NH3 and NH4+ transport by rabbit renal proximal straight tubules.
Isolated perfused S2 proximal straight tubules from rabbits spontaneously secreted ammonia (-1.34 pmol X mm-1 X min-1) and absorbed bicarbonate (49.3 pmol X mm-1 X min-1) when perfusate and bath solutions contained 1 mM NH4Cl and 25 mM bicarbonate (pH 7.4). The NH3 concentration in the collected fluid was on average 40% lower than that of the bath as a consequence of a lower pH in the lumen. To test whether diffusion of NH3 down the bath-to-lumen NH3 concentration gradient could account for the measured ammonia secretion, we measured the permeabilities to NH3 (1.6 X 10(-2) cm/s) and NH4+ (4.5 X 10(-5) cm/s). From these values, we calculated predicted rates of passive NH3 secretion (-3.6 pmol X mm-1 X min-1) and passive NH4+ absorption (0.9 pmol X mm-1 X min-1). The predicted rate of net ammonia secretion exceeded the measured rate, indicating that passive NH3 secretion can fully account for the measured flux. In additional experiments, 10(-4) M acetazolamide in bath and perfusate inhibited net bicarbonate absorption by approximately 80%, but the rate of ammonia secretion was unaffected.


CONCLUSIONS
S2 proximal straight tubules spontaneously secrete ammonia as required for generation of a corticomedullary ammonia concentration gradient by counter-current multiplication. Diffusion of NH3 down a concentration gradient created by luminal acidification can account for the ammonia secretion. A substantial passive lumen-to-bath backflux of NH4+ occurs. Acetazolamide does not inhibit ammonia secretion.
D000086:Acetazolamide / Q000494:pharmacology; D000641:Ammonia / Q000378:metabolism*; D000818:Animals; D001692:Biological Transport; D066298:In Vitro Techniques; D007477:Ions; D007687:Kidney Tubules, Proximal / Q000378:metabolism*; D010539:Permeability; D000644:Quaternary Ammonium Compounds / Q000378:metabolism*; D011817:Rabbits
D007477:Ions; D000644:Quaternary Ammonium Compounds; D000641:Ammonia; D000086:Acetazolamide