Intracellular pH Regulatory Mechanism in Human Atrial Myocardium: Functional Evidence for Na+/H+ Exchanger and Na+/HCO3– Symporter

Abstract

Intracellular pH (pH_i) exerts considerable influence on cardiac contractility and rhythm. Over the last few years, extensive progress has been made in understanding the system that controls pH_i in animal cardiomyocytes. In addition to the housekeeping Na⁺-H⁺ exchanger (NHE), the Na⁺-HCO₃^– symporter (NHS) has been demonstrated in animal cardiomyocytes as another acid extruder. However, whether the NHE and NHS functions exist in human atrial cardiomyocytes remains unclear. We therefore investigated the mechanism of pH_i recovery from intracellular acidosis (induced by NH₄Cl prepulse) using intracellular 2′,7′-bis(2-carboxethyl)-5(6)-carboxy-fluorescein fluorescence in human atrial myocardium. In HEPES (nominally HCO₃^–-free) Tyrode solution, pH_i recovery from induced intracellular acidosis could be blocked completely by 30 µM 3-methylsulfonyl-4-piperidinobenzoyl, guanidine hydrochloride (HOE 694), a specific NHE inhibitor, or by removing extracellular Na⁺. In 3% CO₂-HCO₃^– Tyrode solution, HOE 694 only slowed the pH_i recovery, while addition of HOE 694 together with 4,4′-diisothiocyanatostilbene-2,2′-disulphonic acid (an NHS inhibitor) or removal of extracellular Na⁺ inhibited the acid extrusion entirely. Therefore, in the present study, we provided evidence that two acid extruders involved in acid extrusion in human atrial myocytes, one which is HCO₃^– independent and one which is HCO₃^– dependent, are mostly likely NHE and NHS, respectively. When we checked the percentage of contribution of these two carriers to pH_i recovery following induced acidosis, we found that the activity of NHE increased steeply in the acid direction, while that of NHS did not change. Our present data indicate for the first time that two acid extruders, NHE and NHS, exist functionally and pH_i dependently in human atrial cardiomyocytes.