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

Abstract

The effect of amiloride on cell membrane potentials and intracellular Na activity (Na_i) 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 (PD_pt), the transepithelial potential difference (PD_te) and the Na electrochemical gradient across the peritubular cell membrane (E ^Na_pt ), in the absence and the presence of amiloride (1·10⁻⁴mol·1⁻¹) in both groups of animals. Amiloride did not affect PD_pt and E ^Na_pt in control animals but depolarized PD_pt and E ^Na_pt by about 8 mV in K-adapted animals. Na_i (11.0 ±0.6 mmol·l⁻¹ in early distal cells of control animals) did not change significantly by this maneuver. However, Na_i decreased to extremely low values (2.3±0.2 mmol·l⁻¹) when the luminal cotransport system for Na, Cl and K was inhibited by the luminal application of furosemide (5·10⁻⁵ mol/l) and when the luminal cell membrane was exposed simultaneously to amiloride. The amiloride-induced effects on PD_pt, E ^Na_pt and Na_i 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 ofAmphiuma 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.