Loss of Na+,HCO3−-Cotransporter NBCn1 Inhibits Net Acid Extrusion in the Atria and Causes Hypertension-Associated Cardiac Hypertrophy

Abstract

Aim

Metabolic disturbances challenge pH homeostasis in cardiomyocytes. The electroneutral Na⁺,HCO₃⁻-cotransporter NBCn1/Slc4a7 mediates net acid extrusion, and genetic variation in SLC4A7 contributes to human hypertension and cardiovascular risk. Nonetheless, the cardiac consequences of disrupted NBCn1 expression and function remain unclear. Here, we test the hypothesis that NBCn1, either directly or indirectly, influences cardiac structure, contractile function, and electrophysiological properties.

Methods

Based on mice with global loss of NBCn1, we measure intracellular pH in atria and ventricles of the heart (fluorescence microscopy), membrane potential responses (patch clamping), electro- and echocardiographic variables, blood pressure (telemetry), and cardiac dimensions (in vivo and postmortem analyses).

Results

We find that protein and mRNA expression of NBCn1 are more prominent in atrial than in ventricular cardiomyocytes. Disruption of NBCn1 expression lowers Na⁺,HCO₃⁻-cotransport activity more than 50% in atria without significantly influencing net acid extrusion activity of ventricular cardiomyocytes. Loss of NBCn1 is associated with hypertension (blood pressure increased by ~15 mmHg), cardiac hypertrophy (heart/body weight increased by ~10%), and prolonged ventricular isovolumic relaxation time (increased by ~25%). NBCn1 knockout does not affect cardiomyocyte size, collagen content in the heart wall, overall cardiac contractile function, electrophysiological properties of ventricular cardiomyocytes, or the electrocardiogram.

Conclusion

NBCn1 is a main mechanism of Na⁺,HCO₃⁻-cotransport in atrial tissue and contributes substantially to net acid extrusion during intracellular acidification. NBCn1 does not play any major direct role in ventricular cardiomyocytes of unchallenged mice, but global knockout of NBCn1 increases systemic blood pressure and results in the development of cardiac hypertrophy.