These experiments were designed to determine the relative contributions of active NH4+ transport and voltage-driven NH4+ diffusion to direct NH4+ absorption by the medullary thick ascending limb of the rat. Medullary thick ascending limbs were perfused in vitro with solutions containing 25 mM HCO3 and 4 mM total ammonia. Under steady-state conditions, the lumen-positive transepithelial voltage (V(T)) was not sufficient to account for the observed decrease in lumen NH4+ concentration, consistent with active absorption of NH4+. Flux calculations based on V(T) and measured NH4+ permeability (6 x 10-5 cm/s) indicate that the majority (at least 65%) of total ammonia absorption is due to active transport of NH4+. The remainder of NH4+ absorption can be accounted for by voltage-driven diffusion. Increasing the potassium concentration from 4 to 24 mM in perfusate and bath markedly inhibited total ammonia absorption but did not affect V(T), NH4+ permeability, or HCO3 absorption. These results are consistent with inhibition of the active component of NH4+ absorption by potassium. The active NH4+ absorption is likely mediated by cotransport of Na+, NH4+, and Cl- across the apical cell membrane. Inhibition of active NH4+ absorption by an increase in potassium concentration may be due, in part, to competition between NH4+ and K+ for a common binding site on the Na+-K+-2Cl- cotransport system.
|Original language||English (US)|
|Journal||American Journal of Physiology - Renal Fluid and Electrolyte Physiology|
|Issue number||1 (24/1)|
|State||Published - 1988|
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