Mejia R, Flessner M F, Knepper M A
Laboratory of Kidney and Electrolyte Metabolism, National Heart, Lung, and Blood Institute, National Institutes of Health, Bethesda, Maryland 20892.
Am J Physiol. 1993 Mar;264(3 Pt 2):F397-403. doi: 10.1152/ajprenal.1993.264.3.F397.
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.
从近髓肾单位近端曲管流出的管腔液在流经亨利袢长襻细降支（LDL）时会发生碱化。碱化的三种潜在机制为：1）通过水的重吸收使碳酸氢盐浓缩；2）碳酸氢盐直接进入；3）氨进入。我们使用了LDL的数学模型来研究这些机制。利用对灰鼠LDL各亚段测得的HCO3-、NH3和NH4+的渗透率[M.F.弗莱斯纳、R.梅希亚和M.A.克内珀。《美国生理学杂志》264（肾流体电解质生理学33）：F388 - F396，1993]、各段的渗透水渗透率[周C.-L.和M.A.克内珀。《美国生理学杂志》263（肾流体电解质生理学32）：F417 - F426，1992]以及文献中的适当参数，我们使用该模型计算了在假定间质内管腔液沿LDL下降时的假设pH值、HCO3-浓度和NH3浓度。通过依次将适当的渗透率设为零来消除每种机制后，我们重新计算了轴向分布。我们的结果表明，尽管所有三种机制都分别对LDL碱化有贡献，但氨进入可能起主要作用。
