Ruminal transport and metabolism of short-chain fatty acids (SCFA) in vitro: effect of SCFA chain length and pH

Postnatal Growth and Development of the Rumen: Integrating Physiological and Molecular Insights

The rumen plays an essential role in the physiology and production of agriculturally important ruminants such as cattle. Functions of the rumen include fermentation, absorption, metabolism, and protection. Cattle are, however, not born with a functional rumen, and the rumen undergoes considerable changes in size, histology, physiology, and transcriptome from birth to adulthood. In this review, we discuss these changes in detail, the factors that affect these changes, and the potential molecular and cellular mechanisms that mediate these changes. The introduction of solid feed to the rumen is essential for rumen growth and functional development in post-weaning calves. Increasing evidence suggests that solid feed stimulates rumen growth and functional development through butyric acid and other volatile fatty acids (VFAs) produced by microbial fermentation of feed in the rumen and that VFAs stimulate rumen growth and functional development through hormones such as insulin and insulin-like growth factor I (IGF-I) or through direct actions on energy production, chromatin modification, and gene expression. Given the role of the rumen in ruminant physiology and performance, it is important to further study the cellular, molecular, genomic, and epigenomic mechanisms that control rumen growth and development in postnatal ruminants. A better understanding of these mechanisms could lead to the development of novel strategies to enhance the growth and development of the rumen and thereby the productivity and health of cattle and other agriculturally important ruminants.

Quantitative Differences in Rumen Epithelium Proteins in Lambs Fed Wheat, Perennial Wheat, or Perennial Wheat plus Lucerne

The value of crops such as perennial wheat (PW) for grain and grazing compared to conventional wheat (W), or the addition of lucerne to PW (PWL) is still being determined. This research sought to determine if these diets were associated with changes in the membranebound proteins that transport nutrients in the rumen epithelium (RE). Crossbred ewes (Poll Dorset × Merino) were fed W, PW, or PWL (50:50) fresh-cut forage ad libitum for 4 weeks. Average daily gain (ADG; p < 0.001) was highest in the W-fed lambs compared to the PW and PWL. Metabolisable energy intake (MEI) was higher in lambs fed W (p < 0.001) compared to PW and PWL. In pairwise comparisons of the PW and PWL diet group we found protein abundance was significantly (p < 0.05, FDR < 0.05, Benjamini p < 0.05) different in fatty acid metabolism, oxidative phosphorylation, and biosynthesis of cofactors pathways. There were not any differences in protein abundance related to nutrient transport or energy metabolism in the RE between W- vs. PW- and W- vs. PWL-fed lambs. However, in the PW- vs. PWL-fed lambs, there was a difference in the level of proteins regulating the metabolism of fatty acids and energy production in the mitochondria of the rumen epithelium.

Physiological responses and adaptations to high methane production in Japanese Black cattle

In this study, using enteric methane emissions, we investigated the metabolic characteristics of Japanese Black cattle. Their methane emissions were measured at early (age 13 months), middle (20 months), and late fattening phases (28 months). Cattle with the highest and lowest methane emissions were selected based on the residual methane emission values, and their liver transcriptome, blood metabolites, hormones, and rumen fermentation characteristics were analyzed. Blood β-hydroxybutyric acid and insulin levels were high, whereas blood amino acid levels were low in cattle with high methane emissions. Further, propionate and butyrate levels differed depending on the enteric methane emissions. Hepatic genes, such as SERPINI2, SLC7A5, ATP6, and RRAD, which were related to amino acid transport and glucose metabolism, were upregulated or downregulated during the late fattening phase. The above mentioned metabolites and liver transcriptomes could be used to evaluate enteric methanogenesis in Japanese Black cattle.

Transcriptome Analysis of Bovine Rumen Tissue in Three Developmental Stages

Rumen development is a crucial physiological challenge for ruminants. However, the molecular mechanism regulating rumen development has not been clearly elucidated. In this study, we investigated genes involved in rumen development in 13 rumen tissues from three developmental stages (birth, youth, and adult) using RNA sequencing. We identified that 6,048 genes were differentially expressed among three developmental stages. Using weighted correlation network analysis, we found that 12 modules were significantly associated with developmental stages. Functional annotation and protein–protein interaction (PPI) network analysis revealed that CCNB1, CCNB2, IGF1, IGF2, HMGCL, BDH1, ACAT1, HMGCS2, and CREBBP involved in rumen development. Integrated transcriptome with GWAS information of carcass weight (CW), stomach weight (SW), marbling score (MS), backfat thickness (BFT), ribeye area (REA), and lean meat weight (LMW), we found that upregulated DEGs (fold change 0∼1) in birth–youth comparison were significantly enriched with GWAS signals of MS, downregulated DEGs (fold change >3) were significantly enriched with GWAS signals of SW, and fold change 0∼1 up/downregulated DEGs in birth–adult comparison were significantly enriched with GWAS signals of CW, LMW, REA, and BFT. Furthermore, we found that GWAS signals for CW, LMW, and REA were enriched in turquoise module, and GWAS signals for CW was enriched in lightgreen module. Our study provides novel insights into the molecular mechanism underlying rumen development in cattle and highlights an integrative analysis for illustrating the genetic architecture of beef complex traits.

Effects of lipopolysaccharide exposure on the inflammatory response, butyrate flux, and metabolic function of the ruminal epithelium using an ex vivo model

Acidotic conditions in the rumen have been associated with compromised barrier function of the ruminal epithelium and translocation of microbe-associated molecular patterns (MAMP) such as lipopolysaccharide (LPS). Interaction of MAMP with the ruminal epithelium may also induce a local proinflammatory response. The aim of this study was to evaluate the potential proinflammatory response of the ruminal epithelium following LPS exposure in Ussing chambers, to investigate whether LPS exposure affects the flux and metabolism of butyrate. Ruminal epithelial tissue from 9 Holstein bull calves were mounted into Ussing chambers and exposed to 0, 10,000, 50,000, or 200,000 endotoxin units (EU)/mL LPS for a duration of 5 h. Radiolabeled ¹⁴C-butyrate (15 mM) was added to the mucosal buffer to assess the mucosal-to-serosal flux of ¹⁴C-butyrate. Additional Ussing chambers, without radioisotope, were exposed to either 0 or 200,000 EU/mL LPS and were used to measure the release of β-hydroxybutyrate (BHB) and IL1B into the buffer, and to collect epithelial tissue for analysis of gene expression. Genes associated with inflammation (TNF, IL1B, CXCL8, PTGS2, TGFB1, TLR2, TLR4), nutrient transport (MCT1, MCT4, SLC5A8, GLUT1), and metabolic function (ACAT1, BDH1, MCU, IGFBP3, IGFBP5) were selected and analyzed using quantitative real-time PCR. Butyrate flux was not significantly affected by LPS exposure; however, we detected a tendency for the mucosal-to-serosal butyrate flux to increase linearly with LPS dose. Bidirectional releases of BHB and IL1B were not affected by LPS exposure. Expression of PTGS2, TGFB1, TLR4, and MCU were downregulated following exposure to LPS ex vivo. We detected no effects on the expression of genes associated with nutrient transport. The results of the present study are interpreted to indicate that, although the inflammatory response of the ruminal epithelium was slightly suppressed, exposure to LPS may have altered metabolic function.

Effects of a proinflammatory response on metabolic function of cultured, primary ruminal epithelial cells

Inflammation of ruminal epithelium may occur during ruminal acidosis as a result of translocation and interaction of ruminal epithelial cells (REC) with molecules such as lipopolysaccharide (LPS). Such inflammation has been reported to alter cellular processes such as nutrient absorption, metabolic regulation, and energy substrate utilization in other cell types but has not been investigated for REC. The objectives of this study were to investigate the effects of LPS on metabolism of short-chain fatty acids by primary REC, as well as investigating the effects of media containing short-chain fatty acids on the proinflammatory response. Ruminal papillae from 9 yearling Speckle Park beef heifers were used to isolate and culture primary REC. Cells were grown in minimum essential medium (MEM) for 12 d before use and then reseeded in 24-well culture plates. The study was conducted as a 2 × 2 factorial, where cells were grown in unaltered MEM (REG) or medium containing 2 mM butyrate and 5 mM propionate (SCFA) with (50,000 EU/mL; +LPS) or without LPS (-LPS) for 24 h. Supernatant samples were collected for analysis of glucose and SCFA consumption. Cells were collected to determine the expression of mRNA for genes associated with inflammation (TNF, IL1B, CXCL2, CXCL8, PTGS2, and TLR4), purinergic signaling (P2RX7, ADORAB2, and CD73), nutrient transport [SLC16A1 (MCT1), SLC16A3 (MCT4), SLC5A8, and MCU], and cell metabolism [ACAT1, SLC2A1 (GLUT1), IGFBP3, and IGFBP5]. Protein expression of TLR4 and ketogenic enzymes (BDH1 and HMGCS1) were also analyzed using flow cytometry. Statistical analysis was conducted with the MIXED model of SAS version 9.4 (SAS Institute Inc., Cary, NC) with medium, LPS exposure, and medium × LPS interaction as fixed effects and animal within plate as a random effect. Cells tended to consume more glucose when exposed to LPS as opposed to no LPS exposure (31.8 vs. 28.7 ± 2.7), but consumption of propionate and butyrate was not influenced by LPS. Expression of TNF and IL1B was upregulated when exposed to LPS, and expression of CXCL2 and CXCL8 increased following LPS exposure with SCFA (medium × LPS). For cells exposed to LPS, we found a downregulation of ACAT1 and IGFBP5 and an upregulation of SLC2A1, SLC16A3, MCU, and IGFBP3. Medium with SCFA led to greater expression of MCU. SLC16A1 was upregulated in cells incubated with SCFA and without LPS compared with the other groups. Protein expression of ketogenic enzymes was not affected; however, BDH1 mean fluorescence intensity (MFI) expression tended to be less in cells exposed to LPS. These data are interpreted to indicate that when REC are exposed to LPS, they may increase glucose metabolism. Moreover, transport of solutes was affected by SCFA in the medium and by exposure to LPS. Overall, the results suggest that metabolic function of REC in vitro is altered by a proinflammatory response, which may lead to a greater glucose requirement.

Ruminal epithelium: a checkpoint for cattle health

The reticulorumen, as the main fermentation site of ruminants, delivers energy in the form of short-chain fatty acids (SCFA) for both the animal as well as the ruminal wall. By absorbing these SCFA, the ruminal epithelium plays a major role in the maintenance of intraruminal and intraepithelial acid-base homoeostasis as well as the balance of osmolarity. It takes up SCFA via several pathways which additionally lead to either a reduction of protons in the ruminal lumen or the secretion of bicarbonate, ultimately buffering the ruminal content effectively. Nutrition of the epithelium itself is achieved by catabolism of the SCFA, especially butyrate. Catabolism of SCFA also helps to maintain a concentration gradient across the epithelium to ensure efficient SCFA uptake and stability of the epithelial osmolarity. Furthermore, the ruminal epithelium forms a tight barrier against pathogens, endotoxins or biogenic amines, which may emerge from ruminal microorganisms and feed. Under physiological conditions, it reduces toxin uptake to a minimum. Moreover, the epithelium seems to have the ability to degrade biogenic amines like histamine. Nonetheless, in high performance production animals like dairy cattle, the reticulorumen is confronted with large amounts of rapidly fermentable carbohydrates. This may push the epithelium to its limits, even though it possesses a great capacity to adapt to varying feeding conditions. If the epithelial limit is exceeded, increasing amounts of SCFA lead to an acidotic imbalance that provokes epithelial damage and thereby elevates the entrance of pathogens and other potentially harmful substances into the animal's body. Hence, the ruminal epithelium lays the foundation for the animal's health, and in order to ensure longevity and high performance of ruminant farm animals, it should never be overburdened.

Transcriptomic Changes in the Rumen Epithelium of Cattle after the Induction of Acidosis

The transition from normal forage to a highly fermentable diet to achieve rapid weight gain in the cattle industry can induce ruminal acidosis. The molecular host mechanisms that occur in acidosis are largely unknown. Therefore, the histology and transcriptome profiling of rumen epithelium was investigated in normal and acidosis animals to understand the molecular mechanisms involved in the disease. The rumen epithelial transcriptome from acidosis (n=3) and control (n=3) Holstein steers was obtained using RNA-sequencing. The mean values of clean reads were 70,975,460&amp;plusmn;984,046 and 71,142,189&amp;plusmn;834,526 in normal and acidosis samples, respectively. In total, 1,074 differentially expressed genes were identified in the two groups (P&amp;lt;0.05), of which 624 and 450 genes were up- and down-regulated in the acidosis samples, respectively. Functional analysis indicated that the majority of the up-regulated genes had a function in filament organization, positive regulation of epithelial and muscle fiber concentration, biomineral tissue development, negative regulation of fat cell differential, regulation of ion transmembrane transport, regulation of cell adhesion and butyrate, as well as short-chain fatty acid absorption that was metabolized as an energy source. Functional analysis of the down-regulated genes revealed effects in immune response, positive regulation of T-cell migration, regulation of metabolic processes, and localization. Furthermore, the results showed a differential expression of genes involved in the Map Kinase and Toll-like receptor signaling pathways. The IL1B, CXCL5, IL36A, and IL36B were significantly down-regulated in acidosis rumen tissue samples. The results suggest that rapid shifts to rich fermentable carbohydrates diets cause an increase in the concentration of ruminal volatile fatty acids, tissue damage, and significant changes in transcriptome profiles of rumen epithelial.

Possible influence of free fatty acid receptors on pH regulation in the ruminal epithelium of sheep

High amounts of short-chain fatty acids (SCFAs) occur in the ovine rumen and constitute the animal's main energy source. However, they lead to an acidification of the ruminal epithelium. Therefore, effective intracellular pH (pH_i ) regulation by transport proteins like monocarboxylate transporter 1 (MCT1) and Na⁺ /H⁺ exchangers (NHEs) is pivotal to ruminants to avoid epithelial damage. SCFAs might function not only as nutrients but also as signalling molecules by activating free fatty acid receptors (FFARs) in the ruminal epithelium and thus influence pH_i regulation. FFARs work as nutrient sensors, transducing their information by modulating cyclic adenosine monophosphate (cAMP) levels. We hypothesized that (FFAR-modulated) decreases in cAMP levels stimulate the activity of MCT1 and NHEs in the ruminal epithelium of sheep. We detected two FFARs (GPR109A and FFAR2) immunohistochemically in the ovine ruminal epithelium. Administration of 10 mM butyrate to Ussing chamber-mounted epithelia provoked a significant reduction in intraepithelial cAMP levels. However, application of the GPR109A agonist niacin did not affect cAMP levels. MCT1 activity was analysed by measuring transepithelial ¹⁴ C-acetate fluxes, which were not inhibited by forskolin-induced increased cAMP levels. The recovery of pH_i after acidification was assessed as an indicator of NHE activity in primary cultured ruminal epithelial cells. Recovery was significantly reduced when cells with increased cAMP levels were subjected to the NHE inhibitor 5-(N-ethyl-N-isopropyl)-amiloride (10 µM). Nonetheless, with augmented cAMP levels alone, NHE activity tended to decline. We hypothesize that modulation of cAMP levels by butyrate is accomplished by FFAR2 activation, regulating NHE activity for pH_i homoeostasis at least in part.

Effects of grain processing methods on the expression of genes involved in volatile fatty acid transport and pH regulation, and keratinization in rumen epithelium of beef cattle

Two experiments were carried out to evaluate the effects of corn and sorghum with different processing methods on the expression of genes involved in volatile fatty acids transport and pH regulation, and ruminal keratinization in rumen epithelium of finishing bulls. For Exp. 1, five rumen cannulated Nellore bulls were used in a 5x5 Latin square arrangement, with 14 d for adaptation and 9 d for sample collection. Treatments were: dry ground corn, dry ground sorghum, reconstituted corn, reconstituted sorghum, and control (forage-based diet). Samples of rumen epithelium from ventral sac were excised, rinsed, snap-frozen and stored at -80°C until total RNA isolation and quantitative real-time PCR analysis. In the Exp. 2, 24 Nellore bulls were assigned to a completely randomized design lasting 168 d. Experimental treatments were similar to those at Exp. 1, but without the control treatment. After the experimental period, bulls were slaughtered and rumen epithelium samples were rapidly excised for further histological analysis. Rumen epithelial tissue from animals fed reconstituted corn had lower expression of downregulated-in-adenoma (P = 0.03) and Na+/H+ exchanger 2 (trend; P = 0.09). The expression of Na+/ H+ exchanger 1 (P = 0.10) and putative anion transporter (P = 0.06) tended to be lower in rumen epithelium of bulls fed reconstituted grains. Ruminal concentration of valerate was greater for animals fed reconstituted grain (P = 0.01). Likewise, animals fed reconstituted corn tended to have greater butyrate ruminal concentration (P = 0.08). Keratinized layer thickness did not differ among treatments (P > 0.10). Therefore, reconstituted grains (especially corn) decrease the mRNA expression of genes involved in volatile fatty acids transport and pH control in the rumen epithelium.

1H NMR-Based Identification of Intestinally Absorbed Metabolites by Ussing Chamber Analysis of the Rat Cecum

The large intestine (cecum and colon) is a complex biochemical factory of vital importance to human health. It plays a major role in digestion and absorption by salvaging nutrients from polysaccharides via fermentation initiated by the bacteria that comprise the gut microbiome. We hypothesize that the intestinal epithelium absorbs a limited number of luminal metabolites with bioactive potential while actively excluding those with toxic effects. To explore this concept, we combined ¹H NMR detection with Ussing chamber measurements of absorptive transport by rat cecum. Numerous metabolites transported across the epithelium can be measured simultaneously by ¹H NMR, a universal detector of organic compounds, alleviating the need for fluorescent or radiolabeled compounds. Our results demonstrate the utility of this approach to delineate the repertoire of fecal solutes that are selectively absorbed by the cecum and to determine their transport rates.

Effect of ruminal acidosis and short-term low feed intake on indicators of gastrointestinal barrier function in Holstein steers

The objective of this study was to determine effect of ruminal acidosis (RA) and low feed intake [LFI] on the regional barrier function of the gastrointestinal tract. Twenty-one Holstein steers were fed for ad libitum intake for 5 d (control [CON]), fed at 25% of ad libitum intake for 5 d (LFI), or provided 2 d of ad libitum intake followed by 1-d of feed restriction (25% of ad libitum intake), 1 d where 30% of ad libitum dry matter intake (DMI) was provided as pelleted barley followed by the full allocation (RA) and fed for ad libitum intake the following day. Tissues and digesta from the rumen, omasum, duodenum, jejunum, ileum, cecum, proximal, and distal colon were collected. Permeability was assessed using the mucosal-to-serosal flux of inulin (JMS-inulin) and mannitol (JMS-mannitol). Digesta pH was 0.81, 0.63, and 0.42 pH units less for RA than CON in the rumen, cecum, and proximal colon; while, LFI had pH that was 0.47 and 0.36 pH units greater in the rumen and proximal colon compared to CON. Total ruminal short-chain fatty acid (SCFA) concentration were less for LFI (92 mM; P = 0.010) and RA (87 mM; P = 0.007) than CON (172 mM) steers. In the proximal colon, the proportion of butyrate (P = 0.025 and P = 0.022) and isobutyrate (P = 0.019 and P = 0.019) were greater, and acetate (P = 0.028 and P = 0.028) was less for LFI and RA, respectively, when compared to CON steers. Ruminal papillae length, width, perimeter, and surface area were 1.21 mm, 0.78 mm, 3.84 mm, and 11.15 mm2 less for LFI than CON; while, RA decreased papillae width by 0.52 mm relative to CON. The JMS-mannitol was less for LFI steers than CON in the proximal colon (P = 0.041) and in the distal colon (P = 0.015). Increased gene expression for claudin 1, occludin, tight-cell junction protein 1 and 2, and toll-like receptor 4 were detected for LFI relative to CON in the rumen, jejunum, and proximal colon. For RA steers, expression of toll-like receptor 4 in the rumen, and occludin and tight-cell junction protein 1 were greater in the jejunum than CON. An acute RA challenge decreased pH in the rumen and large intestine but did not increase tissue permeability due to increases in the expression of genes related to barrier function within 1 d of the challenge. Steers exposed to LFI for 5 d had reduced ruminal SCFA concentrations, smaller ruminal papillae dimensions, and increased tissue permeability in the proximal and distal colon despite increases for genes related to barrier function and immune function.

Transcriptome profiling of the rumen epithelium of beef cattle differing in residual feed intake

Background

Feed efficient cattle consume less feed and produce less environmental waste than inefficient cattle. Many factors are known to contribute to differences in feed efficiency, however the underlying molecular mechanisms are largely unknown. Our study aimed to understand how host gene expression in the rumen epithelium contributes to differences in residual feed intake (RFI), a measure of feed efficiency, using a transcriptome profiling based approach.

Results

The rumen epithelial transcriptome from highly efficient (low (L-) RFI, n = 9) and inefficient (high (H-) RFI, n = 9) Hereford x Angus steers was obtained using RNA-sequencing. There were 122 genes differentially expressed between the rumen epithelial tissues of L- and H- RFI steers (p < 0.05) with 85 up-regulated and 37 down-regulated in L-RFI steers. Functional analysis of up-regulated genes revealed their involvement in acetylation, remodeling of adherens junctions, cytoskeletal dynamics, cell migration, and cell turnover. Additionally, a weighted gene co-expression network analysis (WGCNA) identified a significant gene module containing 764 genes that was negatively correlated with RFI (r = −0.5, p = 0.03). Functional analysis revealed significant enrichment of genes involved in modulation of intercellular adhesion through adherens junctions, protein and cell turnover, and cytoskeletal organization that suggest possible increased tissue morphogenesis in the L-RFI steers. Additionally, the L-RFI epithelium had increased expression of genes involved with the mitochondrion, acetylation, and energy generating pathways such as glycolysis, tricarboxylic acid cycle, and oxidative phosphorylation. Further qPCR analysis of steers with different RFI (L-RFI, n = 35; M-RFI, n = 34; H-RFI, n = 35) revealed that the relative mitochondrial genome copy number per cell of the epithelium was positively correlated with RFI (r = 0.21, p = 0.03).

Conclusions

Our results suggest that the rumen epithelium of L-RFI (efficient) steers may have increased tissue morphogenesis that possibly increases paracellular permeability for the absorption of nutrients and increased energy production to support the energetic demands of increased tissue morphogenesis compared to those of H-RFI (inefficient) animals. Greater expression of mitochondrial genes and lower relative mitochondrial genome copy numbers suggest a greater rate of transcription in the rumen epithelial mitochondria of L-RFI steers. Understanding how host gene expression profiles are associated with RFI could potentially lead to identification of mechanisms behind this trait, which are vital to develop strategies for the improvement of cattle feed efficiency.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2935-4) contains supplementary material, which is available to authorized users.

Short-term adaptation of the ruminal epithelium involves abrupt changes in sodium and short-chain fatty acid transport

The objectives of this study were to determine the effect of an increase in diet fermentability on 1) the rate and extent to which short-chain fatty acid (SCFA) absorption pathways adapt relative to changes in Na(+) transport, 2) the epithelial surface area (SA), and 3) the barrier function of the bovine ruminal epithelium. Twenty-five Holstein steer calves were assigned to either the control diet (CON; 91.5% hay and 8.5% supplement) or a moderately fermentable diet (50% hay; 41.5% barley grain (G), and 8.5% supplement) fed for 3 (G3), 7 (G7), 14 (G14), or 21 days (G21). All calves were fed at 2.25% body weight at 0800. Calves were killed (at 1000), and ruminal tissue was collected to determine the rate and pathway of SCFA transport, Na(+) transport and barrier function in Ussing chambers. Tissue was also collected for SA measurement and gene expression. Mean reticular pH decreased from 6.90 for CON to 6.59 for G7 and then increased (quadratic P < 0.001). While effective SA of the ruminal epithelium was not affected (P > 0.10) by dietary treatment, the net Na(+) flux increased by 125% within 7 days (quadratic P = 0.016). Total acetate and butyrate flux increased from CON to G21, where passive diffusion was the primary SCFA absorption pathway affected. Increased mannitol flux, tissue conductance, and tendencies for increased expression of IL-1β and TLR2 indicated reduced rumen epithelium barrier function. This study indicates that an increase in diet fermentability acutely increases Na(+) and SCFA absorption in the absence of increased SA, but reduces barrier function.

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

AIM

This study aimed to assess the role of HCO₃⁻ in the transport of acetate and butyrate across the basolateral membrane of rumen epithelium and to identify transport proteins involved.

METHODS

The effects of basolateral variation in HCO₃⁻ concentrations on acetate and butyrate efflux out of the epithelium and the transepithelial flux of these short-chain fatty acids were tested in Ussing chamber experiments using (14)C-labelled substrates. HCO₃⁻-dependent transport mechanisms were characterized by adding specific inhibitors of candidate proteins to the serosal side.

RESULTS

Effluxes of acetate and butyrate out of the epithelium were higher to the serosal side than to the mucosal side. Acetate and butyrate effluxes to both sides of rumen epithelium consisted of HCO₃⁻-independent and -dependent parts. HCO₃⁻-dependent transport across the basolateral membrane was confirmed in studies of transepithelial fluxes. Mucosal to serosal fluxes of acetate and butyrate decreased with lowering serosal HCO₃⁻ concentrations. In the presence of 25 mm HCO₃⁻, transepithelial flux of acetate was inhibited effectively by p-hydroxymercuribenzoic acid or α-cyano-4-hydroxycinnamic acid, while butyrate flux was unaffected by the blockers. Fluxes of both acetate and butyrate from the serosal to the mucosal side were diminished largely by the addition of NO₃⁻ to the serosal side, with this effect being more pronounced for acetate.

CONCLUSION

Our results indicate the existence of a basolateral short-chain fatty acid/HCO₃⁻ exchanger, with monocarboxylate transporter 1 as a primary candidate for acetate transfer.

Epithelia of the ovine and bovine forestomach express basolateral maxi-anion channels permeable to the anions of short-chain fatty acids

It has long been established that the absorption of short-chain fatty acids (SCFA) across epithelia stimulates sodium proton exchange. The apically released protons are not available as countercations for the basolateral efflux of SCFA anions and a suitable transport model is lacking. Patch clamp and microelectrode techniques were used to characterize an anion conductance expressed by cultured cells of the sheep and bovine rumen and the sheep omasum and to localize the conductance in the intact tissue. Cells were filled with a Na-gluconate solution and superfused with sodium salts of acetate, propionate, butyrate, or lactate. Reversal potential rose and whole cell current at +100 mV decreased with the size of the anion. Anion-induced currents could be blocked by diisothiocyanato-stilbene-2,2'-disulfonic acid (DIDS), NPPB (200 μmol l(-1)), or pCMB (1 mmol l(-1)). In patches of bovine ruminal cells, single channels were observed with a conductance for chloride (327 ± 11 pS), acetate (115 ± 8 pS), propionate (102 ± 10 pS), butyrate (81 ± 2 pS), and gluconate (44 ± 3 pS). Channels expressed by sheep rumen and omasum were similar. Microelectrode experiments suggest basolateral localization. In conclusion, forestomach epithelia express basolateral maxi-anion channels with a permeability sequence of chloride > acetate > propionate > butyrate. SCFA absorption may resemble functionally coupled transport of NaCl, with the Na(+)/K(+)-ATPase driving the basolateral efflux of the anion through a channel. Since protons are apically extruded, the model accurately predicts that influx of buffers with saliva is essential for the pH homeostasis of the ruminant forestomach.

Quantification of Transcriptome Responses of the Rumen Epithelium to Butyrate Infusion using RNA-seq Technology

Short-chain fatty acids (SCFAs), such as butyrate, produced by gut microorganisms, play a critical role in energy metabolism and physiology of ruminants as well as in human health. In this study, the temporal effect of elevated butyrate concentrations on the transcriptome of the rumen epithelium was quantified via serial biopsy sampling using RNA-seq technology. The mean number of genes transcribed in the rumen epithelial transcriptome was 17,323.63 ± 277.20 (±SD; N = 24) while the core transcriptome consisted of 15,025 genes. Collectively, 80 genes were identified as being significantly impacted by butyrate infusion across all time points sampled. Maximal transcriptional effect of butyrate on the rumen epithelium was observed at the 72-h infusion when the abundance of 58 genes was altered. The initial reaction of the rumen epithelium to elevated exogenous butyrate may represent a stress response as Gene Ontology (GO) terms identified were predominantly related to responses to bacteria and biotic stimuli. An algorithm for the reconstruction of accurate cellular networks (ARACNE) inferred regulatory gene networks with 113,738 direct interactions in the butyrate-epithelium interactome using a combined cutoff of an error tolerance (ɛ = 0.10) and a stringent P-value threshold of mutual information (5.0 × 10⁻¹¹). Several regulatory networks were controlled by transcription factors, such as CREBBP and TTF2, which were regulated by butyrate. Our findings provide insight into the regulation of butyrate transport and metabolism in the rumen epithelium, which will guide our future efforts in exploiting potential beneficial effect of butyrate in animal well-being and human health.

Transepithelial transport and intraepithelial metabolism of short-chain fatty acids (SCFA) in the porcine proximal colon are influenced by SCFA concentration and luminal pH

Short-chain fatty acids (SCFA) are end products of bacterial fermentation in the colon and cecum of monogastric animals. As SCFA serve as relevant energy suppliers for colonocytes and various tissues, it is important to reveal fundamental mechanistic characteristics of their transepithelial transport subjected to transient variations of fermentations rates. We performed Ussing chamber studies with porcine (Sus scrofa) colon epithelium under physiological conditions and examined individual mucosal disappearance, metabolized loss, tissue concentrations and serosal release of acetate, propionate and butyrate by gas chromatography. Reduction of initial SCFA concentrations from 80 to 40 mmol/L resulted in diminished absolute flux rates, but the relative proportions of mucosal disappearance and intracellular metabolization of individual SCFA were slightly enhanced. Simulation of high fermentation rates by lowering the mucosal pH induced an increase in mucosal disappearance and serosal release of all SCFA, while their tissue contents trended to lower levels. With respect to the metabolization at lowered pH we found increased acetate concentrations and a decrease of propionate and butyrate. Our data indicate that the colon epithelium possesses a high adaptive capacity to ensure its energetic maintenance under various intraluminal fermentation rates by utilizing the unique features of individual SCFA as energy sources.

Modulation of urea transport across sheep rumen epithelium in vitro by SCFA and CO2

Urea transport across the gastrointestinal tract involves transporters of the urea transporter-B group, the regulation of which is poorly understood. The classical stimulatory effect of CO(2) and the effect of short-chain fatty acids (SCFA) on the ruminal recycling of urea were investigated by using Ussing chamber and microelectrode techniques with isolated ruminal epithelium of sheep. The flux of urea was found to be phloretin sensitive and passive. At a luminal pH of 6.4, but not at 7.4, the addition of SCFA (40 mmol/l) or CO(2)/HCO3- (10% and 25 mmol/l) led to a fourfold increase in urea flux. The stepwise reduction of luminal pH in the presence of SCFA from 7.4 to 5.4 led to a bell-shaped modification of urea transport, with a maximum at pH 6.2. Lowering the pH in the absence of SCFA or CO(2) had no effect. Inhibition of Na(+)/H(+) exchange increased urea flux at pH 7.4, with a decrease being seen at pH 6.4. In experiments with double-barreled, pH-sensitive microelectrodes, we confirmed the presence of an apical pH microclimate and demonstrated the acidifying effects of SCFA on the underlying epithelium. We confirm that the permeability of the ruminal epithelium to urea involves a phloretin-sensitive pathway. We present clear evidence for the regulation of urea transport by strategies that alter intracellular pH, with permeability being highest after a moderate decrease. The well-known postprandial stimulation of urea transport to the rumen in vivo may involve acute pH-dependent effects of intraruminal SCFA and CO(2) on the function of existing urea transporters.

The vacuolar-type H-ATPase in ovine rumen epithelium is regulated by metabolic signals

In this study, the effect of metabolic inhibition (MI) by glucose substitution with 2-deoxyglucose (2-DOG) and/or application of antimycin A on ovine rumen epithelial cells (REC) vacuolar-type H⁺-ATPase (vH⁺-ATPase) activity was investigated. Using fluorescent spectroscopy, basal pH_i of REC was measured to be 7.3 ± 0.1 in HCO₃⁻-free, glucose-containing NaCl medium. MI induced a strong pH_i reduction (−0.44 ± 0.04 pH units) with a more pronounced effect of 2-DOG compared to antimycin A (−0.30 ± 0.03 versus −0.21 ± 0.03 pH units). Treatment with foliomycin, a specific vH⁺-ATPase inhibitor, decreased REC pH_i by 0.21 ± 0.05 pH units. After MI induction, this effect was nearly abolished (−0.03 ± 0.02 pH units). In addition, membrane-associated localization of vH⁺-ATPase B subunit disappeared. Metabolic control of vH⁺-ATPase involving regulation of its assembly state by elements of the glycolytic pathway could provide a means to adapt REC ATP consumption according to energy availability.

Epithelial capacity for apical uptake of short chain fatty acids is a key determinant for intraruminal pH and the susceptibility to subacute ruminal acidosis in sheep

Subacute ruminal acidosis (SARA) is a common digestive disorder occurring in ruminants, with considerable variation in the severity of SARA observed among animals fed the same diet. Our aim in this study was to determine whether differences in the capacity of the ruminal epithelium for the apical uptake of acetate and butyrate (determined in Ussing chambers after slaughter) explains differences observed for the severity of a preceding episode of SARA in vivo. Adult sheep with an indwelling small ruminant ruminal pH measurement system (SRS) were randomly assigned to either a SARA induction treatment (oral drench containing 5 g glucose/kg body weight; n = 17) or a sham treatment (SHAM; n = 7; 12 mL water/kg body weight). Sheep receiving the glucose drench were further classified as nonresponders (NR; n = 7) or responders (RES; n = 7) according to their ruminal pH profile for the 3 h following the oral drench. Mean ruminal pH for the 3 h following the drench differed among groups (P < 0.001), with it being highest for SHAM (6.67 +/- 0.08), intermediate for NR (5.97 +/- 0.05), and lowest for RES (5.57 +/- 0.08) sheep. The apical uptake of acetate and butyrate did not differ between SHAM and RES sheep. However, NR sheep had greater in vitro apical uptake of acetate and butyrate and a higher plasma beta-hydroxybutyrate concentration than RES sheep, suggesting greater absorptive capacity for NR. Differences between NR and RES were attributed to greater bicarbonate-independent, nitrate-sensitive uptake of acetate (P = 0.007), a tendency for greater bicarbonate-dependent uptake of acetate (P = 0.071), and greater bicarbonate-independent uptake of butyrate (P = 0.022). These data indicate that differences in the rates and pathways for the uptake of acetate and butyrate explain a large proportion of the individual variation observed for the severity of SARA.

Effect of dietary forage to concentrate ratio on volatile fatty acid absorption and the expression of genes related to volatile fatty acid absorption and metabolism in ruminal tissue

The objective of the study was to investigate the fractional rate of volatile fatty acid (VFA) absorption and the expression of genes encoding for transporters and enzymes involved in the absorption and metabolism of VFA in ruminal tissue when cattle were fed high or low concentrate diets. Twelve ruminally cannulated Holstein cows were used in a randomized complete block design. The low concentrate (LC) and high concentrate (HC) diets contained 8 and 64% dietary concentrate (dry matter basis), respectively. Cows were fed their respective diet for at least 28 d, following which data and samples were collected over 6 d. Ruminal pH was measured continuously for 72 h, and the in vivo VFA absorption and passage rates were measured using Co-EDTA and n-valeric acid as markers. Ruminal tissue was collected postslaughter from the ventral sac of the rumen, and gene expression was evaluated using quantitative real-time PCR. Dry matter intake was not affected by treatment, averaging 14.9 kg/d, but cows fed HC had lower mean ruminal pH (6.03 vs. 6.48), and a greater duration (376 vs. 10 min/d) that ruminal pH was <5.8. Ruminal VFA concentration was 24 mM higher for cows fed HC compared with LC; however, the fractional rate of VFA absorption and passage from the rumen was not affected by dietary treatment, averaging 23.4 and 9.6%/h, respectively. The expression of genes encoding for enzymes involved in VFA activation and ketogenesis were not affected by treatment. Cows fed HC tended to have a relative abundance of pyruvate dehydrogenase lipoamide alpha 1 mRNA transcripts that was 1.4 times lower than that of cows fed LC, but other enzymes involved in pyruvate metabolism or regulation of the citric acid cycle were not affected. Collectively, these results suggest that the dietary forage to concentrate ratio does not affect the fractional rate of VFA absorption in vivo, but potentially alters energy metabolism in ruminal tissue.

Absorption of short-chain fatty acids, sodium and water from the forestomach of camels

In camelids the ventral parts of compartments 1 and 2 (C1/C2) and the total surface of compartment 3 of the forestomach are lined with tubular glands, whereas in ruminants the surface of the forestomach is composed entirely of stratified, squamous epithelium. Thus, differences in absorption rates between these foregut fermenters can be expected. In five camels C1/C2 was temporarily isolated, washed and filled with buffer solutions. Absorption of short-chain fatty acids (SCFA) and net absorption of sodium and water were estimated relative to Cr-ethylenediaminetetraacetic acid as a fluid marker. SCFA were extensively absorbed in the forestomach; clearance rates of SCFA with different chain lengths were equal. After lowering the pH of solutions SCFA absorption rates increased, but much less than the increase of the non-ionized fraction. Absorption of propionate was lower when acetate had been added. Findings suggest that most of the SCFA in camels are transported in the ionized form, most likely via an anion exchange mechanism. Net water absorption is closely related to net sodium absorption. Apparently water absorption results from an iso-osmotic process. Differences between absorption mechanisms of SCFA from the forestomach of camelids and ruminants are discussed.

Splanchnic metabolism of volatile fatty acids in the dairy cow

Volatile fatty acids (VFA) are quantitatively important substrates for dairy cows and other ruminants. It has been a central dogma in the nutritional physiology of ruminants that the ruminal epithelium metabolizes a large fraction of VFA during theirabsorption and consequently a relatively small fraction of VFA is available for peripheral tissues including the mammary gland. New data on splanchnic metabolism of VFA indicate that the ruminal epithelium metabolizes none or small amounts of acetate and propionate absorbed from the rumen. However, the ruminal epithelium has a large fractional uptake of butyrate and valerate during their absorption from the rumen. The liver takes up proportionately 0·9 or more of the absorbed propionate, however multiple factors are involved in regulation of hepatic metabolism and propionate does not determine glucose availability to the cowper se. In light of the quantitative importance of VFA to the dairy cow it is important that future research attempts to bridge the gap between the biology of food degradation/digestion in the gastro-intestinal tract and availability of specific nutrients to the cow which impact intermediary metabolism and nutrient utilizationin productive tissues.

Expression and localization of monocarboxylate transporters and sodium/proton exchangers in bovine rumen epithelium

Monocarboxylate-H+ cotransporters, such as monocarboxylate transporter (MCT) SLC16A, have been suggested to mediate transruminal fluxes of short-chain fatty acids, ketone bodies, and lactate. Using an RT-PCR approach, we demonstrate expression of MCT1 (SLC16A1) and MCT2 (SLC16A7) mRNA in isolated bovine rumen epithelium. cDNA sequence from these PCR products combined with overlapping expressed sequence tag data allowed compilation of the complete open reading frames for MCT1 and MCT2. Immunohistochemical localization of MCT1 shows plasma membrane staining in cells of the stratum basale, with intense staining of the basal aspects of the cells. Immunostaining decreased in the cell layers toward the rumen lumen, with weak staining in the stratum spinsoum. Immunostaining in the stratum granulosum and stratum corneum was essentially negative. Since monocarboxylate transport will load the cytosol with acid, expression and location of Na+/H+ exchanger (NHE) family members within the rumen epithelium were determined. RT-PCR demonstrates expression of multiple NHE family members, including NHE1, NHE2, NHE3, and NHE8. In contrast to MCT1, immunostaining showed that NHE1 was predominantly localized to the stratum granulosum, with a progressive decrease toward the stratum basale. NHE2 immunostaining was observed mainly at an intracellular location in the stratum basale, stratum spinosum, and stratum granulosum. Given the anatomic localization of MCT1, NHE1, and NHE2, the mechanism of transruminal short-chain fatty acid, ketone body, and lactate transfer is discussed in relation to a functional model of the rumen epithelium comprising an apical permeability barrier at the stratum granulosum, with a cell syncitium linking the stratum granulosum to the blood-facing stratum basale.

Integration of Ruminal Metabolism in Dairy Cattle

An important objective is to identify nutrients or dietary factors that are most critical for advancing our knowledge of, and improving our ability to predict, milk protein production. The Dairy NRC (2001) model is sensitive to prediction of microbial protein synthesis, which is among the most important component of models integrating requirement and corresponding supply of metabolizable protein or amino acids. There are a variety of important considerations when assessing appropriate use of microbial marker methodology. Statistical formulas and examples are included to document and explain limitations in using a calibration equation from a source publication to predict duodenal flow of purine bases from measured urinary purine derivatives in a future study, and an improved approach was derived. Sources of specific carbohydrate rumen-degraded protein components probably explain microbial interactions and differences among studies. Changes in microbial populations might explain the variation in ruminal outflow of biohydrogenation intermediates that modify milk fat secretion. Finally, microbial protein synthesis can be better integrated with the production of volatile fatty acids, which do not necessarily reflect volatile fatty acid molar proportions in the rumen. The gut and splanchnic tissues metabolize varying amounts of volatile fatty acids, and propionate has important hormonal responses influencing milk protein percentage. Integration of ruminal metabolism with that in the mammary and peripheral tissues can be improved to increase the efficiency of conversion of dietary nutrients into milk components for more efficient milk production with decreased environmental impact.

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

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.

Functional organization of the bovine rumen epithelium

The functional organization of the bovine rumen epithelium has been examined by electron and light microscopy combined with immunocytochemistry to define a transport model for this epithelium. Expression of connexin 43, an integral component of gap junctions, the tight-junction molecules claudin-1 and zonula occludens 1 (ZO-1), and the catalytic α-subunit of Na+-K+-ATPase was demonstrated by SDS-PAGE and Western blotting. From the lumen surface, four cell layers can be distinguished: the stratum corneum, the stratum granulosum, the stratum spinosum, and the stratum basale. Both claudin-1 and ZO-1 immunostaining showed plasma membrane staining, which was present at the stratum granulosum with decreasing intensity through the stratum spinosum to the stratum basale. The stratum corneum was negative for claudin-1 immunostaining. Transmission electron microscopy confirmed that occluding tight junctions were present at the stratum granulosum. Plasma membrane connexin 43 immunostaining was most intense at the stratum granulosum and decreased in intensity through stratum spinosum and stratum basale. There was intense immunostaining of the stratum basale for Na+-K+-ATPase, with weak staining of the stratum spinosum. Both the stratum granulosum and the stratum corneum were essentially negative. Stratum basale cells also displayed a high mitochondrial density relative to more apical cell layers. We conclude that epithelial barrier function may be attributed to the stratum granulosum and that cell-cell gap junctions allow diffusion to interconnect the barrier cell layer with the stratum basale where Na+-K+-ATPase is concentrated.

Anion-dependent Mg2+ influx and a role for a vacuolar H+-ATPase in sheep ruminal epithelial cells

The K+-insensitive component of Mg2+ influx in primary culture of ruminal epithelial cells (REC) was examined by means of fluorescence techniques. The effects of extracellular anions, ruminal fermentation products, and transport inhibitors on the intracellular free Mg2+ concentration ([Mg2+]i), Mg2+ uptake, and intracellular pH were determined. Under control conditions (HEPES-buffered high-NaCl medium), the [Mg2+]i of REC increased from 0.56 +/- 0.14 to 0.76 +/- 0.06 mM, corresponding to a Mg2+ uptake rate of 15 microM/min. Exposure to butyrate did not affect Mg2+ uptake, but it was stimulated (by 84 +/- 19%) in the presence of CO2/HCO(-)3. In contrast, Mg2+ uptake was strongly diminished if REC were suspended in HCO(-)3-buffered high-KCl medium (22.3 +/- 4 microM/min) rather than in HEPES-buffered KCl medium (37.5 +/- 6 microM/min). After switching from high- to low-Cl- solution, [Mg2+]i was reduced from 0.64 +/- 0.09 to 0.32 +/- 0.16 mM and the CO2/HCO(-)3-stimulated Mg2+ uptake was completely inhibited. Bumetanide and furosemide blocked the rate of Mg2+ uptake by 64 and 40%, respectively. Specific blockers of vacuolar H+-ATPase reduced the [Mg2+]i (36%) and Mg2+ influx (38%) into REC. We interpret this data to mean that the K+-insensitive Mg2+ influx into REC is mediated by a cotransport of Mg2+ and Cl- and is energized by an H+-ATPase. The stimulation of Mg2+ transport by ruminal fermentation products may result from a modulation of the H+-ATPase activity.

Transfer of energy substrates across the ruminal epithelium: implications and limitations

The ruminal epithelium has an enormous capacity for the absorption of short-chain fatty acids (SCFAs). This not only delivers metabolic energy to the animal but is also an essential regulatory mechanism that stabilizes the intraruminal milieu. The epithelium itself, however, is endangered by the influx of SCFAs because the intracellular pH (pHi) may drop to a lethal level. To prevent severe cytosolic acidosis, the ruminal epithelium is able to extrude (or buffer) protons by various mechanisms: (i) a Na+/H+ exchanger, (ii) a bicarbonate importing system and (iii) an H+/monocarboxylate cotransporter (MCT). Besides pHi regulation, the MCT also provides the animal with ketone bodies derived from the intraepithelial breakdown of SCFAs. Ketone bodies, in turn, can serve as an energy source for extrahepatic tissues. In addition to SCFA uptake, glucose absorption has recently been identified as a potential way of eliminating acidogenic substrates from the rumen. At least with respect to SCFAs, absorption rates can be elevated when adapting animals to energy-rich diets. Although they are very effective under physiological conditions, the absorptive and regulatory mechanisms of the ruminal epithelium also have their limits. An increased number of protons during the state of ruminal acidosis can be eliminated neither from the lumen nor the cytosol, thus worsening dysfermentation and finally leading to functional and morphological alterations of the epithelial lining.

Portal recovery of short-chain fatty acids infused into the temporarily-isolated and washed reticulo-rumen of sheep

The present study was undertaken to study the metabolism of short-chain fatty acids (SCFA) by the reticulo-ruminal epithelium and the portal-drained viscera (PDV) under in vivo conditions with no interference from the metabolism of the rumen microbes. The technique of temporary isolation of the reticulo-rumen was applied to wethers implanted with catheters in a mesenteric artery, the hepatic portal vein and the right ruminal vein. Portal blood flow was measured by downstream dilution of p-aminohippuric acid; the PDV uptake of arterial acetate, as well as the whole-body irreversible loss rate (ILR) of acetate, was estimated by [2-(13)C]acetate infusion into the right ruminal vein. The sheep were maintained with a bicarbonate-buffered solution of SCFA in the reticulo-rumen along with continuous intraruminal infusion of SCFA for 4 h. The portal appearance of SCFA of non-reticulo-ruminal origin was estimated before and after the infusion protocol. Of the acetate absorbed by the sheep, 89 (SE 5), 109 (SE 7) and 101 (SE 7)% was recovered as portal net appearance of acetate, portal net appearance of acetate corrected for PDV uptake of arterial acetate and increase in the ILR of acetate respectively. Of the propionate, isobutyrate, butyrate, isovalerate and valerate absorbed by the sheep, 95 (SE 7), 102 (SE 9), 23 (SE 3), 48 (SE 5) and 32 (SE 4)% respectively was recovered as portal net appearance. In contrast to current concepts, the present study showed that the reticulo-ruminal epithelium metabolizes none (or only a small proportion) of the acetate and propionate absorbed from the rumen. This observation could lead to the more efficient use of results obtained with multi-catheterized animals to quantify the net metabolite output of the rumen microbes.

Transport of butyrate across the isolated bovine rumen epithelium--interaction with sodium, chloride and bicarbonate

The Ussing chamber technique was used for studying unidirectional fluxes of 14C-butyrate across the bovine rumen epithelium in vitro. Significant amounts of butyrate were absorbed across the bovine rumen epithelium in vitro, without any external driving force. The paracellular pathway was quantitatively insignificant. The transcellular pathway was predominately voltage-insensitive. The serosal to mucosal (SM) pathway was regulated by mass action, whereas the mucosal to serosal (MS) pathway further includes a saturable process, which accounted for 30 to 55% of the MS flux. The studied transport process for 14C-butyrate across the epithelium could include metabolic processes and transport of 14C-labelled butyrate metabolites. The transport of butyrate interacted with Na+, Cl- and HCO3-, and there was a linear relationship between butyrate and sodium net transport. Lowering the sodium concentration from 140 to 10 mmol l-1 decreased the butyrate MS flux significantly. Amiloride (1 mmol l-1) did, however, not reduce the butyrate flux significantly. Chloride concentration in itself did not seem to influence the transport of butyrate, but chloride-free conditions tended to increase the MS and SM flux of butyrate by a DIDS-sensitive pathway. DIDS (bilateral 0.5 mmol l-1) did further decrease the butyrate SM flux significantly at all chloride concentrations. Removing bicarbonate from the experimental solutions decreased the MS and increased the SM flux of butyrate significantly, and abolished net butyrate flux. There were no significant effects of the carbonic anhydrase inhibitor Acetazolamide (bilateral 1.0 mmol l-1). The results can be explained by a model where butyrate and butyrate metabolites are transported both by passive diffusion and by an electroneutral anion-exchange with bicarbonate. The model couples sodium and butyrate via CO2 from metabolism of butyrate, and intracellular pH.