Good D W
Department of Internal Medicine, University of Texas Medical Branch, Galveston.
Miner Electrolyte Metab. 1990;16(5):291-8.
Absorption of ammonium by the loop of Henle provides a mechanism by which ammonium produced and secreted by proximal tubules is transferred to collecting ducts in the renal medulla. Loop ammonium absorption is largely a result of active absorption of NH4+ by the thick ascending limb. Ammonium absorption in the thick ascending limb is mediated primarily by apical membrane Na(+)-NH4(+)-2Cl- cotransport, and provides a 'single effect' for countercurrent multiplication of ammonium in the renal medulla. Countercurrent multiplication causes ammonium to accumulate to high concentrations in the medullary interstitial fluid and generates a transepithelial concentration difference for NH3 that favors secretion of ammonium into medullary collecting ducts. Changes in systemic potassium balance and chronic metabolic acidosis influence ammonium excretion by regulating ammonium absorption in the medullary thick ascending limb and altering transfer of ammonium to medullary collecting ducts.
亨氏袢对铵的重吸收提供了一种机制，近端小管产生和分泌的铵可借此转运至肾髓质的集合管。袢对铵的重吸收主要是由于髓袢升支粗段对NH4+的主动重吸收。髓袢升支粗段对铵的重吸收主要由顶端膜Na(+)-NH4(+)-2Cl-共转运介导，为肾髓质中铵的逆流倍增提供了一种“单一效应”。逆流倍增使铵在髓质间质液中积聚至高浓度，并产生有利于铵分泌到髓质集合管的NH3跨上皮浓度差。全身钾平衡的变化和慢性代谢性酸中毒通过调节髓袢升支粗段对铵的重吸收以及改变铵向髓质集合管的转运来影响铵的排泄。
