Abstract
Regulation of intracellular pH (pH(i) ) in cardiomyocytes is crucial for cardiac function; however, currently known mechanisms for direct or indirect extrusion of acid from cardiomyocytes seem insufficient for energetically efficient extrusion of the massive H(+) loads generated under in vivo conditions. In cardiomyocytes, voltage-sensitive H(+) channel activity mediated by the HVCN1 proton channel would be a highly efficient means of disposing of H(+) , while avoiding Na(+) loading, as occurs during direct acid extrusion via Na(+) /H(+) exchange or indirect acid extrusion via Na(+) -HCO(3)(-) cotransport. PCR and immunoblotting demonstrated expression of HVCN1 mRNA and protein in canine heart. Patch clamp analysis of canine ventricular myocytes revealed a voltage-gated H(+) current that was highly H(+) -selective. The current was blocked by external Zn(2+) and the HVCN1 blocker 5-chloro-2-guanidinobenzimidazole. Both the gating and Zn(2+) blockade of the current were strongly influenced by the pH gradient across the membrane. All characteristics of the observed current were consistent with the known hallmarks of HVCN1-mediated H(+) current. Inhibition of HVCN1 and the NHE1 Na(+) /H(+) exchanger, singly and in combination, showed that either mechanism is largely sufficient to maintain pH(i) in beating cardiomyocytes, but that inhibition of both activities causes rapid acidification. These results show that HVCN1 is expressed in canine ventricular myocytes and provides a major H(+) extrusion activity, with a capacity similar to that of NHE1. In the beating heart in vivo, this activity would allow Na(+) -independent extrusion of H(+) during each action potential and, when functionally coupled with anion transport mechanisms, could facilitate transport-mediated CO(2) disposal. KEY POINTS: Intracellular pH (pH(i) ) regulation is crucial for cardiac function, as acidification depresses contractility and causes arrhythmias. H(+) ions are generated in cardiomyocytes from metabolic processes and particularly from CO(2) hydration, which has been shown to facilitate CO(2) venting from mitochondria. Currently, the NHE1 Na(+) /H(+) exchanger is viewed as the dominant H(+) extrusion mechanism in cardiac muscle. We show that the HVCN1 voltage-gated proton channel is present and functional in canine ventricular myocytes, and that HVCN1 and NHE1 both contribute to pH(i) regulation. HVCN1 provides an energetically efficient mechanism of H(+) extrusion that would not cause Na(+) loading, which can cause pathology, and that could contribute to transport-mediated CO(2) disposal. These results provide a major advance in our understanding of pH(i) regulation in cardiac muscle.