Cardiac chloride and pH regulation in health and disease

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Heart disease is the leading cause of mortality in the United States and causes more deaths than all cancers combined. Coronary heart disease (or ischemic heart disease, IHD), the most common type of heart disease, is accompanied by a major decline of local pH in myocardium. However, the mechanisms of pH regulation and the homeostasis of H+ neutralizing buffers, such as HCO3- and Cl- in cardiomyocytes remain incompletely understood, making it difficult to design therapeutic strategies targeting pH regulation. Recently, we have identified and cloned different isoforms of a solute carrier, Slc26a6, from cardiac myocytes. Slc26a6 is the predominant Cl-/HCO3- exchanger in the heart. We demonstrated that Slc26a6 mediates electrogenic Cl-/HCO3- exchange activities in both atrial and ventricular myocytes. Our findings raise the possibility that Slc26a6 may represent the predominant Cl-/HCO3- regulatory mechanism in the heart. We have obtained exciting data to support the critical roles of Slc26a6 in cardiac excitability and contractility. We documented that null deletion of Slc26a6 in mice results in shortened action potentials (APs), sinus bradycardia, fragmented QRS complexes and impaired cardiac function compared to wild type littermates. We have identified and characterized two isoforms of human SLC26A6 in human heart, which are also electrogenic, akin to mouse cardiac Slc26a6. In addition, we recently identified and reported a dynamic beat-to-beat intracellular pH (pHi) regulation system, termed “pHi transients”, which dovetails with the prevailing three known dynamic systems, namely electrical, Ca2+, and mechanical systems. However, critical questions remain unanswered. How do Slc26a6 activities affect not only pHi, but also cardiac AP and contractility? The goal of study is to determine the mechanistic links between the Slc26a6 activities and cardiac AP and contractility. Contributions of Slc26a6-mediated Cl-/HCO3- towards the pHi transients will also be tested. Taken together, we hypothesize that the activities of Slc26a6 on pHi will directly contribute towards intracellular Na+ homeostasis, through Na+/HCO3- cotransporter (NBCe) and Na+/H+ exchanger (NHE), and subsequently regulate intracellular Ca2+ concentration through sarcolemmal Na+- Ca2+ exchanger (NCX). Therefore, ablation of Slc26a6 will result in a reduction in intracellular Na+ and Ca2+ through the actions of NHE/NBCe and NCX, respectively. We further hypothesize that Slc26a6 plays important roles in the dynamic pHi regulation in the heart regulating cardiac pacemaking activities and contractility. We will test our hypothesis using multidisciplinary approaches including functional electrophysiological recordings, imaging, biochemical, molecular and genetic approaches as well as ex vivo and in vivo functional studies. Wild type and cardiac-specific Slc26a6 knockout mouse model as well as human cardiomyocytes will be tested. Three specific aims are: 1. To determine the regulatory mechanisms of Slc26a6 on cardiac pHi and function. We will test how Slc26a6 regulates dynamic cardiac pHi, Na+ and Ca2+ homeostasis, hence, excitability and contractility. The relationship between pHi and cardiac function will be directly tested to gain mechanistic insights into the functional roles of Slc26a6 in the heart. We will use novel techniques including multimodal second harmonic generation (SHG) microscopy and our recently established dynamic pH recording techniques. 2. To determine the mechanistic roles of Slc26a6 in cardiac ischemia/reperfusion (I/R). We will test the contributions of Slc26a6 to cardiac function in the I/R mouse model. Mechanistic roles of Slc26a6 in cardiac I/R injury will be tested using ex vivo confocal imaging of pHi, intracellular Na+ and Ca2+ concentrations. I/R injury will be employed in control and Slc26a6-/- mice. 3. To determine the functional roles and regulatory mechanisms of Slc26a6 in cardiac pacemaking activities. We will test the mechanistic roles of Slc26a6 in the regulation of AP firing frequency, pacemaker currents, Ca2+ signaling, and pHi in SAN cells. Additionally, ECG telemetry will be used to test the roles of Slc26a6 in conscious control and SAN-specific Slc26a6-/- mice. Our studies will unravel a missing molecular link between pHi regulation and Na+, and Ca2+ homeostasis in the heart. The anticipated results will provide novel insights into the roles of Slc26a6 in cardiac pHi regulation, cardiac excitability, and function under physiological and pathological conditions. At the translational level, Slc26a6 may represent a novel therapeutic target for cardioprotection in cardiac ischemia and arrhythmia.