Abdoun Khalid, Wolf Katarina, Arndt Gisela, Martens Holger
Department of Physiology, Faculty of Veterinary Sciences, University of Khartoum, Sudan.
Br J Nutr. 2003 Oct;90(4):751-8. doi: 10.1079/bjn2003957.
The cellular uptake of ammonia affects the intracellular pH (pHi) of polar and non-polar cells. A predominant uptake of NH3 and its intracellular protonation tend to alkalinise the cytoplasm, whereas a predominant uptake of NH4(+) acidifies the cytoplasm by reversing this reaction. Hence, the well-known absorption of ammonia across the rumen epithelium probably causes a change in the pHi. The magnitude and direction of this change in pHi (acid or alkaline) depends on the relative transport rates of NH3 and NH4(+). Consequently, the intracellular availability of protons will influence the activity of the Na(+)-H(+) exchanger, which could affect transepithelial Na(+) transport. The aim of the present study has been to test this possible interaction between ruminal ammonia concentrations and Na(+) transport. The term ammonia is used to designate the sum of the protonated (NH4(+)) and unprotonated (NH3) forms. Isolated ruminal epithelium of sheep was investigated by using the Ussing-chamber technique in vitro. The present results indicate that ammonia inhibits Na(+) transport across the rumen epithelium of hay-fed sheep, probably by binding intracellular protons and thus inhibiting Na(+)-H(+) exchange. By contrast, ammonia stimulates Na(+) transport in concentrate-fed and urea-fed sheep, which develop an adaptation mechanism in the form of an increased metabolism of ammonia in the rumen mucosa and/or an increased permeability of rumen epithelium to the charged ammonium ion (NH4(+)). Intracellular dissociation of NH4(+) increases the availability of protons, which stimulate Na(+)-H(+) exchange. This positive effect of ruminal ammonia on Na(+) absorption may significantly contribute to the regulation of osmotic pressure of the ruminal fluid, because intraruminal ammonia concentrations up to 40 mmol/l have been reported.
氨的细胞摄取会影响极性和非极性细胞的细胞内pH值（pHi）。NH3的主要摄取及其细胞内质子化倾向于使细胞质碱化，而NH4(+)的主要摄取则通过逆转该反应使细胞质酸化。因此，众所周知的氨跨瘤胃上皮的吸收可能会导致pHi发生变化。pHi这种变化的幅度和方向（酸性或碱性）取决于NH3和NH4(+)的相对转运速率。因此，细胞内质子的可用性将影响Na(+)-H(+)交换体的活性，这可能会影响跨上皮Na(+)转运。本研究的目的是测试瘤胃氨浓度与Na(+)转运之间这种可能的相互作用。术语“氨”用于表示质子化形式（NH4(+)）和未质子化形式（NH3）的总和。采用Ussing室技术在体外对绵羊的离体瘤胃上皮进行了研究。目前的结果表明，氨抑制了干草喂养绵羊瘤胃上皮的Na(+)转运，可能是通过结合细胞内质子从而抑制Na(+)-H(+)交换。相比之下，氨刺激了精料喂养和尿素喂养绵羊的Na(+)转运，这些绵羊形成了一种适应机制，其形式为瘤胃黏膜中氨的代谢增加和/或瘤胃上皮对带电铵离子（NH4(+)）的通透性增加。NH4(+)的细胞内解离增加了质子的可用性，从而刺激Na(+)-H(+)交换。瘤胃氨对Na(+)吸收的这种积极作用可能对瘤胃液渗透压的调节有显著贡献，因为据报道瘤胃内氨浓度高达40 mmol/l。
