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

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+) exceeds the amount found in plasma by a factor of 2 to 3, while the secretion of bicarbonate (HCO3-) is 6 to 8 times higher than the amount of HCO3- in the total extracellular space. This implies a need for efficient absorptive mechanisms across forestomach epithelia to allow for an early recycling. While Na+ is absorbed from all forestomachs via Na+/H+ exchange and a non-selective cation channel that shows increased conductance at low concentrations of Mg2+, Ca2+ or H+ in the luminal microclima and at low intracellular Mg2+, HCO3- is secreted by the rumen for the buffering of ingesta but absorbed by the omasum to prevent liberation of CO2 in the abomasum. Fermentation provides short chain fatty acids and ammonia (NH3) 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 Mg2+ from the diet. Other ions can be absorbed, if delivered in sufficient amounts (Ca2+, Pi, K+, Cl- and NH4+). 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+-ATPase, differently expressed anion exchangers and monocarboxylate transporters.