Transport of cations and anions across forestomach epithelia: conclusions from in vitro studies

Abstract

Secretion of saliva as well as absorptive and secretory processes across forestomach epithelia ensures an optimal environment for microbial digestion in the forestomachs. Daily salivary secretion of sodium (Na 1 ) exceeds the amount found in plasma by a factor of 2 to 3, while the secretion of bicarbonate (HCO 3 2 ) is 6 to 8 times higher than the amount of HCO 3 2 in the total extracellular space. This implies a need for efficient absorptive mechanisms across forestomach epithelia to allow for an early recycling. While Na 1 is absorbed from all forestomachs via Na 1 /H 1 exchange and a non-selective cation channel that shows increased conductance at low concentrations of Mg 21 , Ca 21 or H 1 in the luminal microclima and at low intracellular Mg 21 , HCO 3 2 is secreted by the rumen for the buffering of ingesta but absorbed by the omasum to prevent liberation of CO 2 in the abomasum. Fermentation provides short chain fatty acids and ammonia (NH 3 ) that have to be absorbed both to meet nutrient requirements and maintain ruminal homeostasis of pH and osmolarity. The rumen is an important location for the absorption of essential minerals such as Mg 21 from the diet. Other ions can be absorbed, if delivered in sufficient amounts (Ca 21 , P i , K 1 , Cl 2 and NH 4 1 ). Although the presence of transport mechanisms for these electrolytes has been described earlier, our knowledge about their nature, regulation and crosstalk has increased greatly in the last years. New transport pathways have recently been added to our picture of epithelial transport across rumen and omasum, including an apical non-selective cation conductance, a basolateral anion conductance, an apical H 1 -ATPase, differently expressed anion exchangers and monocarboxylate transporters.Keywords: ruminants, sodium potassium magnesium and calcium, short chain fatty acids, chloride and bicarbonate, channels transporters and exchangers ImplicationsThe improved knowledge of ruminal ion transport clearly underlines its physiological meaning for the whole animal. For example, magnesium absorption is markedly reduced at low Mg and high K intake and the effect of potassium is diminished at high Mg intake. This variable interaction has been quantified recently permitting the prediction of Mg absorption. Great progress has also been made in understanding the interactions between the absorptive pathways for Na, short chain fatty acids and ammonium. The new findings on structure and regulation of various ion transporters will allow a better understanding of the challenges that different diets pose to the maintenance of homeostatic conditions within the rumen and within the cells of the surrounding epithelium, with the implications for the investigation of ruminal adaptation to different diets. Future studies on transport pathways should include the barrier function of rumen epithelium and its possible impairment under harsh feeding conditions.

Calcium transport in bovine rumen epithelium as affected by luminal Ca concentrations and Ca sources

“…For the rumen epithelium, we could show recently the existence of a nonspecific cation conductance which might contribute to the absorption of Ca across the rumen wall (Leonhard‐Marek ; Leonhard‐Marek et al. , ).…”

“…Using the everted gut sac assay, no differences in duodenal active Ca transport could be detected between wild-type, TRPV6 knockout, CaBP 9K knockout, and TRPV6/CaBP 9K double knockout mice fed a standard rodent chow diet (Benn et al 2008). For the rumen epithelium, we could show recently the existence of a nonspecific cation conductance which might contribute to the absorption of Ca across the rumen wall (Leonhard-Marek 2002;Leonhard-Marek et al 2005, 2010.…”

Dietary modulation of the expression of genes involved in short-chain fatty acid absorption in the rumen epithelium is related to short-chain fatty acid concentration and pH in the rumen of goats

“…Ruminal short-chain FA (SCFA) can be efficiently absorbed by the rumen epithelium via diffusion or a protein-mediated pathway (Leonhard-Marek et al, 2010;Aschenbach et al, 2011). Past research has demonstrated that feeding a high-concentrate diet could lead to enhanced SCFA absorptive capacity of rumen epithelium (Gäbel et al, 1991), even when no change occurred in the absorptive surface area of the rumen papillae (Sehested et al, 2000).…”

Bicarbonate‐dependent transport of acetate and butyrate across the basolateral membrane of sheep rumen epithelium

“…, Leonhard‐Marek et al . ). However, the quantitative contributions and identities of these transport proteins are still under discussion.…”

“…() and Leonhard‐Marek et al . () assumed a solely electrogenic transfer via an anion channel, Connor et al . () and Kirat et al .…”