Effects of intracellular signals on Na+/K(+)-ATPase pump activity in the frog skin epithelium.

The effects of intracellular signals (pHi, Na+i, Ca2+i, and the electrical membrane potential), on Na+ transport mediated by the Na+/K+ pump were investigated in the isolated Rana esculenta frog skin. In particular we focussed on pHi sensitivity since protons act as an intrinsic regulator of transepithelial Na+ transport (JNa) by a simultaneous control of the apical membrane Na+ conductance (gNa) and the basolateral membrane K+ conductance (gK). pHi changes which modify JNa, gNa and gK, do not affect the Na+ transport mediated by the pump as shown by kinetic and electrophysiological studies. In addition, no changes were observed in the number of 3H-ouabain binding sites in acid-loaded epithelia. Our attempts to modify cellular Ca2+ (by using Ca(2+)-free/EGTA Ringer solution or A23187 addition) also failed to produce any significant effects in the Na+ pump turnover rate or the number of 3H-ouabain binding sites. The Na+ pump current was found to be sensitive to the basolateral membrane potential, saturating for very positive (cell) potentials and a reversal potential of -160 mV was calculated from I-V relationships of the pump. Changes in Na+i considerably affected the Na+ pump rate. A saturating relationship was found between pump rate and Nai+ with maximal activation at Nai+ greater than 40 mmol/l; a high dependence of the pump rate and of the number of 3H-ouabain binding sites was observed in the physiological range of Nai+. We conclude that protons (in the physiological pH range) which act directly and simultaneously on the passive transport pathways (gNa and gK), have no direct effect on the Na+/K+ pump rate. After an acid load, the inhibition of JNa is primarily due to the reduction of gNa. This results in a reduction of Nai and the pump turnover rate then becomes dependent on other pathways of Na+ entry such as the basolateral membrane Na+/H+ exchanger.