Electrochemical profile of K+ and Na+ in the amphibian early distal tubule.

Double-barreled microelectrodes selective to either potassium or sodium were used to determine the transepithelial potential difference (VTE) and the intraluminal activity of potassium (alpha LuK) or sodium (alpha LuNa) in the early distal tubule (EDT) of Triturus waltlii in vivo; luminal activities were compared with the corresponding plasma ion activities, alpha PtK and alpha PtNa. The transepithelial equilibrium potentials for potassium (EK(TE] and sodium (ENa(TE] were computed from the respective transmural chemical distributions: they were used to assess the transepithelial electrochemical potential differences [(V-EK)TE and (V-ENa)TE]. By dividing the raw data into three groups of 30% total tubular length (0-30, 31-60, 61-90%), the following results were obtained. 1) VTE increases from +15 to +20 mV (lumen positive) between the first and second portion of the EDT but remains constant thereafter. 2) The alpha LuK/alpha PtK ratio decreases steadily along the EDT from 1.92 to 1.66 and then to 1.32. 3) The values of alpha LuNa/alpha PtNa in the same three subdivisions are 0.79, 0.44, and 0.45. 4) The (V-EK)TE difference is largely positive along the whole EDT: +32, +33, and +27 mV. 5) The (V-ENa)TE difference declines from +9 mV (first portion) to values statistically not different from zero in the last two thirds of the EDT.(ABSTRACT TRUNCATED AT 250 WORDS)