Absorption of short-chain fatty acids across ruminal epithelium of sheep

Abstract

Investigations on the absorption of shortchain fatty acids across ruminal epithelium of sheep were performed both in vitro (Ussing chamber technique, using propionic acid representatively for short-chain fatty acids) and in vivo (washed, isolated reticulorumen). A pH-induced, nearly tenfold increase in the concentration of undissociated propionate led to an only twofold increase in mucosal-to-serosal flux of propionate (in vitro). Neither amiloride (1 mmol.l-1, in vitro) nor theophylline (10 mmol.l-1, in vivo), inhibitors of the ruminal Na+/H+ exchanger, exerted any significant influence on propionate fluxes or short-chain fatty acids absorption, respectively. Total replacement of luminal Na+ (by choline) did not alter short-chain fatty acids absorption (in vivo). Mucosal 4,4'-diisothiocyanatostilbene-2,2'-disulfonic acid (0.1 mmol.l-1) or mucosal nitrate (40 mmol.l-1) markedly reduced propionate net flux (in vitro). Increasing mucosal Cl- concentration brought about a significant drop in mucosal-to-serosal flux of propionate (in vitro) and in short-chain fatty acids net absorption (in vivo), respectively. The results obtained suggest that short-chain fatty acids are absorbed both as anions and as undissociated acids across ruminal epithelium of sheep. It is concluded that short-chain fatty acids anions either compete with Cl- for binding sites at a common anion-exchange mechanism or that they are absorbed by an short-chain fatty acids anion/ HCO3- exchanger indirectly coupled to a Cl-/HCO3- exchanger via intracellular bicarbonate.

Gene expression in the digestive tissues of ruminants and their relationships with feeding and digestive processes

“…Studies of vH 1 -ATPase protein distribution in bovine rumen epithelium indicated expression in small groups primarily within the cell membranes, but also within the cytosol in the epithelial layers of the stratum spinosum and granulosum, and in a few epithelia of the stratum basale (Albrecht et al, 2008). On the basis of the results of this and earlier functional studies by Kramer et al (1996), it was concluded that the vH 1 -ATPase also contributes to the transport of SCFA through the rumen epithelium by supplying lumenal H 1 for protonation of SCFA, facilitating their uptake. Figure 2 summarizes the proteins involved in maintenance of rumen intracellular pH and their putative roles.…”

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

“…Changing from hay to concentrate diets raised net bicarbonate secretion as well as net SCFA absorption, while the replacement of SCFA by gluconate and mannitol reversed net bicarbonate secretion into net absorption, which was further enhanced by the additional removal of luminal chloride. These studies gave rise to the concept of bicarbonate secretion via anion exchangers that would also accept SCFA 2 or Cl 2 anions (Figures 3a and 4, Kramer et al, 1996). Under physiological, low chloride conditions, secretion of HCO 3 2 is apparently driven by the uptake of SCFA anions (Aschenbach et al, 2009).…”

“…The major fraction of SCFA crosses the forestomach epithelia unmetabolized (Kiddle et al, 1951;Gä bel et al, 2002), although SCFA, in particular butyrate, are also used by the epithelial cells themselves for cell metabolism, which could support active transport of other electrolytes (Weigand et al, 1975;Sehested et al, 1999;Kristensen et al, 2000;Gä bel et al, 2002;Gä bel and Aschenbach, 2006). Apical uptake mechanisms for SCFA Previous studies and reviews have focussed on two possible mechanisms of SCFA uptake across the apical membrane: diffusion of the protonated, undissociated form through the lipid bilayer and exchange of the dissociated SCFA 2 anion for bicarbonate via a transport protein that exchanges SCFA 2 for HCO 3 2 anions under physiological conditions (Figure 4, Gä bel et al, 1991a;Kramer et al, 1996;Gä bel et al, 2002;Aschenbach et al, 2009).…”

“…In the reticulorumen of sheep in vivo, that is in the presence of SCFA gradients, the disappearance of butyrate was not affected by Cl 2 , while butyrate slightly reduced the disappearance of chloride (Aschenbach et al, 2009), pointing to a small additional contribution of anion exchangers to butyrate absorption and a possible competition between butyrate and Cl 2 absorption. The absorptive flux of propionate across sheep rumen epithelium could be stimulated by a decrease in pH -in line with uptake of undissociated acid via lipid diffusion or via a specific transport protein -but also by a reduction in mucosal Cl 2 concentration -in line with uptake via anion exchange (Kramer et al, 1996). In vivo, the negative effects of Cl 2 on SCFA disappearance from the washed reticulorumen were most pronounced for propionate, which vice versa had a high negative impact on the disappearance of Cl 2 (Aschenbach et al, 2009).…”

“…In vivo studies showed negative impacts of acetate or chloride on the disappearance of the respective other anion. Nitrate that had been shown to block ruminal Cl 2 and propionate absorption in the presence of bicarbonate (Wü rmli et al, 1987;Kramer et al, 1996) blocked ruminal acetate absorption in the presence and in the absence of bicarbonate to almost the same final level (Aschenbach et al, 2009). From these observations the investigators conclude the presence of bicarbonate dependent and independent anion exchangers in addition to lipophilic uptake as mechanisms of acetate absorption.…”