Function of the thick ascending limb of Henle's loop.

BURG, MAURICE, AND NORDICA GREEN. Function of the thick ascending limb of Henle’s loop. Am. J. Physiol. 224(3) : 659-668. 1973.-The thick ascending limb of Henle’s loop was dissected from rabbit kidneys and perfused in vitro. There was net NaCl absorption from the tubule lumen with resultant lowering of lumen NaCl concentration below that of the bath. Since the electrical PD was positive in the lumen, Cl is absorbed actively. Na absorption is largely passive as indicated by comparison of 1) Na flux ratio and PD (at fast perfusion rates); and 2) Na concentration ratio and PD (at slow perfusion rates). However, the possibility of some active Na transport is not ruled out. The PD results from Cl transport. It is eliminated reversibly by removal of Cl from bath and lumen (SO4 substitution) and by ouabain. The PD is increased by removal of Na from bath and lumen (choline substitution). Na permeability is greater than Cl permeability, as indicated by I) the NaCl diffusion potential that is lumen positive when lumen NaCl concentration is low, and 2) radioisotope measurements of ion fluxes. Water permeability (Lp) is low, which combined with the NaCl transport accounts for the dilution of the urine in this segment.