Publikationsdatenbank

Ionic Conductances of the Ruminal Epithelium (2011)

Art

Hochschulschrift

Autor

Stumpff, Friederike (WE 2)

Quelle

Berlin: Mensch und Buch Verlag, 2011 — [6], 141 Seiten

ISBN: 978-3-86664-961-3

Sprache

Englisch

Verweise

URL (Volltext): https://refubium.fu-berlin.de/handle/fub188/10504

Kontakt

Institut für Veterinär-Physiologie

Oertzenweg 19 b
14163 Berlin
+49 30 838 62600
physiologie@vetmed.fu-berlin.de

Abstract / Zusammenfassung

Introduction: Postprandial fermentational processes in the rumen set free large quantities of SCFA, protons, K+, and NH3, whereas Na+, HCO3- , HPO32-, and Cl- enter with saliva, and urea is secreted across the ruminal wall. Absorptive processes across the ruminal wall are necessary to restore osmolarity and ruminal pH. Methods: The rumen and the omasum were studied by using various methods at the level of both the tissue and the cell (patch- clamp double-barreled pH-sensitive microelectrodes, Ussing chamber, confocal laser scanning microscopy, Western blot, PCR). A method for the isolation and cultivation of omasal cells was established. Results and Conclusions: Based on the findings and a careful study of the literature, a model for the efflux of osmotically active particles from the rumen was developed. Sodium, magnesium, and potassium: we present data demonstrating that apical non-selective cation channels gated by Ca2+ and Mg2+ mediate the efflux of cations from the rumen (199). When K+ concentrations in the rumen rise, the cells are depolarized. The divalent cations blocking the pore of the channel are repelled by the potential, and the pore of the channel is open for the influx of Na+, so that the absorption of this cation from the rumen is enhanced, and osmolarity is restored. The resulting transepithelial potential limits the efflux of K+ from the rumen and into the blood, thus facillitating potassium homeostasis. Apical depolarization also limits channel-mediated uptake of Mg2+, explaining the concomittant reduction in Mg2+ digestibility. However, the negative impact of K+ on the uptake of Mg2+ is balanced by the positive effects on the absorption of Na+, thus reducing ruminal osmolarity while maintaining systemic potassium homeostasis (351, 358, 359). Urea and ammonium: SCFA acidify the epithelium and stimulate the secretion of urea into the rumen via a protein-mediated pathway (probably UT-B) that is regulated by cytosolic pH (2). The efflux of ammonium from the rumen can occur both as NH3 and as NH4+, with differing impact on the absorption of sodium (4). Efflux of NH4+ occurs through apical non-selective cation channels and basolateral potassium-selective channels. At high pH, apical uptake occurs primarily as NH3, inhibiting sodium absorption via NHE. At acidic pH, uptake occurs as NH4+ through non-selective cation channels. Since cytosolic pH is higher than ruminal pH, apical recirculation (in as NH4+ out as NH3) will follow and can lead to the stimulation of Na+ absorption via NHE, which may play a role in ruminal osmoregulation. At all values of ruminal pH, the bulk of ammonium will remain protonated and leave through potassium channels in the form of NH4+, removing a proton from the cytosol. The findings suggest that the postprandial stimulation of urea secretion into the rumen occurs in response to changes of cytosolic pH and serves to the meet the nitrogen requirements of ruminal microbial populations, to regulate the speed of ruminal fermentation, and to buffer ruminal content (1). Cl- and SCFA-: The uptake of SCFA may be influenced by an apical microclimate, and occurs in a manner that acidifies the tissue (2). An anion exchanger can serve as an apical uptake pathway for both Cl- and SCFA- in exchange for HCO3-(15). The basolateral efflux of anions (Cl-, SCFA-) is mediated by a maxi-anion channel, coupled to the charge of Na+ leaving via the Na+/K+-ATPase (360). The data confirm the classical notion that electrically silent absorption of NaCl across epithelia requires the expression of a basolateral chloride channel. The data also confirm that maxi-anion channels allow the passage of large anions. The demonstration of these channels in a transporting epithelium is new and explains why protons freed by fermentational processes have to be extensively buffered by saliva, whereas large quantities of SCFA cross the rumen coupled to the transport of Na+ in an electrically silent manner. The permeability sequence of the channel, with p(Cl¬-) > p(acetate-) > p(propionate-) > p(butyrate-) might explain the well- known observation that acetate and propionate enter the portal blood to meet the energy requirements of the animal, whereas butyrate is extensively metabolized within the epithelium. It is suggested that protons taken up with SCFA are apically returned in exchange for Na+ via NHE, where they are buffered by saliva so that epithelial function is not endangered by acidification. When ruminal pH drops, protons have to be basolaterally extruded via NHE1, or buffered by ruminal secretion of HCO3- entering apically in exchange for SCFA, and basolaterally via Na-HCO3 cotransport. In all cases, the efflux of Na+ taken up by pH regulatory proteins occurs via the Na+/K+-ATPase, thus efficiently energizing the efflux of SCFA- anions through a large-conductance anion channel. A cytosolic accumulation of protons or SCFA is not required.