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

Validation of an in vivo dual permeability marker technique to characterize regional gastrointestinal tract permeability in mid lactation Holstein cows during short-term feed restriction

This study evaluated the impact of short-term feed restriction in lactating dairy cows on regional permeability of the gastrointestinal tract (GIT), and the recovery of DMI, ruminal pH, and milk yield. In addition, sampling methods for a novel dual marker technique to characterize total GIT and post ruminal permeability were validated. Six ruminally cannulated lactating Holstein cows were blocked by parity (3 primiparous, 3 multiparous; 189 DIM ± 25.2) and enrolled in a crossover design. Experimental periods included a 5-d baseline phase (BASE), 5-d challenge phase (CHAL), and 2 weeks of recovery (REC1 and REC2). During CHAL cows received either 100% ad libitum feed intake (AL) or 40% of ad libitum feed intake (FR). To assess, total-tract and post-ruminal permeability, equimolar doses of Cr-EDTA and Co-EDTA were infused on d 3 of CHAL into the rumen and abomasum (0.369 mmol/kg BW). Following infusions, total urine and feces were collected every 8 h over 96 h, and blood samples were collected at h 0, 1, 2, 3, 4, 6, 8, 12, 16, 20, 24, 32, 40, 48, and 64. The plasma area under the curve (AUC) for Cr and Co were calculated. By design, DMI for FR was reduced by 60% during CHAL and remained 19% lower than AL during REC1 but was not different from AL in REC2. Mean ruminal pH for FR was greatest during CHAL and the least during REC1, with no differences detected between AL and FR in REC2. The duration that pH was < 5.8 was the least for FR during CHAL and greatest during REC1 which were different from AL and were no longer different between treatments in REC2. Milk yield was the least for FR during CHAL and REC1 and no longer different from AL in REC2. Feed restriction reduced milk fat, protein, and lactose yields by 26, 31% and 31%, respectively. Plasma Cr AUC was 34% greater and Co AUC tended to be 35% greater for FR than AL on d 3 of CHAL. Urinary Cr recovery after 48-h was not affected by treatment; however, urinary Co recovery was 36% greater for FR than AL. Positive correlations between plasma AUC and urinary recovery for Cr and Co were detected. It was determined that blood samples collected at h 2, 8, 20, 40, and 48 could predict the total plasma Cr and Co AUC within 1.9% and 6.2%, respectively. In summary, short-term FR in lactating dairy cows increases permeability of the total GIT and may increase permeability of the post-ruminal regions with more than 60% of the permeability occurring post-ruminally. After FR, cows experienced low ruminal pH and a sustained reduction in milk yield. When utilizing Cr- and Co-EDTA to evaluate regional GIT permeability, plasma AUC can be used as an alternate to urinary Cr and Co excretion. In addition, blood samples collected at h 2, 8, 20, 40, and 48 result in adequate prediction accuracy, at least when comparing GIT permeability for lactating dairy cows exposed to AL and FR.

Effects of offering free-choice hay for the first 5 days postpartum on productivity, serum inflammatory markers, gut permeability, and colon gene expression in fresh dairy cows

The objective of the present study was to evaluate the effects of offering free-choice hay to cows during the first 5 d immediately after calving on feed intake, milk yield, plasma metabolites, serum inflammatory markers, rumination, gut permeability, and colon gene expression. It was hypothesized that cows offered free-choice hay would have lower gut permeability, lower inflammation, and higher milk production, compared with cows not offered hay. Thirty-two multiparous cows were fed a closeup total mixed ration (TMR; 21.5% starch, 32.1% forage neutral detergent fiber [NDF] on a dry matter basis) until calving. In the postpartum period, all cows were fed a fresh cow TMR (26.8% starch and 23.4% forage NDF) from calving until 21 DIM, and were assigned randomly to receive 1 of 2 treatments as follows: (1) free-choice timothy hay (61.6% NDF; 9.6% crude protein), offered outside of the TMR in a separate manger, for the first 5 d postpartum (FCH; n = 20), or 2) no free-choice hay (NH; n = 12). The FCH cows tended to have lower serum haptoglobin concentration on d 3, compared with NH (0.95 vs. 1.52 mg/mL). Within the FCH group, cows with greater hay intake had a smaller increase in serum amyloid A from d 1 to 3 after calving (r = 0.37), and tended to have a smaller increase in serum haptoglobin concentration (r = 0.36). Cows in the FCH group had a lower ratio of starch intake (kg) to forage NDF intake (kg) on d 1 and 2, compared with NH (0.91 vs. 1.14 ± 0.03), and cows that had a lower starch:forage NDF ratio tended to have a smaller increase in serum haptoglobin concentration from d 1 to 3 after calving (r = 0.32). Cows in the FCH group had lower TMR dry matter intake (DMI; 15.0 vs. 17.1 ± 0.93 kg/d) and lower total DMI (TMR + hay DMI; 15.9 vs. 17.1 ± 0.87 kg/d), from d 1 to 5 when free-choice hay was offered, compared with NH. However, the hay treatment did not affect plasma energy metabolite concentration, gut permeability, colon gene expression, milk yield, rumination time, or change in body weight or body condition score. Overall, these findings suggest that offering free-choice hay for the first 5 d after calving may reduce serum inflammatory marker concentration, but milk yield may not increase, due to lower intake.

Effect of sodium concentration and mucosal pH on apical uptake of acetate and butyrate, and barrier function of the isolated bovine ruminal epithelium

This study was conducted to investigate the role of Na+ on ruminal short-chain fatty acid (SCFA) absorption and barrier function when isolated ruminal epithelium was exposed to high and low pH ex vivo. Nine Holstein steer calves (322 ± 50.9 kg of body weight) consuming 7.05 ± 1.5 kg dry matter of a total mixed ration were euthanized and ruminal tissue was collected from the caudal-dorsal blind sac. Tissues were mounted between 2 halves of Ussing chambers (3.14 cm2) and exposed to buffers that contained low (10 mM) or high (140 mM) Na+ with low (6.2) or high (7.4) mucosal pH. The same buffer solutions were used on the serosal side except that pH was maintained at 7.4. Buffers used to evaluate SCFA uptake contained bicarbonate to determine total uptake or excluded bicarbonate and included nitrate to determine noninhibitable uptake. Bicarbonate-dependent uptake was calculated as the difference between the total and noninhibitable uptake. Acetate (25 mM) and butyrate (25 mM) were spiked with 2-3H-acetate and 1-14C-butyrate, respectively, and were then added to the mucosal side, incubated for 1 min, and tissues were analyzed to evaluate rates of SCFA uptake. Tissue conductance (Gt) and the mucosal-to-serosal flux of 1-3H-mannitol were used to assess barrier function. There were no Na+ × pH interactions for butyrate or acetate uptake. Decreasing mucosal pH from 7.4 to 6.2 increased total acetate and butyrate uptake, and bicarbonate-dependent acetate uptake. Flux of 1-3H-mannitol was not affected by treatment. However, high Na+ concentration reduced Gt and prevented an increase in Gt from flux period 1 to flux period 2. The results of this study indicate that although providing more Na+ to the ruminal epithelium does not affect SCFA uptake or mannitol flux, it may help stabilize tissue integrity.

Effect of a molasses-based liquid supplement on gastrointestinal tract barrier function, inflammation, and performance of newly received feedlot cattle before and after a transport stress

The objective of this study was to determine the effect of a dry versus a molasses-based liquid supplement on ruminal butyrate concentration, gastrointestinal tract (GIT) barrier function, inflammatory status, and performance of newly received feedlot cattle. In experiment 1, 60 mixed breed steers (234 ± 2.1 kg) were weaned, held overnight at a sale barn, then transported 14 h to Purdue University. After arrival, steers were weighed, blocked by body weight, and allotted within block to treatments (six pens per treatment and five steers per pen). Diets consisted of 45% roughage and 55% concentrate (dry matter basis). Treatments differed in the supplement source as follows: DRY: 10% dry supplement or LIQUID: 10% liquid molasses-based supplement. Feed intake, average daily gain (ADG), and gain:feed were determined for the three 21-d periods and overall. In experiment 2, 16 crossbred heifers (246 ± 7.5 kg) were used (8 heifers per treatment). Diets were the same as in experiment 1 and were fed for 60 d. On d 56 ruminal fluid samples were collected at 0, 3, 6, and 9 h after feeding. To mimic a stress event, heifers were transported for 4 h on d 61, rested overnight, and transported 12 h on d 62. Blood was collected from heifers immediately prior to transport and immediately upon their return. Gut barrier function using a Cr-EDTA marker was determined after transportation. Data were analyzed using the GLIMMIX procedure of SAS. Steers fed the liquid supplement had greater (P ≤ 0.03) ADG through d 42 and overall compared to steers fed the dry supplement. Feed intake did not differ (P = 0.25) between treatments from d 0 to d 21. However, steers fed the liquid supplement showed greater (P < 0.001) dry matter intake after d 21 and overall compared to those fed the dry supplement. Steers fed the liquid supplement tended (P < 0.09) to have reduced serum haptoglobin and lipopolysaccharide-binding protein (LBP) compared to those fed the dry supplement. Heifers fed the liquid supplement had greater (P = 0.02) Cr in urine and tended (P = 0.07) to have lower serum LBP after transport compared to those fed the dry supplement. Heifers fed the liquid supplement had 72% lower serum haptoglobin before, but only a 19% lower serum haptoglobin after transport compared to animals fed the dry supplement (treatment × time; P = 0.07). Therefore, the liquid supplement altered GIT barrier function, and improved inflammatory status, resulting in increased growth of receiving cattle.

Subacute ruminal acidosis in dairy herds: Microbiological and nutritional causes, consequences, and prevention strategies

Dairy cattle are frequently fed high-concentrate (HC) diets in modern intensive feeding systems, especially in the transition period. During this period, cows face many alterations that include hormonal changes and shifting to a lactating state. Switching to a HC diet that may disrupt the ruminal microbiota balance can lead to subacute ruminal acidosis (SARA). Moreover, the main factor shaping the rumen microbiota is dietary composition, especially the ratio of starch to fibrous carbohydrates. Feeding highly fermentable carbohydrate diets after adaptation to forage diets leads to a rumen fermentation rate that exceeds rumen absorption and buffering rates, resulting in a reduction in ruminal pH. As a result of Gram-negative bacterial cell lysis, an increase in harmful ruminal bacterial metabolites, including lipopolysaccharide, lactic acid, and histamine, is observed. The interactions between the host immune system and the ruminal microbiota play an essential role in many physiological processes and the development of the disorder. Progress in DNA sequencing and bioinformatics platforms provides new opportunities to investigate the composition of ruminal microbes and yields unique advances in understanding ecology of the rumen. Subacute ruminal acidosis is linked with a change in the ruminal microbiota structure and richness and with other metabolic disorders; such as rumenitis, milk fat depression, laminitis, and liver abscesses. Therefore, this review aims to explore a better understanding of the crosstalk between diet and microbiota in the prevalence of rumen acidosis and its consequences, which is crucial for control strategies such as feeding management, and supplementation with thiamine, prebiotics, and probiotics.

Effect of aspirin to intentionally induce leaky gut on performance, inflammation, and carcass characteristics of feedlot cattle

The negative impacts of stress on gastrointestinal tract (GIT) barrier function can result in compromised animal growth and health. Aspirin is known to cause mucosal injury leading to increased gut permeability and tight junction damage and can be used as a model to study leaky gut in cattle. The objective of this study was to determine the long-term impact of aspirin-induced chronic leaky gut on cattle growth and carcass attributes. Two treatments were evaluated in two studies: control (no aspirin) or 0.25% of the diet dry matter (DM) aspirin fed daily. Diets consisted of 50% corn, 24% dried distillers grains, 20% corn silage, and 6% supplement on a DM basis. In experiment 1, sixteen Angus × Simmental heifers, allotted by body weight (BW) and breed composition, were fed diets for 154 d. On day 155, heifers were dosed with 1 liter of a 180-mM Cr-ethylenediaminetetraacetic acid solution using an esophageal tube and had urine collected every 1.5 to 3 h for 48 h for analysis of Cr as a measure of gut leakiness. In experiment 2, ninety-six Simmental × Angus steers (355.0 ± 14.8 kg) were allotted by BW and breed composition and fed treatment diets for 159 d. Weight was recorded monthly and serum was collected on day 159 and analyzed for lipopolysaccharide-binding protein (LBP), interleukin-6 (IL-6), serum amyloid A (SAA), haptoglobin, and aspartate aminotransferase (AST). Data were analyzed using the MIXED procedure of SAS. Heifers fed 0.25% aspirin in experiment 1 excreted more Cr into urine compared with heifers not fed aspirin (overall treatment effect, P = 0.01). In experiment 2, aspirin tended to increase serum LBP (P = 0.06) but had no effect on concentrations of IL-6, haptoglobin, SAA, or AST (P ≥ 0.25). Aspirin tended to decrease average daily gain (P = 0.10), decreased hot carcass weight and rib-eye area (P ≤ 0.05), and increased fat thickness, marbling score, and yield grade (P ≤ 0.02). Aspirin tended to increase kidney, pelvic, and heart fat percentage (P = 0.10) and had no effect on liver abscesses (P ≥ 0.80). This study indicates that leaky gut induced by long-term administration of aspirin has negative impacts on feedlot performance and carcass composition. The negative impact of aspirin-induced leaky gut on animal performance suggests that chronic leaky gut caused by other factors (subacute acidosis, stress) may be a significant problem for the feedlot industry.

Potential protective effects of thiamine supplementation on the ruminal epithelium damage during subacute ruminal acidosis

In ruminants, the ruminal epithelium not only has the function of absorbing nutrients but also is an important tissue to prevent harmful substances in the rumen from entering the blood circulation. Thus, the normal function of ruminal epithelium is critical for ruminants. However, subacute ruminal acidosis induced by high-concentrate diets often damages the barrier function of ruminal epithelium in ruminants. Recently, many studies have shown that dietary supplementation with thiamine is an effective method to alleviate subacute ruminal acidosis. In order to provide theoretical reference for the in-depth study of subacute ruminal acidosis and the application of thiamine in the future, this review introduces the effects of subacute ruminal acidosis on morphological structure, inflammatory response, and tight junction of ruminal epithelium. In addition, this paper summarizes the role of thiamine in maintaining ruminal epithelial function of ruminants during subacute ruminal acidosis challenge.

Use of aspirin to intentionally induce gastrointestinal tract barrier dysfunction in feedlot cattle

Stress negatively affects the gastrointestinal tract (GIT) barrier function, resulting in compromised animal health. A deeper understanding of how diet and stress impacts the GIT barrier function in feedlot cattle is needed. Aspirin decreases mucus production and mucosal repair in the GIT and could be used as a model for GIT barrier dysfunction research. The objective of this study was to evaluate the effectiveness of aspirin to induce GIT barrier dysfunction in beef cattle. In experiment 1, sixteen crossbred heifers (425.0 ± 8.6 kg) were allotted to 0, 50, 100, or 200 mg/kg body weight (BW) aspirin doses based on BW. Experiment 1 consisted of two periods separated by 4 wk where four heifers per treatment received the same aspirin dose during each period. Heifers were fed a 49.4% corn silage and 50.6% concentrate diet. The 200 mg/kg BW aspirin treatment was dosed as a 100 mg/kg BW aspirin oral bolus 36 and 24 h prior to Cr-ethylenediaminetetraacetic acid (EDTA) dosing (1 liter; 180 mM). The 50 and 100 mg/kg BW aspirin treatments were dosed as an oral bolus 24 h prior to Cr-EDTA dosing. Urine was collected every 3 h for 48 h and analyzed for Cr. Serum was collected at 0 and 48 h and analyzed for lipopolysaccharide-binding protein (LBP), interleukin-6, serum amyloid A (SAA), haptoglobin, and aspartate aminotransferase. In experiment 2, sixteen crossbred steers (576.0 ± 14.2 kg) fed a similar diet were allotted by BW to the 0 and 200 mg/kg BW aspirin treatments (eight steers/treatment) and were slaughtered 24 h after the last dose. Jejunal tissues were collected, and claudin (CLDN) 1, 2, and 3, occludin, and zonula occludens tight junction messenger ribonucleic acid (mRNA) expression was determined. Data were analyzed using the MIXED procedure of SAS. Urinary Cr excretion increased linearly at hours 3, 6, 9, and 12 (P ≤ 0.04) as aspirin dose increased from 0 to 200 mg/kg. Aspirin linearly increased Cr absorption (P = 0.02) and elimination (P = 0.04) rates and linearly decreased mean retention time of Cr (P = 0.02). Aspirin increased SAA (P = 0.04) and tended to increase LBP (P = 0.09) in serum but did not affect any other serum inflammatory marker (P ≥ 0.19). Aspirin tended to increase jejunal CLDN-1 mRNA expression (P = 0.10) but did not affect the mRNA expression of other genes regulating tight junction function (P ≥ 0.20). Results from this study indicate that aspirin disrupts the GIT barrier function in beef cattle and has a potential as a model in GIT permeability research.

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.

The Regulation of Ruminal Short-Chain Fatty Acids on the Functions of Rumen Barriers

The rumen barriers, constituted by the microbial, physical and immune barrier, prevent the transmission of pathogens and toxins to the host tissue in the maintenance of host-microbe homeostasis. Ruminal short-chain fatty acids (SCFAs), which are the important signaling molecules derived from the rumen microbiota, regulate a variety of physiological functions of the rumen. So far, how the ruminal SCFAs regulate the function of rumen barriers is unclear. By the combined methods of transcriptome sequencing, 16S rRNA gene sequencing, and metagenome shotgun sequencing, we have investigated the regulatory effects of ruminal SCFAs on the functions of rumen barriers, by determining the composition and functions of epimural microbiota and on the structure and immunity of the rumen epithelium in goats receiving a 10% (LC group), 35% (MC group), or 65% concentrate diet (HC group). We found that, when the dietary concentrate shifted from 10 to 35%, the increase of total SCFA is associated with the diversification of epimural microbiota and the diversity of its gene pool. Within the microbial community, the relative abundance of genera Sphingobium, Acinetobacter, and Streptococcus increase mostly. Meantime, the signals on pathways concerning the mechanical connections and growth homeostasis in the rumen epithelium were upregulated. Under these conditions, the responses of immune components in the rumen epithelium decrease. However, when the dietary concentrate shifted from 35 to 65%, the increase of acetate and reduction of pH decrease the diversity of epimural microbiota and the diversity of its gene pool. Within the microbial community, the relative abundance of genera Sphingobium, Acinetobacter, and Streptococcus significantly decrease. Concomitantly, the signals on pathways concerning the cell growth and tight junction disruption were upregulated, while the signals on pathways concerning paracellular permeability were downregulated. Under these conditions, the signals on the pathways relating to the immune components increase. Our data thus indicates that diet-SCFA axis maintains the host-microbe homeostasis via promoting the diversification of epimural microbiota and maintaining the integrity of rumen epithelium in healthy animals, while via enhancing the activities of immune barrier in animal with lower rumen pH.

Acute exposure to ergot alkaloids from endophyte-infected tall fescue does not alter absorptive or barrier function of the isolated bovine ruminal epithelium

Ergot alkaloids in endophyte-infected (Neotyphodium coenophialum) tall fescue (Lolium arundinaceum) have been shown to cause a reduction in blood flow to the rumen epithelium as well as a decrease in volatile fatty acids (VFA) absorption from the washed rumen of steers. Previous data also indicates that incubating an extract of endophyte-infected tall fescue seed causes an increase in the amount of VFA absorbed per unit of blood flow, which could result from an alteration in the absorptive or barrier function of the rumen epithelium. An experiment was conducted to determine the acute effects of an endophyte-infected tall fescue seed extract (EXT) on total, passive or facilitated acetate and butyrate flux across the isolated bovine rumen as well as the barrier function measured by inulin flux and tissue conductance (G t ). Flux of ergovaline across the rumen epithelium was also evaluated. Rumen tissue from the caudal dorsal sac of Holstein steers (n=6), fed a common diet, was collected and isolated shortly after slaughter and mounted between two halves of Ussing chambers. In vitro treatments included vehicle control (80% methanol, 0.5% of total volume), Low EXT (50 ng ergovaline/ml) and High EXT (250 ng ergovaline/ml). Results indicate that there is no effect of acute exposure to ergot alkaloids on total, passive or facilitated flux of acetate or butyrate across the isolate bovine rumen epithelium (P>0.51). Inulin flux (P=0.16) and G t (P>0.17) were not affected by EXT treatment, indicating no alteration in barrier function due to acute ergot alkaloid exposure. Ergovaline was detected in the serosal buffer of the High EXT treatment indicating that the flux rate is ~0.25 to 0.44 ng/cm2 per hour. Data indicate that specific pathways for VFA absorption and barrier function of the rumen epithelium are not affected by acute exposure to ergot alkaloids from tall fescue at the concentrations tested. Ergovaline has the potential to be absorbed from the rumen of cattle that could contribute to reduced blood flow and motility and lead to reduced growth rates of cattle.

Acidic pH and short-chain fatty acids activate Na+ transport but differentially modulate expression of Na+/H+ exchanger isoforms 1, 2, and 3 in omasal epithelium

Low sodium content in feed and large amounts of salivary sodium secretion are essential requirements to efficient sodium reabsorption in the dairy cow. It is already known that Na(+)/H(+) exchange (NHE) of the ruminal epithelium plays a key role in Na(+) absorption, and its function is influenced by the presence of short-chain fatty acids (SCFA) and mucosal pH. By contrast, the functional role and regulation of NHE in omasal epithelium have not been completely understood. In the present study, we used model studies in small ruminants (sheep and goats) to investigate NHE-mediated Na(+) transport and the effects of pH and SCFA on NHE activity in omasal epithelium and on the expression of NHE isoform in omasal epithelial cells. Conventional Ussing chamber technique, primary cell culture, quantitative PCR, and Western blot were used. In native omasal epithelium of sheep, the Na(+) transport was electroneutral, and it was inhibited by the specific NHE3 inhibitor 3-[2-(3-guanidino-2-methyl-3-oxo-propenyl)-5-methyl-phenyl]-N-isopropylidene-2-methyl-acrylamide dihydrochloride, which decreased mucosal-to-serosal, serosal-to-mucosal, and net flux rates of Na(+) by 80% each. The application of low mucosal pH (6.4 or 5.8) in the presence of SCFA activated the Na(+) transport across omasal epithelium of sheep compared with that at pH 7.4. In cultured omasal epithelial cells of goats, mRNA and protein of NHE1, NHE2, and NHE3 were detected. The application of SCFA increased NHE1 mRNA and protein expression, which was most prominent when the culture medium pH decreased from 7.4 to 6.8. At variance, the mRNA and protein expression of NHE2 and NHE3 were decreased with low pH and SCFA, which was contrary to the published data from ruminal epithelial studies. In conclusion, this paper shows that (1) NHE1, NHE2, and NHE3 are expressed in omasal epithelium; (2) NHE3 mediates the major portion of transepithelial Na(+) transport in omasal epithelium; and (3) SCFA and acidic pH acutely activate Na(+) transport but suppress the expression of NHE2 and NHE3 in the longer term. By contrast, the expression of NHE1 is increased by SCFA and acidic pH, indicating a prominent role for NHE1 in the regulation of intracellular pH of omasal epithelium. Our results suggest a regulatable Na(+) absorption in ruminal and omasal epithelium. It is of benefit for intracellular pH homeostasis and highly relevant to dairy cows fed on high-concentrate diets.

The influence of age and weaning on permeability of the gastrointestinal tract in Holstein bull calves

Fourteen Holstein bull calves were used in a randomized complete block design to investigate the effect of calf age and weaning on permeability of the gastrointestinal tract (GIT). Calves were randomly assigned to 1 of 2 treatments: (1) a weaning protocol that was initiated on d 35; WN; n=7), or (2) a control treatment where calves were not weaned (CON; n=7). Calves were bottle-fed milk replacer (150 g/L), in 3 equal portions/d targeting 15% of their body weight (BW) in liquid milk intake [approximately 21.1g/kg of BW/d, dry matter (DM) basis]. On d 35, the amount of milk replacer offered to WN calves was reduced to 7.5% of BW for 7 d before calves were weaned on d 42. On d 14, 28, and 42, calves were orally dosed with 500 mL of Cr-EDTA (179 mM Cr-EDTA solution) and housed in a metabolism crate to enable total urine collection and determination of total urinary Cr recovery as an indicator of total-tract permeability. On d 44, calves were killed and tissues from the rumen, omasum, duodenum, jejunum, ileum, cecum, and proximal and distal colon were collected, rinsed, and transported in buffer solution (pH 7.4 at 38.5°C). Tissues were incubated in Ussing chambers under short-circuit conditions with buffer solutions designed to mimic the mucosal and serosal energy source that would be available in vivo (glucose for tissues from the small intestine and short-chain fatty acids for tissues that would be exposed to fermentation; rumen, omasum, and large intestinal tissues). The serosal to mucosal flux of (14)C-mannitol and (3)H-inulin was measured for each region. Although we detected treatment × period interactions for BW and starter intake, dietary treatments did not differ within a week. Overall, the time that ruminal pH was <5.5 was less before weaning than after weaning. We observed a differential response for the appearance of Cr in urine for WN and CON calves, where the appearance of Cr (mg/48 h) in urine decreased for both treatments from d 14 to 28, but increased from d 28 to 42 for WN, whereas Cr appearance continued to decrease for CON. The flux of mannitol and inulin did not differ between treatments but did differ among region of the GIT, with rumen, duodenum, and jejunum having the greatest permeability. These data suggest that permeability of the GIT decreases with age but weaning may disrupt this process. The rumen, duodenum, and jejunum appear to be the regions with greatest permeability.

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.

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

Alterations in rumen epithelial tight junctions (TJs) at the tissue and molecular levels during high-grain (HG) diet feeding are unknown. Here, 10 male goats were randomly assigned to either a hay diet (0% grain; n = 5) or HG diet group (65% grain; n = 5) to characterize the changes in ruminal epithelial structure and TJ protein expression and localization using scanning and transmission electron microscopy, quantitative real-time PCR, Western blot analysis, and immunofluorescence. After 7 wk of feeding, ruminal free LPS in HG group increased significantly (P < 0.001) compared with the hay group, and free LPS in the peripheral blood was detectable with concentrations of 0.8 ± 0.20 EU/ml, while not detectable in the control, suggesting a leakage of LPS into the blood in the HG group. Correspondingly, the HG-fed goats showed profound alterations in ruminal epithelial structure and TJ proteins, depicted by marked epithelial cellular damage and intercellular junction erosion, down-regulation of TJ proteins claudin-4, occludin, and zonula occludens-1 mRNA and protein expression, as well as redistribution of claudin-1, claudin-4, and occludin. Furthermore, these changes in TJ proteins in the HG group were coupled with the upregulation of mRNA levels for the cytokines TNF-α and IFN-γ in the ruminal epithelia. These results demonstrated for the first time that the HG diet feeding caused disruption of ruminal epithelial TJs that was associated with a local inflammatory response in the rumen epithelium. These findings may provide new insights into understanding the role of TJ proteins in the ruminal epithelial immune homeostasis of ruminants.

Supplemental butyrate does not enhance the absorptive or barrier functions of the isolated ovine ruminal epithelia

Our objective was to determine if increasing the ruminal butyrate concentration would improve the selective permeability of ruminal epithelia. Suffolk wether lambs (n = 18) with an initial BW of 47.4 ±1.4 kg were housed in individual pens (1.5 × 1.5 m) with rubber mats on the floor. Lambs were blocked by initial BW into 6 blocks and, within block, were randomly assigned to either the control (CON) or 1 of 2 butyrate supplementation amounts (i.e., 1.25% or 2.50% butyrate as a proportion of DMI). With the exception of butyrate supplementation, all lambs were fed a common diet (90% concentrate and 10% barley silage). After a 14-d feeding period, lambs were killed, and ruminal epithelia from the ventral sac were mounted in Ussing chambers. To facilitate the Ussing chamber measurements, only 1 lamb was killed on an individual day. Thus, the starting date was staggered so that all lambs were exposed to the same experimental protocol. In Ussing chambers, epithelia were incubated using separate mucosal (pH 6.2) and serosal (pH 7.4) bathing solutions. Then 1-14C-butyrate (74 kBq/10 mL) was added to the mucosal side and was used to measure the mucosal-to-serosal flux (J(ms-butyrate)) in 2 consecutive 60-min flux periods with simultaneous measurement of transepithelial conductance (G(t)). During the first (challenge) flux period, the mucosal buffer solution was either acidified to pH 5.2 (ACID) or used as a control (pH 6.2; SHAM). Buffer solutions bathing the epithelia were replaced before the second flux period (recovery). Total ruminal short-chain fatty acid and butyrate concentrations were greater (P = 0.001) in lambs fed 2.50% compared with those fed 0% or 1.25% butyrate. The J(ms-butyrate) was less for lambs fed 1.25% and 2.50% butyrate [3.00 and 3.12 μmol/(cm2·h), respectively] than for CON [3.91 μmol/(cm2· h)]. However, no difference (P = 0.13)was observed for G(t). An ex vivo treatment × flux period interaction was detected (P = 0.003) for J(ms-butyrate), where no differences were present between ACID and SHAM during the challenge period, but the Jms-butyrate was less for ACID than for SHAM during recovery. These results indicate that large increases in the ruminal butyrate concentration decrease the selective permeability of the isolated ruminal epithelia.

Evidence for NHE3-mediated Na transport in sheep and bovine forestomach

Na absorption across the cornified, multilayered, and squamous rumen epithelium is mediated by electrogenic amiloride-insensitive transport and by electroneutral Na transport. High concentrations of amiloride (>100 μM) inhibit Na transport, indicating Na+/H+ exchange (NHE) activity. The underlying NHE isoform for transepithelial Na absorption was characterized by mucosal application of the specific inhibitor HOE642 for NHE1 and S3226 for NHE3 in Ussing chamber studies with isolated epithelia from bovine and sheep forestomach. S3226 (1 μM; NHE3 inhibitor) abolished electroneutral Na transport under control conditions and also the short-chain fatty acid-induced increase of Na transport via NHE. However, HOE642 (30 μM; NHE1 inhibitor) did not change Na transport rates. NHE3 was immunohistochemically localized in membranes of the upper layers toward the lumen. Expression of NHE1 and NHE3 has been previously demonstrated by RT-PCR, and earlier experiments with isolated rumen epithelial cells have shown the activity of both NHE1 and NHE3. Obviously, both isoforms are involved in the regulation of intracellular pH, pHi. However, transepithelial Na transport is only mediated by apical uptake via NHE3 in connection with extrusion of Na by the basolaterally located Na-K-ATPase. The missing involvement of NHE1 in transepithelial Na transport suggests that the proposed “job sharing” in epithelia between these two isoforms probably also applies to forestomach epithelia: NHE3 for transepithelial transport and NHE1 for, among others, pHi and volume regulation.

A single mild episode of subacute ruminal acidosis does not affect ruminal barrier function in the short term

Twenty-four German Merino sheep (72.3±10.1 kg of body weight) were fed an all-hay diet and assigned to either the subacute ruminal acidosis (SARA) treatment (n=17) or sham treatment (n=7). The SARA sheep were orally dosed with a 2.2 M glucose solution to supply 5 g of glucose/kg of body weight, whereas sham sheep received an equal volume of water. Ruminal pH was measured for 48 h before and 3 h after the oral dose. Sheep were then killed and ruminal epithelia from the ventral sac were mounted in Ussing chambers. The serosal-to-mucosal flux rate of partially (3)H-labeled mannitol (J(mannitol-SM)), an indicator of barrier function, was measured while epithelia were exposed to 3 sequential in vitro measurement periods lasting 1 h each. The measurement periods consisted of baseline, challenge, and recovery periods and were interspersed by 30-min periods for treatment equilibration. Baseline conditions were pH 6.1 (mucosal solution) and pH 7.4 (serosal solution) with a bilateral osmolarity of 293 mOsm/L. During the challenge period, the mucosal side of the epithelia was exposed to either an acidotic challenge (pH 5.2, osmolarity 293 mOsm/L) or an osmotic challenge (pH 6.1, osmolarity 450 mOsm/L); a third group served as control (pH 6.1, osmolarity 293 mOsm/L). The mucosal buffer solution was replaced for the recovery period. In vivo, sheep on the SARA treatment had lower mean (5.77 vs. 6.67) and nadir (5.48 vs. 6.47) ruminal pH for the 3h following the oral drench compared with sham sheep, indicating the successful induction of SARA with the oral glucose dose. Despite the marked reduction in pH in vivo, induction of SARA had no detectable effects on the baseline measurements of J(mannitol-SM), tissue conductance (G(t)), and short-circuit current (I(sc)) in vitro. However, reducing mucosal pH to 5.2 in vitro had negative effects on epithelial barrier function in the recovery period, including increased J(mannitol-SM), increased G(t), and decreased I(sc). The osmotic challenge increased J(mannitol-SM) and G(t) and decreased I(sc) during the challenge period, which was reversible in the recovery period except for slight reduction in I(sc). Interactions between the in vitro treatment and measurement period were detected for J(mannitol-SM), G(t), and I(sc). These data indicate that a mild episode of SARA (nadir pH, 5.48; duration ruminal pH <5.8, 111 min relative to the 180-min measurement period) does not affect ruminal epithelial barrier function immediately after the episode but that a rapid and more severe acidification (pH 5.2) in vitro increases epithelial permeability following the insult.

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.

Change of ruminal sodium transport in sheep during dietary adaptation

Rumen adaptation plays an important role in the productive cycle of dairy cattle. In this study, the time course of functional rumen epithelium adaptation after a change from hay feeding (ad libitum) to a mixed hay/concentrate diet was monitored by measuring Na+ transport rates in Ussing chamber experiments. A total of 18 sheep were subjected to different periods of mixed hay/concentrate feeding ranging from 0 weeks (control; hay ad libitum) to 12 weeks (800 g hay plus 800 g concentrate per day in two equal portions). For each animal, the net absorption of sodium was measured following the mixed hay/concentrate feeding period. Net Na transport, Jnet, significantly rose from 2.15 +/- 0.43 (control) to 3.73 +/- 1.02 microeq x cm(-2) x h(-1) after one week of mixed hay/ concentrate diet, reached peak levels of 4.55 +/- 0.50 microEq x cm(-2) x h(-1) after four weeks and levelled out at 3.92 +/- 0.36 microeq x cm(-2) x h(-1) after 12 weeks of mixed feeding. Thus, 73% of functional adaptation occurred during the first week after diet change. This is in apparent contrast to findings that morphological adaptation takes approximately six weeks to reach peak levels. Hence, early functional adaptation to a mixed hay/concentrate diet is characterised by enhanced Na absorption rates per epithelial cell. Absorption rates are likely to be further enhanced by proliferative effects on the rumen epithelium (number and size of papillae) when concentrate diets are fed over longer periods of time. Early functional adaptation without surface area enlargement of the rumen epithelium appears to be the first step in coping with altered fermentation rates following diet change.

Ste20-related proline/alanine-rich kinase (SPAK) regulated transcriptionally by hyperosmolarity is involved in intestinal barrier function

The Ste20-related protein proline/alanine-rich kinase (SPAK) plays important roles in cellular functions such as cell differentiation and regulation of chloride transport, but its roles in pathogenesis of intestinal inflammation remain largely unknown. Here we report significantly increased SPAK expression levels in hyperosmotic environments, such as mucosal biopsy samples from patients with Crohn's disease, as well as colon tissues of C57BL/6 mice and Caco2-BBE cells treated with hyperosmotic medium. NF-kappaB and Sp1-binding sites in the SPAK TATA-less promoter are essential for SPAK mRNA transcription. Hyperosmolarity increases the ability of NF-kappaB and Sp1 to bind to their binding sites. Knock-down of either NF-kappaB or Sp1 by siRNA reduces the hyperosmolarity-induced SPAK expression levels. Furthermore, expression of NF-kappaB, but not Sp1, was upregulated by hyperosmolarity in vivo and in vitro. Nuclear run-on assays showed that hyperosmolarity increases SPAK expression levels at the transcriptional level, without affecting SPAK mRNA stability. Knockdown of SPAK expression by siRNA or overexpression of SPAK in cells and transgenic mice shows that SPAK is involved in intestinal permeability in vitro and in vivo. Together, our data suggest that SPAK, the transcription of which is regulated by hyperosmolarity, plays an important role in epithelial barrier function.

Chloride, gluconate, sulfate, and short-chain fatty acids affect calcium flux rates across the sheep forestomach epithelium

In ruminants, more than 50% of overall gastrointestinal Ca absorption can occur preintestinally, and the anions of orally applied Ca salts are thought to play an important role in stimulating ruminal Ca absorption. This assumption is based mainly on ion-exchange studies that have used gluconate as the control anion, which may bind Ca2+ ions and interfere with treatment effects. In the present study, we investigated the distinct effects of different anions on Ca absorption across the sheep rumen and on the concentration of free Ca2+ ions ([Ca2+]ion). We showed that gluconate, sulfate, and short-chain fatty acids (SCFA) remarkably reduced [Ca2+]ion in buffer solutions. Nevertheless, increasing the Cl or SCFA concentration by 60 mM stimulated net ruminal Ca absorption 5- to 7-fold, but these effects could be antagonized by gluconate. Therefore, ion-exchange experiments must be (re)evaluated very carefully, because changes in [Ca2+]ion in the presence of gluconate, sulfate, or SCFA not only might entail an underestimation of Ca flux rates, but also might have effects on other cellular pathways that are Ca2+ dependent. Concerning the optimal Ca supply for dairy cows, the present study suggests that CaCl2 formulations and Ca salts of the SCFA stimulate Ca absorption across the rumen wall and are beneficial in preventing or correcting a Ca deficiency.

Impact of Bacillus thuringiensis toxin Cry1Ab on rumen epithelial cells (REC) - a new in vitro model for safety assessment of recombinant food compounds

The growing use of genetically modified crops necessitates viable screening methods for safety evaluation of recombinant feed, particularly for ruminants. A new sheep rumen epithelial cell culture is introduced as an in vitro cell system for safety evaluation especially focussing on feed and food compounds. We used lactate dehydrogenase (LDH) release, WST-1 conversion, ATP content and caspase 3/7 activity to evaluate cytotoxicity of Cry1Ab, one of the newly expressed Bt-proteins in transgene maize. The results were compared to the effects of valinomycin, a potassium ionophore known to induce cytotoxic effects on a wide range of cells. Whereas no toxicity of Cry1Ab was observed in short as well as in long term experiments, even at non-physiological high concentrations, exposure to valinomycin induced apoptosis and a significant response of all viability parameters after a number of hours. The ATP content and the WST-1 conversion reflecting the energy metabolism of the cells appear to be more sensitive indicators of valinomycin toxicity than the LDH release, a parameter which reflects the membrane integrity. This study presents an in vitro model system, that may be useful as a supplementary tool in toxicity screening before testing substances on animals in vivo.

Ammonia and urea transport across the rumen epithelium: a review

The transport of nitrogen across the rumen epithelium is characterized by absorption of ammonia from the rumen and by an influx of urea into the rumen. The transport rates of both compounds are large and exhibit wide variation. The transport of ammonia occurs in two forms: in the lipophilic form as NH3, the magnitude of which is linearly related to the pH in the ruminal fluid at pH values above 7, while at a physiological pH of 6.5 or lower, ammonia is predominantly absorbed as NH4+via putative potassium channels in the apical membrane. The uptake of NH4+depends on the potential difference of the apical membrane, Pda, and shows competition with K uptake. The pathway for basolateral exit of NH4+is unknown. Hence, the relative transport rates of NH3or NH4+are determined by the ruminal pH according to the Henderson–Hasselbalch equation. Transport of ammonia interacts with the transport of Na and Mg mainly via changes of the intracellular pH. Urea recycling into the rumen has been known for many years and the transport across the rumen epithelium is mediated via urea transporters in the luminal and basolateral membrane of the epithelium. Transport of urea occurs by simple diffusion, but is highly variable. A significant increase of urea influx is caused by the fermentation products CO2and short-chain fatty acids. Conversely, there is some evidence of inhibition of urea influx by ruminal ammonia. The underlying mechanisms of this modulation of urea transport are unknown, but of considerable nutritional importance, and future research should be directed to this aspect of ruminal transport.

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

In this study, the existence and functional activity of a vacuolar-type H(+)-ATPase (vH(+)-ATPase) was explored in primary cultures of sheep ruminal epithelial cells (REC). The mRNA transcripts of the E and B subunits of vH(+)-ATPase were detectable in RNA from REC samples by RT-PCR. Immunoblotting of REC protein extractions with antibodies directed against the B subunit of yeast vH(+)-ATPase revealed a protein band of the expected size (60 kDa). Using the fluorescent indicator BCECF and selective inhibitors (foliomycin, HOE 694, S3226), the contribution of vH(+)-ATPase and Na(+)/H(+) exchanger (NHE) subtype 1 and 3 activity to the regulation of intracellular pH (pH(i)) was determined in nominally HCO(3)(-)-free, HEPES-buffered NaCl medium containing 20 mM of the short-chain fatty acid butyrate as well as after reduction of the extracellular Cl(-) concentration ([Cl(-)](e)) from 136 to 36 mM. The initial pH(i) of REC was 7.4 +/- 0.1 in nominally HCO(3)(-)-free, HEPES-buffered NaCl medium and 7.0 +/- 0.1 after acid loading with butyrate. Selective inhibition of the vH(+)-ATPase with foliomycin decreased pH(i) by 0.19 +/- 0.03 pH units. On the basis of the observed decreases in pH(i) resulting from inhibition of vH(+)-ATPase as well as of subtypes 1 and 3 of NHE, vH(+)-ATPase activity appears to account for approximately 30% of H(+) extrusion, whereas the activities of NHE subtypes 3 and 1 account for 20 and 50% of H(+) extrusion, respectively. Lowering of [Cl(-)](e) induced a pH(i) decrease (-0.51 +/- 0.03 pH units) and impaired pH(i) recovery from butyrate-induced acid load. Moreover, reduction of [Cl(-)](e) abolished the inhibitory effect of foliomycin and markedly reduced the HOE 694- and S3226-sensitive components of pH(i), indicating a role of Cl(-) in the function of these H(+) extrusion mechanisms. We conclude that a vH(+)-ATPase is expressed in ovine REC and plays a considerable role in the pH(i) regulation of these cells.

Effects of diet and osmotic pressure on Na+transport and tissue conductance of sheep isolated rumen epithelium

The intention of this study was to determine the effects of mucosal osmotic pressure on transport and barrier functions of the rumen epithelium of sheep, which were fed various diets: hay ad libitum, or 600, 1200 or 1800 g day(-1) of a supplemented diet plus hay ad libitum. The experiments were conducted by using the conventional Ussing chamber technique. Mucosal osmolarity was adjusted to 300 (control), 375 or 450 mosmol l(-1). Feeding of a supplemented diet led to a significant increase of mucosal to serosal Na+ transport and net Na+ transport, probably because of an increase of apical Na+-H+ exchange activity. An increase in mucosal osmotic pressure: (a) reduced net Na+ transport in all feeding groups, the remaining net Na+ transport being higher in tissues of sheep fed a supplemented diet; (b) increased transepithelial tissue conductance, this rise being smallest with a high intake of the supplemented diet; and (c) enhanced the serosal to mucosal Na+ transport in tissues of hay-fed sheep and sheep fed with 600 g day(-1) of the supplemented diet, while higher intakes of the supplemented diet (1200 and 1800 g) did not produce any effect. All these changes indicate a diet-dependent adaptation to luminal hypertonicity.