Evidence for an amiloride sensitive Na+ pathway in the amphibian diluting segment induced by K+ adaptation.

The effect of amiloride on cell membrane potentials and intracellular Na activity (Nai) was tested in early distal tubules of the isolated perfused kidney of control and of K-adapted (high-K diet) Amphiuma. Conventional and Na-sensitive liquid ion-exchanger microelectrodes were employed to measure the peritubular cell membrane potential (PDpt), the transepithelial potential difference (PDte) and the Na electrochemical gradient across the peritubular cell membrane (ENapt), in the absence and the presence of amiloride (1 X 10(-4) mol X 1(-1] in both groups of animals. Amiloride did not affect PDpt and ENapt in control animals but depolarized PDpt and ENapt by about 8 mV in K-adapted animals. Nai (11.0 +/- 0.6 mmol X 1(-1) in early distal cells of control animals) did not change significantly by this maneuver. However, Nai decreased to extremely low values (2.3 +/- 0.2 mmol X 1(-1] when the luminal cotransport system for Na, Cl and K was inhibited by the luminal application of furosemide (5 X 10(-5) mol/l) and when the luminal cell membrane was exposed simultaneously to amiloride. The amiloride-induced effects on PDpt, ENapt and Nai occurred within seconds and were fully reversible. We conclude that high-K diet (K adaptation) induces an amiloride-sensitive pathway in the luminal cell membrane of early distal cells of Amphiuma which exists in parallel with the furosemide-sensitive cotransport system located in this cell barrier. The results suggest a luminal amiloride-sensitive Na/H exchange mechanism which regulates the luminal K permeability.