Pump electrogenesis in mammalian “twitch” muscle fibers

Abstract

1. 1. The electrogenic contribution of the Na +-K + exchange pump on the membrane potential was studied in the isolated extensor digitorum longus (EDL) muscles of hypokalemic rats. 2. 2. At 2.5 to 15 mM [K] o, the peak hyperpolarization of “Na-rich” muscle fibers exceeded the levels of the steady-state value achieved the end of 90 min recovery in 10 mM K + Krebs solution, the membrane potential in “fresh” muscle fibers of normal rats, or even E k. After peaking, the hyperpolarization declined gradually with decreasing [Na] i and increasing E k. The steady-state membrane potential at the end of recovery became equal to E k. 3. 3. There was a high correlation between the amount of hyperpolarization beyond the steady-state value of membrane potential and the active Na + efflux. 4. 4. On increasing [K] o from 0.5 to 5 mM the K +-sensitive hyperpolarization was increased while either time to peak or the duration of hyperpolarization shortened. 5. 5. The K +-sensitive hyperpolarization was abolished by removing [K] o, by cooling below 4°C by substituting Li + for Na +, or by adding ouabain. On cooling or adding ouabain the membrane potential agreed well with that predicted by a modified “constant-field” equation. 6. 6. The K + sensitive hyperpolarization was reproducible when exposure of K +deficient muscle to 5 mM K + Krebs was kept less than 15 min each hr with the balance of the hr in K + free Krebs solution. 7. 7. Reducing [Na] o had no effect on the pump activation at given K + concentrations. [Cl] o had a short-circuiting effect on the hyperpolarization generated by the pump electrogenesis. 8. 8. The pH optimum for operating the electrogenic Na-pump lay between 7.0 and 7.2. 9. 9. The Na-pump was activated by increases in [Na] i of only a few mmol beyond the [Na] i level of “fresh” muscles and was further enhanced with increasing [Na] i. 10. 10. From the present in vitro results it is concluded that the EDL muscle behaves in a similar way as the SOL muscle as far as activation of the Na-pump is concerned.