NA-K+ EXCHANGE IN FROG EARLY DISTAL TUBULE - EFFECT OF ALDOSTERONE ONTHE SET-POINT()

1. Intracellular pH (pH(i)) regulation was investigated in frog early distal tubule. Single tubules were dissected and perfused, such that t he compositions of apical and basolateral solutions could be varied in dependently. pH(i) was measured using the fluorescent probe 2',7'-bis( carboxyethyl)-5,6-carboxyfluorescein (BCECF). 2. Brief exposure to NH4 + on the basolateral aspect of the tubules elicited an intracellular a cidification, followed by an active recovery. The recovery was inhibit ed by amiloride and its analogue 5-(N-ethyl-N-isopropyl) amiloride (EI PA) when added to the basolateral, but not the apical, solution. Omiss ion of Na+ from the basolateral solution alone completely inhibited pH (i) recovery. Thus the Na+-H+ exchangers appear to be located on the b asolateral membrane. 3. Neither amiloride nor EIPA had any effect on p H(i) under control conditions, suggesting that the activity of the Na-H+ exchangers at the resting pH(i) is low. However, removal of basola teral Na+ caused an acidification that was blocked by amiloride, indic ating that the Na+-K+ exchangers can be activated from the resting sta te. 4. Intrinsic buffering power (beta(i)) was determined by stepwise removal of ammonium from the cells in Na+-free conditions, to prevent pH regulation, and in the presence of Ba2+ and furosemide (frusemide), to inhibit ammonium transport. beta(i) was a function of pH(i), incre asing as pH(i) decreased. 5. Proton efflux was calculated during the r ecovery from an acid load in tubules from normal and K+-loaded frogs a nd in tubules which had been incubated for 30 min with aldosterone. Po tassium loading produces a chronic increase in plasma aldosterone. Bot h acute and chronic aldosterone treatment caused an intracellular alka linization. This was due to an alkaline shift in the set-point of the basolateral Na+-H+ exchanger, with no change in the density and/or tur nover rate.