Activation of the electrogenic sodium pump in guinea-pig auricles by internal sodium ions.

1. The effect of various intracellular Na concentrations ([Na](i)) on the membrane potential after hypothermia was studied in guinea-pig auricles.2. For varying [Na](i), the atria were cooled for 4 hr at 4-6 degrees C in a K-poor solution with different concentrations of NaCl. The auricles were rewarmed in normal Tyrode solution at 35 degrees C.3. Extracellular space (ECS), intracellular Na and K concentrations ([Na](i) and [K](i)) and membrane potential of the atria were measured before and after hypothermia.4. The ECS, measured as inulin space, amounted to 350 ml./kg wet wt. at 35 degrees C and to 300 ml./kg wet wt. at 4-6 degrees C.5. [K](i) decreased during cooling and increased during rewarming the auricles. [Na](i) increased during hypothermia in bathing fluids containing NaCl, but decreased in NaCl- and Na-free solutions. At the beginning of rewarming a net Na transport occurred from cells with high [Na](i), while a net Na uptake took place in atria with low [Na](i).6. At the same time, the membrane potential of auricles with increased [Na](i) hyperpolarized beyond the steady-state value recorded at the end of rewarming, or even beyond the calculated K(+) equilibrium potential (E(K)). Afterwards, the hyperpolarization levelled off, while the E(K) values increased further. The membrane potential of atria with decreased [Na](i) showed no transitory hyperpolarization during rewarming.7. The hyperpolarization beyond the steady-state value of membrane potential in rewarmed auricles was significantly correlated to the active Na efflux.8. From these results it is concluded that the membrane potential of guinea-pig atria after hypothermia is affected by an active, electrogenic Na pump activated by intracellular Na ions.