overloads that cause cardiac dysfunction and tissue damage. Ischemia elevates Na⁺_i by two mechanisms; the acidosis that occurs during the first few minutes of ischemia leads to an influx of Na⁺ by activation of the NHE, and the reduced ATP levels depress Na⁺,K⁺‐ATPase (NKA) activity which normally extrudes Na⁺_i. Indeed, the NHE accounts for as much as 50% of the cardiac membrane's basal permeability to Na⁺ following intracellular acidification 36. Other Na⁺ carriers, such as Na⁺‐bicarbonate cotransporters (NBC), Na⁺‐K⁺‐2Cl⁻ cotransporters (NKCC) and Na⁺ channels (I_Na), can further exacerbate the Na⁺ overload. Furthermore, the reduction in ATP levels also causes a decrease in the pH_i sensitivity of the Na⁺/H⁺ exchanger; thereby impairing its ability to fully restore pH_i to neutral. The net result is a chronic state of cellular acidosis. The elevation of Na⁺_i reduces the transmembrane (TM) Na⁺ gradient, thereby inhibiting Na⁺/Ca²⁺ exchangers (NCX) which, under normal conditions, extrude Ca²⁺_i in exchange for Na⁺_o. Moreover, if Na⁺_i increases sufficiently, the NCX could reverse and mediate Ca²⁺ influx. This Ca²⁺_i overload is heightened by diminished activities of energy‐dependent plasma membrane and sarcoplasmic reticulum Ca²⁺‐ATPase pumps (PMCA and SERCA, respectively) which cannot adequately extrude excess cytoplasmic Ca²⁺. This leads to Ca²⁺‐induced mitochondrial damage, cardiac arrhythmias and, if untreated, contributes to contractile failure. Reperfusion of the failing heart with physiological fluids to restore pH_i is the standard approach to rescuing the tissue, but may lead to further tissue damage. Rapid removal of the acidic extracellular fluid generates a large TM pH gradient that drives NHE. This further augments Na⁺_i and markedly elevates Ca²⁺_i, causing reperfusion arrhythmias, contractile failure, and cellular necrosis. Thus, the NHE appears to play a central role in injuries caused by ischemia and reperfusion.