Luminal hyperosmolarity decreases Na transport and impairs barrier function of sheep rumen epithelium

Abstract

The effects of luminal hyperosmolarity on Na and Cl transport were studied in rumen epithelium of sheep. An increase of luminal osmotic pressure with mannitol (350 and 450 mosm/l) caused a significant increase of tissue conductance, G (T), which is linearly correlated with flux rates of (51)Cr-EDTA and indicates an increase of passive permeability. Studies with microelectrodes revealed, that an increase of the osmotic pressure caused a significant increase of the conductance of the shunt pathway from 1.23 +/- 0.10 (control) to 1.92 +/- 0.14 mS cm(-2) (450 mosm/l) without a change of fractional resistance. Hyperosmolarity significantly increased J (sm) and reduced J (net) Na. The effect of hyperosmolarity on J (ms) Na is explained by two independent and opposed effects: increase of passive permeability and inhibition of the Na(+)/H(+) exchanger. Hypertonic buffer solution induced a decrease of the intracellular pH (pH(i)) of isolated ruminal cells, which is consistent with an inhibition of Na(+)/H(+) exchange, probably isoform NHE-3, because NHE-3-mRNA was detectable in rumen epithelium. These data are in contrast to previous reports and reveal a disturbed Na transport and an impaired barrier function of the rumen epithelium, which predisposes translocation of rumen endotoxins and penetration of bacteria.

A vH+-ATPase is present in cultured sheep ruminal epithelial cells

“…However, HEPESbuffered, nominally HCO 3 Ϫ -free medium was used in the present study. Under these conditions, the pH i recovery induced by butyrate is mainly due to stimulation of NHE activity in these cells (36,40,42), but there is only sparse information on the contribution of specific NHE subtypes to this process. Recently, our group showed the expression of the NHE subtypes 1 and 3 in REC (40).…”

“…Under these conditions, the pH i recovery induced by butyrate is mainly due to stimulation of NHE activity in these cells (36,40,42), but there is only sparse information on the contribution of specific NHE subtypes to this process. Recently, our group showed the expression of the NHE subtypes 1 and 3 in REC (40). In this study, selective inhibition of NHE1 (by use of HOE 694, 100 mol/l) and of NHE3 (by use of S3226, 10 mol/l) led to a 24 and 27% reduction of the pH i recovery, respectively.…”

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

“…To date, nine isoforms of NHE have been identified in different tissues (Lam et al, 2009). Experiments with isolated sheep rumen epithelial cells (RECs) have shown that the specific inhibitors HOE 694 and S3226 of NHE-1 and NHE-3, respectively, are able to inhibit an acid-induced pH i recovery in these cells (Schweigel et al, 2005). In the same study, the mRNA of NHE-3 and NHE-1 was detected in samples of sheep rumen epithelium by reverse transcription-PCR.…”

A high-grain diet causes massive disruption of ruminal epithelial tight junctions in goats

“…Rapidly fermentable nonstructural carbohydrates increase the rate of fermentation acid production in the rumen, and the accumulation of the acids leads to a decrease in ruminal pH, hyperosmolarity, and an increase in ruminal toxin concentration (2,29). Previous studies have investigated the specific effect of low pH (9,26), hyperosmolarity (20,34), or an exposure to toxins (7) on ruminal epithelial barrier function in vitro. It should be acknowledged that any one or the combination of these factors may affect epithelial barrier function.…”