Flessner M F, Knepper M A
National Institutes of Health, Bethesda, Md.
Miner Electrolyte Metab. 1990;16(5):299-307.
In vivo studies have shown that ammonium is secreted into the lumen of each of the major collecting duct segments. This secretion occurs by passive diffusion of NH3 in parallel with active H+ secretion. In vitro measurements in each segment have established that the NH3 permeability is relatively high (10(-3) to 10(-2) cm/s) while the NH4+ permeability is essentially zero. Interstitium to lumen NH3 concentration gradients have been observed in vivo in the inner medulla and are presumed to exist in other segments. The active H+ secretion causes a pH disequilibrium in segments which lack carbonic anhydrase and enhances the NH3 gradient driving NH3 secretion by decreasing the luminal NH3 concentration. The low NH4+ permeability prevents NH4+ backflux.
体内研究表明，铵被分泌到每个主要集合管节段的管腔中。这种分泌是通过NH₃的被动扩散与主动H⁺分泌同时进行的。对每个节段的体外测量已确定，NH₃的通透性相对较高（10⁻³至10⁻²cm/s），而NH₄⁺的通透性基本为零。在内髓质中已在体内观察到间质到管腔的NH₃浓度梯度，并且推测在其他节段中也存在。主动H⁺分泌会在缺乏碳酸酐酶的节段中导致pH不平衡，并通过降低管腔NH₃浓度来增强驱动NH₃分泌的NH₃梯度。低NH₄⁺通透性可防止NH₄⁺回流。
