Transport of acetate and sodium in sheep omasum: mutual, but asymmetric interactions

Abstract

We have studied the transport of acetate across the isolated epithelium of sheep omasum; no net transport was observed (J(ms) approximately = J(sm)) under Ussing chamber conditions. Low mucosal pH (pH 6.4) significantly enhanced J(ms) acetate and the transport rates of acetate increased linearly and significantly (r2=0.99) with the luminal acetate concentration. The presence of another short chain fatty acid (propionate) did not affect J(ms) acetate significantly. Neither addition of 1 mmol l(-1) DIDS to the mucosal side nor HCO3 replacement caused changes of J(ms) acetate; this does not support the assumption of acetate transport via anion exchange. Addition of 1 mmol l(-1) amiloride to the mucosal side significantly decreased acetate fluxes at high mucosal acetate concentration (100 mmol l(-1)) and low pH (6.4) indicating interaction between acetate uptake in the undissociated form, intracellular release of protons and activation of Na+/H+ exchange (NHE). However, the mutual interaction between Na transport via NHE and acetate transport is asymmetric. Stimulation or inhibition of Na transport via NHE is much more pronounced than the corresponding changes of acetate fluxes. Thus, the obtained results support the conclusion that acetate is transported via simple diffusion and probably predominantly in the protonated form, thereby explaining the positive and mutual interaction between Na transport and short chain fatty acids.

Sheep rumen and omasum primary cultures and source epithelia: barrier function aligns with expression of tight junction proteins

“…In vivo and in vitro data support an extensive role for the omasum in the absorption of Na + , SCFA and accompanying water from the digesta (Masson and Philipson, 1952;Edrise et al, 1986;Ali et al, 2006). However, related to its greater size (~16l in sheep and >100l in cows) and easier accessibility for in vivo studies, the rumen has historically received greater attention and the tissue has played a pivotal role in studying the epithelial transport of SCFA (Phillipson and McAnnally, 1942;Aschenbach et al, 2009;Stumpff et al, 2009), chloride (Sperber and Hyden, 1952), ammonia (McDonald, 1948), urea (Decker et al, 1961;Harmeyer et al, 1973;Abdoun et al, 2010), Na + (Dobson, 1959) and Mg 2+ (Martens and Harmeyer, 1978), large quantities of which are physiologically absorbed.…”

“…Because acetate enters the epithelium either directly coupled to a proton [via what is probably lipid diffusion (Ali et al, 2006)] or in exchange for buffering HCO 3 - ), but leaves the tissue without a proton, the amount of acetate that can be transported without damage is limited by the capacity of pH regulatory mechanisms. Accordingly, breakdown of barrier function is frequently observed in situations with high ruminal SCFA concentrations and low pH (Plaizier et al, 2008).…”

Absorption and metabolism of volatile fatty acids by rumen and omasum

“…According to VFA pKa is minor than 4.9, over 95% of the VFA should be in ionized form at the ruminal pH around 6 to 7. In the omasum, VFA seems to be absorbed predominantly under protonated form (ALI et al 2006). Furthermore, unlike the rumen, the omasum absorbs bicarbonate, through chloride exchange (ALI, et al 2006).…”

Establishment and Characterization of an Omasal Epithelial Cell Model Derived from Dairy Calves for the Study of Small Peptide Absorption

“…The forestomach, particularly the omasum (bible), plays an important role in the absorption of nutrients from the ingesta, such as water, volatile fatty acids, minerals, electrolytes [2], amino acids (AA) [3], [4] and small peptides [3]–[5]. Interestingly, compared to the rumen, the omasum has a stronger ability to absorb small peptides [3], [6].…”

“…However, many previous studies on the absorption of small peptides in the ruminant gastrointestinal tract have been performed on animals, tissue models or in cell lines [2]–[6], [8]–[10], [13]–[15]. However, most of these cell lines have lost their organ-specific function due to their differentiated status [16].…”