Cohen R M, Stephenson R L, Feldman G M
Department of Medicine, Veterans Administration Medical Center, University of Pennsylvania School of Medicine, Philadelphia 19104.
Am J Physiol. 1988 May;254(5 Pt 2):F657-67. doi: 10.1152/ajprenal.1988.254.5.F657.
Although the mammalian colon is thought to absorb large quantities of total ammonia, principally in the form of NH3, quantitative support for this hypothesis is lacking. In rat distal colon, we observed that NH3 was approximately 400 times more permeant than NH+4. In addition, colonic HCO-3 secretion influenced total ammonia (NH3 plus NH+4) absorption; that is, alteration of HCO-3 secretion caused a parallel change in total ammonia absorption. Perfusion with total ammonia also caused net HCO-3 secretion to switch to net absorption, and, in the setting of preexisting HCO-3 absorption, perfusate containing total ammonia enhanced HCO-3 absorption. These events suggest that colonic HCO-3 secretion titrates luminal NH+4 to NH3, permitting NH3 to diffuse from the lumen, while HCO-3 is titrated to carbon dioxide and also diffuses from the lumen. In support of titration of NH+4 and HCO-3, the magnitude of induced HCO-3 absorption approximated total ammonia absorption. This titration relationship suggests that, in kinetic studies, total ammonia absorption will be limited by a fixed rate of HCO-3 secretion. A model was developed that simulated these events.
尽管哺乳动物的结肠被认为能吸收大量的总氨，主要是以NH3的形式，但缺乏对这一假设的定量支持。在大鼠远端结肠，我们观察到NH3的渗透性比NH4+大约高400倍。此外，结肠HCO3-分泌影响总氨（NH3加NH4+）的吸收；也就是说，HCO3-分泌的改变会导致总氨吸收发生平行变化。用总氨灌注也会使HCO3-的净分泌转变为净吸收，并且，在预先存在HCO3-吸收的情况下，含有总氨的灌注液会增强HCO3-的吸收。这些现象表明，结肠HCO3-分泌将管腔内的NH4+滴定为NH3，使NH3能够从管腔扩散出去，而HCO3-则被滴定为二氧化碳并也从管腔扩散出去。为了支持NH4+和HCO3-的滴定，诱导的HCO3-吸收量近似于总氨吸收量。这种滴定关系表明，在动力学研究中，总氨吸收将受到HCO3-分泌固定速率的限制。开发了一个模拟这些现象的模型。
