Volume 90, Issue 4, October (2003), pp. 751-758 © The Author 2003
doi:10.1079/BJN2003957

Medline/PubMed Citation | Related Articles in PubMed | Download to Citation Matcher  

Effect of ammonia on Na+ transport across isolated rumen epithelium of sheep is diet dependent

Khalid Abdoun¹, Katarina Wolf², Gisela Arndt³, Holger Martens^2
1Department of Physiology, Faculty of Veterinary Sciences, University of Khartoum, , Khartoum, Sudan
²Institute of Veterinary Physiology, Free University of Berlin, Oertzenweg 19b, 14163 Berlin, Germany
³Department of Biometrics and Statistics, Free University of Berlin, Berlin, Germany

 (Received 7 May 2002–Revised 19 March 2003–Accepted 5 June 2003)

The cellular uptake of ammonia affects the intracellular pH (pH_i) 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 pH_i. The magnitude and direction of this change in pH_i (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.

Keywords:
Rumen, Sheep, Na⁺ transport, Ammonia

Abbreviations:
I_sc, short-circuit current, J_ms, mucosal-to-serosal flux, J_net, net flux, ME, metabolisable energy, NHE, Na⁺–H⁺exchange, PD, potential difference, pH_i, intracellular pH