Permselectivity for cations over anions in the upper portion of descending limbs of Henle's loop of long-loop nephron isolated from hamsters

Abstract

The permselectivity of the upper portion of long descending limb of Henle (LDLu) was investigated with electrophysiological methods in the isolated perfused tubule preparation of hamster kidney. The diffusion potential (V _t) was determined in three different protocols. In protocol 1, the tubules were initially perfused with modified Krebs Ringer's solution on the both sides of the epithelium. Then the bath solution was exchanged consecutively with another solution in which 50 mmol/l NaCl replaced by 50 mmol/l KCl, RbCl, NH₄Cl, CsCl, LiCl, NaBr, NaNO₃, NaI, Na acetate or 75 mmol/l NaCl replaced by mannitol. The permeabilities for these ions relative to chloride were calculated by Goldman's constant field equation. The segment was found to be cation selective, with all cations being 5–9 times more permeable than all anions. The sequence of permeability was K⁺>Rb⁺>Li⁺>NH ⁺₄ =Cs⁺≧ Na⁺≫Cl⁻≧Br⁻≧NO ⁻₃ ≧I⁻>Acetate⁻. In protocol 2, pure 150 mM NaCl was used for the basal solution to avoid interference by other ions. The bathing solution was exchanged by other solutions which contained 150 mmol/l KCl, NH₄Cl, CsCl, RbCl, LiCl, NaI, NaBr, NaNO₃, Na acetate or 75 mmol/l NaCl with mannitol. Thus simple biionic substitution was performed. Again, the segment was found to be cation selective, with all cations being 4–10 times more permeable than all anions. The sequence of ion permeability was NH ⁺₄ >K⁺>Rb⁺>Na⁺>Cs⁺> Li⁺≫NO ⁻₃ ≧Cl⁻≧I⁻≧Br⁻≧Acetate⁻. In protocol 3,75 mmol/l of NaCl with mannitol was applied either in the bath or in the lumen and the potential difference was compared on the same tubule. The diffusion potential across the epithelium was found to be completely symmetrical, suggesting that it reflects mainly, if not entirely, the permselectivity of the paracellular pathway. It is concluded that the upper portion of descending limb of Henle of long-loop nephron is highly permselective for cations.