Wall S M
Department of Medicine, Emory University School of Medicine, Atlanta, GA 30322, USA.
Acta Physiol Scand. 2003 Dec;179(4):325-30. doi: 10.1046/j.0001-6772.2003.01212.x.
Along the collecting duct, secretion of ammonium (NH) is thought to occur through active H+ secretion in parallel with the non-ionic diffusion of ammonia (NH3). Thus NH3 is secreted into the collecting duct lumen down its concentration gradient. Moreover, the low NH permeability and high NH3 permeability observed in collecting duct epithelia minimizes back diffusion of NH. In general, an increase in the NH3 concentration gradient between the interstitium and the collecting duct lumen correlates with increased NH secretion. However, our laboratory and others have shown an important role of direct NH transport by the Na,K-ATPase. As K+ and NH compete for a common extracellular binding site on the Na,K-ATPase, reduced interstitial K+ concentration, such as during hypokalemia, augments NH uptake. Na,K-ATPase-mediated NH uptake provides an important source of H+ for net acid secretion during hypokalemia and contributes to the increase in NH excretion and metabolic alkalosis observed in this treatment model.
在集合管中，铵（NH）的分泌被认为是通过主动分泌氢离子（H⁺）与氨（NH₃）的非离子扩散同时进行的。因此，NH₃顺着其浓度梯度分泌到集合管腔中。此外，在集合管上皮细胞中观察到的低NH通透性和高NH₃通透性使NH的反向扩散降至最低。一般来说，间质与集合管腔之间NH₃浓度梯度的增加与NH分泌的增加相关。然而，我们实验室和其他研究表明，钠钾ATP酶对NH的直接转运具有重要作用。由于K⁺和NH竞争钠钾ATP酶上的一个共同细胞外结合位点，间质K⁺浓度降低，如在低钾血症期间，会增加NH的摄取。钠钾ATP酶介导的NH摄取为低钾血症期间的净酸分泌提供了重要的H⁺来源，并导致在该治疗模型中观察到的NH排泄增加和代谢性碱中毒。
