Transport of ammonia in the rabbit cortical collecting tubule.

Abstract

Nonionic diffusion and diffusion equilibrium of ammonia have been generally accepted as the mechanism of urinary ammonium excretion. However, these characteristics have not been examined directly in vitro. In the present studies, nonionic diffusion and diffusion equilibrium of ammonia were examined in rabbit cortical collecting tubules perfused in vitro. Collected fluid ammonium and pH were measured in tubules exposed to chemical gradients of NH3/Nrn. In tubules perfused with an acid perfusate free of ammonia and bathed with solutions containing NH4CI, collected fluid ammonia failed to equilibrate across the epithelium except at slow flow rates. The estimated apparent permeability coefficient to NH3 was -5 X 10-3 cm/s. Predominant nonionic diffusion of NH3, rather than transport of NW, was indicated by alkalinization of luminal fluid in tubules exposed to peritubular NH4C1 and by the relative influence of peritubular NHZ and NH3 on ammonia entry. In tubules perfused with an acid solution containing NH4CI, little loss of ammonium was detectable, indicating a low permeability to NHZ.In contrast to the restricted diffusion of NH3 in cortical collecting tubules, proximal convoluted tubules exhibited a much higher apparent permeability to NH3. In conclusion, nonionic diffusion of NH3 accounted for most ammonium transport in the proximal convoluted tubule and in the cortical collecting tubule. However, there was relatively restricted diffusion in the collecting tubules; this may account for the failure of whole kidney ammonium excretion to obey quantitatively the predictions of nonionic diffusion and diffusion equilibrium of ammonia.

Effect of reduced renal mass on renal ammonia transporter family, Rh C glycoprotein and Rh B glycoprotein, expression

“…Finally, the present study identifies increased apical and basolateral polarization of Rhcg. Because collecting duct ammonia secretion involves parallel H ϩ and NH 3 transport (14,15,19,29,30), the identification of increased apical plasma membrane polarization of both H ϩ -ATPase and Rhcg suggests that coordinated changes in H ϩ and NH 3 transport contribute to the increased individual nephron ammonia metabolism.…”

Expression of the Ammonia Transporter, Rh C Glycoprotein, in Normal and Neoplastic Human Kidney

“…In animal models, luminal acidification, including the generation of a luminal disequilibrium pH, results from apical H ϩ secretion by H ϩ -ATPase and H ϩ -K ϩ -ATPase and is both necessary for and regulates transepithelial ammonia secretion (31)(32)(33)(34)(35)(36). Collecting duct ammonia transport seems to involve both diffusive and transporter-mediated components (26,37), and the transporter-mediated component of apical transport is stimulated by luminal H ϩ (37).…”

Neonatal rabbit juxtamedullary proximal convoluted tubule acidification.

“…The bathing solution was then rapidly changed to one containing 20 mM NH3/ NH' (low-Na UF solution, +NH4CI, pH = 7.40). Because of the large NH3 permeability (32), the cells immediately alkalinize. This is followed by a slower acidification as the cell defends its pHi.…”