Evidence for electrogenic Na + pumping in human atrial myocardium

The resting potential of ‘sodium-loaded’ cardiac cells can transiently hyperpolarize to levels negative to the steady state resting potential [ 3, 5, 6]. Hyperpolarization is associated with the coupled efflux of Na + and influx of K + driven by an active transport process and may results from an increased K + equilibrium potential (E K), an outward pump current or both. Using conventional microelectrode techniques, we found that Na +-loaded human atrial myocardium can also transiently hyperpolarize. Na + loading was induced by cooling to 2° to 3°C. Upon rewarning to 37°C in a 20 m mK + solution, the resting potential transiently hyperpolarized to levels at least 11 mV negative to the calculated E K and 29±2 mV (mean±S.E.) negative to the steady state level (−33±2 mV) recorded some 15–20 minutes later. An increase in K + conductance induced by acetylcholine exposure [ 2, 7, 10] during the transient hyperpolarization caused a depolarization, indicating that the resting potential was indeed negative to E K. These findings cannot be explained by either conductance changes or electroneutral Na + pumping and concomittant extracellular K + depletion. We conclude that the Na +-loaded human atrium can generate net pump current.