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Active NH4+ absorption by the thick ascending limb.

J L Garvin·M B Burg·M A Knepper
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Abstract

Isolated perfused medullary thick ascending limbs from rabbits were studied to determine the mechanism of ammonium ion absorption. Under control conditions, thick ascending limbs spontaneously absorbed NH4+ and generated a lumen-positive potential. When these tubules were chemically voltage clamped to lumen-negative potentials by lowering the bath NaCl concentration, NH4+ absorption persisted. Thus NH4+ was absorbed against an electrochemical gradient. The active flux accounts for most of the net flux under control conditions, the remainder being due to passive paracellular NH4+ diffusion. The NH4+ permeability, measured in separate experiments, was high (1.50 +/- 0.25 x 10(-4) cm/s) compared with values in other segments. The NH3 permeability was relatively low (3.1 +/- 0.5 x 10(-3) cm/s). Luminal furosemide (10(-4) M) eliminated most of the active NH4+ flux, indicating that a major fraction of the active flux is dependent on apical entry of NH4+ via the Na+ -K+ -2Cl- cotransporter (presumably by substitution for K+). The remaining active flux was completely inhibited by 10(-4) M ouabain in the bath. Active chloride absorption was maintained when NH4+ entirely replaced K+ in bath and perfusate, indicating that NH4+ substitutes for K+ on the apical cotransporter and the basolateral Na+ -K+ -ATPase. Ammonium absorption provides an active "single effect" for countercurrent multiplication of NH4+ in the renal medulla.

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AbsorptionAlgorithmsAmmoniaAnimalsChloridesFurosemideKidney MedullaOuabainPerfusionPermeabilityRabbitsJournal ArticleResearch Support, U.S. Gov't, P.H.S.

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BibTeXRIS

J L Garvin, M B Burg, M A Knepper. 1988. Active NH4+ absorption by the thick ascending limb.. https://doi.org/10.1152/ajprenal.1988.255.1.f57

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