Active NH4+ absorption by the thick ascending limb.

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.