Physiological relevance of isoform‐specific voltage dependence in the Na/K pump

The Na/K pump is a heterodimeric (αβ) protein that generates Na and K gradients essential for excitability and secondary active transport. Cardiac muscle presents at least two types of Na/K pump α‐subunits (α1 and α2) that combine with two β‐subunits (β1 and β2), with the physiological combinations believed to be α1β1 and α2β2. In membrane preparations, different isoforms present slightly different kinetic parameters. We performed voltage‐clamp experiments at physiological concentrations of Na o and K o in rat ventricular myocytes and in Xenopus oocytes expressing several human Na/K pump isoforms. Our results indicate that the distinct voltage‐dependent characteristics of the Na/K pump isoforms may be more relevant than previously thought. In particular, we demonstrate that while α1 pumps are fully functional at all voltages, α2 pump function is drastically reduced and nearly zero at resting membrane potentials (~‐75 mV), but maximally‐active at depolarized potentials. This observation provides a simple explanation for the physiological relevance of these pumps (~20% of total pumps in rat ventricle), acting as a reservoir for extra Na/K pumping during the long‐lasting cardiac action potential, where most of the Na and Ca entry occur through ion channels. Furthermore, by measuring transient currents without K o , we demonstrate that these effects correlate with the increased affinity of α2 pumps for Na o . Our results also help explain the beneficial effects of cardiotonic steroids by only inhibiting these reservoir pumping pool and suggest that post‐translational modification of Na/K pump voltage dependent reactions maybe an important regulatory mechanism. NIH R15‐NS081570‐01