Distribution of ouabain-binding sites along the dog nephron

Na,K pump stimulation by intracellular Na in isolated, intact sheep cardiac Purkinje fibers

Regulation of the Na,K pump in intact cells is strongly associated with the level of intracellular Na+. Experiments were carried out on intact, isolated sheep Purkinje strands at 37 degrees C. Membrane potential (Vm) was measured by an open-tipped glass electrode and intracellular Na+ activity (aNai) was calculated from the voltage difference between an Na+-selective microelectrode (ETH 227) and Vm. In some experiments, intracellular potassium (aiK) or chloride (aCli) was measured by a third separate microelectrode. Strands were loaded by Na,K pump inhibition produced by K+ removal and by increasing Na+ leak by removing Mg++ and lowering free Ca++ to 10(-8) M. Equilibrium with outside levels of Na+ was reached within 30-60 min. During sequential addition of 6 mM Mg++ and reduction of Na+ to 2.4 mM, the cells maintained a stable aNai ranging between 25 and 90 mM and Vm was -30.8 +/- 2.2 mV. The Na,K pump was reactivated with 30 mM Rb+ or K+. Vm increased over 50-60 s to -77.4 +/- 5.9 mV with Rb+ activation and to -66.0 +/- 7.7 mV with K+ activation. aiNa decreased in both cases to 0.5 +/- 0.2 mM in 5-15 min. The maximum rate of aiNa decline (maximum delta aNai/delta t) was the same with K+ and Rb+ at concentrations greater than 20 mM. The response was abolished by 10(-5) M acetylstrophantidin. Maximum delta aNai/delta t was independent of outside Na+, while aKi was negatively correlated with aNai (aKi = 88.4 - 0.86.aNai). aCli decreased by at most 3 mM during reactivation, which indicates that volume changes did not seriously affect aNai. This model provided a functional isolation of the Na,K pump, so that the relation between the pump rate (delta aNai/delta t) and aiNa could be examined. A Hill plot allowed calculation of Vmax ranging from 5.5 to 27 mM/min, which on average is equal to 25 pmol.cm-2.s-1.K 0.5 was 10.5 +/- 0.6 mM (the aNai that gives delta aNai/delta t = Vmax/2) and n equaled 1.94 +/- 0.13 (the Hill coefficient). These values were not different with K+ or Rb+ as an external activator. The number of ouabain-binding sites equaled 400 pmol.g-1, giving a maximum Na+ turnover of 300 s-1. The Na,K pump in intact Purkinje strands exhibited typical sigmoidal saturation kinetics with regard to aNai as described by the equation upsilon/Vmax = aNai(1.94)/(95.2 + aNai(1.94)). The maximum sensitivity of the Na,K pump to aiNa occurred at approximately 6 mM.