Model of ammonium and bicarbonate transport along LDL: implications for alkalinization of luminal fluid.

Luminal fluid exiting the proximal convoluted tubule of a juxtamedullary nephron is alkalinized as it passes through the long-loop thin descending limb of Henle (LDL). Three potential mechanisms of alkalinization are: 1) concentration of bicarbonate by water abstraction, 2) direct bicarbonate entry, and 3) NH3 entry. We have used a mathematical model of the LDL to investigate these mechanisms. With permeabilities of HCO3-, NH3, and NH4+ measured for subsegments of the chinchilla LDL [M. F. Flessner, R. Mejia, and M. A. Knepper. Am. J. Physiol. 264 (Renal Fluid Electrolyte Physiol. 33):F388-F396, 1993], the osmotic water permeability of each segment [C.-L. Chou and M. A. Knepper. Am. J. Physiol. 263 (Renal Fluid Electrolyte Physiol. 32):F417-F426, 1992], and appropriate parameters from the literature, we have used the model to calculate hypothetical pH, HCO3- concentration, and NH3 concentration of the luminal fluid as it descends the LDL within an assumed interstitium. After eliminating each mechanism in turn by setting the appropriate permeability to zero, we recalculated the axial profiles. Our results suggest that, although all three mechanisms individually contribute to LDL alkalinization, NH3 entry likely plays the dominant role.