CHANGE OF NA-CELLS WITH VARYING FREE-ENERGY OF ATP HYDROLYSIS( PUMP CURRENT REVERSAL POTENTIAL IN SHEEP CARDIAC PURKINJE)

1. The Na+-K+ pump current, I-p, of cardioballs from isolated sheep ca rdiac Purkinje cells was measured at 30-34 degrees C by means of whole -cell recording.2. Under physiological conditions I-p is an outward cu rrent. Experimental conditions which cause a less negative free energy of intracellular ATP hydrolysis (Delta G(ATP)) and steeper sarcolemma l gradients for the pumped Na+ and Cs+ ions evoked an I-p in the inwar d direction over a wide range of membrane potentials. The reversal of the I-p direction was reversible, 3. The inwardly directed I-p increas ed with increasingly negative membrane potentials and amounted to -0.1 3 +/- 0.03 mu A cm(-2) (mean +/- S.E.M.; n = 6) at -95 mV. 4. The reve rsal potential (E(rev)) of I-p was studied as a function of Delta G(AT P) at constant sarcolemmal gradients of the pumped cations. 5. In orde r to vary Delta G(ATP) the cell interior was dialysed with patch pipet te solutions containing 10 mM ATP and different concentrations of ADP and inorganic phosphate. The media were composed to produce Delta G(AT P) levels of about -58, -49 and -39 kJ mol(-1). 6. A less negative Del ta G(ATP) shifted E(rev) to more positive membrane potentials. From me asurements of I-p as a function of membrane potential E(rev) was estim ated to be -195, -115 and -60 mV at Delta G(ATP) levels of approximate ly -58, -49 and -39 kJ mol(-1), respectively. The calculated E(rev) am ounted to -224 mV at Delta G(ATP) approximate to -58 kJ mol(-1), -126 mV at Delta G(ATP) approximate to -49 kJ mol(-1) and -24 mV at Delta G (ATP) approximate to -39 kJ mol(-1). 7. Possible reasons for the discr epancy between estimated and calculated E(rev) values are discussed. 8 . Shifting Delta G(ATP) to less negative values not only altered E(rev ) but also diminished I-p at each membrane potential tested. The maxim al I-p(I-p,I-max), which can be activated by external Cs+ (Cs-o(+)), d ecreased under these conditions, whereas [Cs+](o) causing half-maximal I-p activation remained unchanged. Similarly, the voltage dependence of I-p activation by Cs-o(+) was unaffected. 9. It is concluded that E (rev) of I-p varies with Delta G(ATP) at constant sarcolemmal gradient s of the a pumped cations. This agrees with thermodynamic consideratio ns.