NaCl transport in mouse medullary thick ascending limbs. II. ADH enhancement of transcellular NaCl cotransport; origin of transepithelial voltage.

We measured the relations between tubular perfusion rate and the rate of net NaCl transport in medullary thick ascending limbs of Henle (mTALH) either in the presence or absence of ADH. These data, together with the known Na+, Cl-, and water permeability characteristics of the mTALH, were used to calculate tau NaCl (mol . s-1 . cm-2), the rate of conservative Cl- transport from lumen through cells to interspaces; and CNaCl, the effective NaCl concentration in lateral intercellular spaces. The experimental results indicate that in these tubules the rate of net Cl- absorption increases monotonically with perfusion rate, and that at a given perfusion rate ADH increases the rate of net salt absorption. The theoretical calculations show clearly that the ADH-mediated increase in salt absorption depends on an increase in the rate of conservative transcellular Cl- transport. However, the present analytical data do not permit a distinction between wholly electroneutral apical membrane NaCl entry with respect to a process in which apical membrane Na+/Cl- entry has a stoichiometry less than unity, and electrogenic Na+ transport accounts for the remaining component of net Na+ absorption. Identification of the stoichiometry of the Na+/Cl- apical membrane entry step will depend, among other factors, on identifying explicitly the diffusion resistance of paracellular fluid and the mode of passive ion transport across junctional complexes.