Functional Evidence for Intracellular Acid Extruders in Human Ventricular Myocardium

Intracellular pH (pHi) is a major homeostatic system within the cell. Changes in pHi exert great influence on cardiac contractility and rhythm. Both the housekeeping Na+–H+ exchanger (NHE) and the Na+–HCO3− symporter (NHS) have been confirmed as major transporters for the active acid extrusion mechanism in animal cardiomyocytes. However, whether the NHE and NHS functionally coexist in human ventricular cardiomyocytes remains unclear. We therefore examined the mechanism of pHi recovery following an NH4Cl-induced intracellular acidosis in the human ventricular myocardium. The pHi was monitored by microspectrofluorimetry by the use of intracellular 2',7'-bis(2-carboxyethyl)-5(6)-carboxy-fluorescein (BCECF)–fluorescence. HOE 694 (30 μM), a specific NHE inhibitor could block pHi recovery from induced intracellular acidosis completely in nominally HCO3−-free HEPES Tyrode solution, but it only partially inhibited the pHi recovery in 5% CO2/HCO3− Tyrode solution. In 5% CO2/HCO3− Tyrode solution, the addition of HOE 694 together with DIDS (an NHS inhibitor) or the removal of [Na+]o could entirely inhibit the acid extrusion. We conclude for the first time that two different acid extruders, HCO3−-independent and -dependent, were most likely the NHE and NHS, respectively, that functionally coexisted in the human ventricular cardiomyocytes.