Bicarbonate exporting transporters in the ovine ruminal epithelium

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.

Heat stress promotes adaptive physiological responses and alters mrna expression of ruminal epithelium markers in Bos taurus indicus cattle fed low- or high-energy diets

This research aimed to evaluate the impact of temperature and energy status on the thermal indices, physiological parameters, and ruminal papilla mRNA expression levels of Zebu beef heifers (Bos taurus indicus). In this trial, we used six ruminal-cannulated Nellore females. The experimental design was a 6 × 6 Latin square, with six treatments and six periods. The research used a 2 × 2 + 2 factorial scheme. The arrangement comprised: two thermal conditions [thermoneutrality (TN; 21.6 °C) or heat stress (HS, 34 °C)]; two dietary energy levels (low or high-energy); and two additional treatments, with heifers exposed to the TN, but pair-fed with females exposed to HS (PFTN). For our purposes, body temperature, heart and respiratory rates were measured and the relative mRNA expression was quantified using the PCR-RT technique. Compared to TN or PFTN, the HS increased the body temperature measurements in the morning and evening (p ≤ 0.04). Heart rate was 22% greater for heifers under HS than for TN (p < 0.01) and 13% higher for those under HS than PFTN (p = 0.03) in the morning. Respiratory rates increased with HS exposure compared to TN or PFTN (p < 0.01). Heifers submitted to HS and fed low-energy diets had and tended to have lower caspase 3 (CASP3, p <i=></i> 0.001) and sodium-glucose cotransporter type 1 (SGLT1; p = 0.17) mRNA expressions, respectively. Heat-stressed heifers fed low-energy diets also increased the putative anion transporter (PAT1; p ≤ 0.01) mRNA expressions by 60%. Heifers under HS-fed high-energy diets had greater kallikrein-related peptidase (KLK) 9 expressions (p = 0.02), while KLK10 (p = 0.11) tended to be up-regulated in heifers in TN-fed a low-energy diets. In conclusion, heat stress down-regulated the mRNA expression of rumen markers related to short-chain fatty acids transport and pH modulation.

Sex differences in rumen fermentation and microbiota of Tibetan goat

BACKGROUND

The gut microbiota play an important role in maintaining host metabolism, the immune system and health, while sex, genotype, diet and health have specific effects on the composition of the gut microbiota. Therefore, to explore the sex differences in the structure and function of rumen microbiota in Tibetan goats, herein we analyzed sex differences in rumen fermentation parameters, rumen microbiota and the expression of genes related to VFA transport in Tibetan goats.

RESULTS

The results showed that the contents of acetic acid and propionic acid in the rumen of TGM (Tibetan goat male) were significantly higher than those in TGFm (Tibetan goat female) (P < 0.05), and total VFAs was significantly higher in TGM than TGFm (P < 0.05). Expression of the VFA transport-related genes DRA, AE2, MCT-1, NHE1, and NHE2 in the rumen epithelium of TGFm was significantly higher than that in TGM. Analysis of the composition and structure of the rumen microbiota revealed significant sex differences. At the phylum level, Firmicutes and Bacteroidetes were the dominant phyla in Tibetan goats. In addition, Fibrobacteres and Spirochaetes had significantly greater relative abundances in TGFm than in TGM (P < 0.05). At the genus level, the relative abundance of Fibrobacter, Ruminococcus_1 and Pyramidobacter was significantly higher in TGFm than in TGM (P < 0.05). The functional prediction results showed that replication, recombination and repair, RNA processing and modification were mainly enriched in TGFm (P < 0.05).

CONCLUSIONS

Correlation analysis revealed significant associations of some rumen microbiota with the fermentation product VFAs and VFA transport-related genes. We concluded that yearling TGM and TGFm have distinct fermentation and metabolism abilities when adapting to the plateau environment, which provides a certain sex reference basis for Tibetan goat adaptation to the plateau environment.

Effects of whole corn high-grain diet feeding on ruminal bacterial community and epithelial gene expression related to VFA absorption and metabolism in fattening lambs

The objective of this study was to investigate the effects of whole corn high-grain diet feeding on growth performance, ruminal bacterial community, and epithelial morphology and gene expression related to VFA absorption and metabolism in fattening lambs. Fourteen male (castrated) lambs were randomly assigned to either a group fed a ground corn high-grain diet (50.4% grain; HGC, n = 7) or a group fed a whole corn high-grain diet (50.4% grain; HWC, n = 7). After 7 wk of feeding, HWC group increased the average daily gain (ADG) (P = 0.036) and decreased the feed: gain value (P = 0.010) significantly. HWC group had a greater crude protein apparent digestibility (P = 0.028) in the third week and dry matter and neutral detergent fiber apparent digestibility (P < 0.05) in the seventh week. Pyrosequencing of the 16S ribosomal RNA gene revealed that HWC feeding increased the relative abundance of genera Anaerovibrio, Schwartzia and Unclassified Veillonellaceae in the rumen content and Howardella, Schwartzia and Unclassified Veillonellaceae in the rumen epithelia (P<0.05), while decreased the proportion of Lachnospira and Unclassified Synergistaceae in the rumen content and Anaerovorax, Papillibacter, Ruminococcus, Fibrobacter, Unclassified Lachnospiraceae, Unclassified Bacteroidales and Unclassified Prevotellaceae in the rumen epithelia (P < 0.05). HWC group increased the rumen papilla length (P = 0.001) and surface area (P = 0.002). Furthermore, HWC diet feeding up-regulated the relative mRNA expression of putative anion transporter isoform 1 (PAT1) (P = 0.032) in the rumen epithelia. In summary, compared with ground corn high-grain diet feeding, whole corn high-grain diet feeding improved animal performance, changed ruminal bacterial composition and diversity, and increased VFA absorption of epithelial papilla in fattening lambs. These findings provided theoretical guidance for the actual application of whole corn high-grain diet in ruminants.

Effect of Tea Tree Oil on the Expression of Genes Involved in the Innate Immune System in Goat Rumen Epithelial Cells

Simple Summary

Subacute rumen acidosis (SARA) often causes significant losses on commercial farms. SARA is mainly caused by endotoxin (LPS) produced by the lysis of Gram-negative bacteria, which causes an inflammatory response. To alleviate the inflammatory response mediated by LPS, it is important to improve animal production performance. Tea tree oil (TTO) is a plant extract that possesses good bactericidal and anti-inflammatory effects. According to this study, LPS can significantly induce inflammatory responses in goat rumen epithelial cells (GRECs), while the addition of TTO could markedly mitigate inflammatory responses mediated by LPS in GRECs. Therefore, it may be useful for the treatment of SARA.

Abstract

In subacute rumen acidosis (SARA), the rumen epithelium is frequently attacked by endotoxin (LPS), which is caused by the lysis of dead Gram-negative bacteria. However, the rumen epithelium innate immune system can actively respond to the infection. Previous studies have demonstrated that tea tree oil (TTO) has good bactericidal and anti-inflammatory effects. Therefore, the aim of this study was to investigate the effect of TTO on the expression of genes involved in the inflammatory cytokines in goat rumen epithelial cells (GRECs) triggered by LPS. Our study shows that rumen epithelial cells isolated from goat rumen tissue can be cultured in vitro in 0.25% trypsin for a long time. These cells were identified as epithelial cells by the expression of cytokeratin 18, monocarboxylate transporter 4 (MCT4), Na[+]/H[+] hydrogen exchanger 1 (NHE1), putative anion transporter 1 (PAT1), vH⁺ ATPase B subunit (vH⁺ ATPase), and anion exchanger 2 (AE2). The mRNA expression of IL-1β, IL-6, TNF-α, TLR-2, NF-κB, CXCL6 and CXCL8 genes was significantly increased when LPS was used compared to untreated controls. In addition, mRNA expression of IL-1β, IL-6, TNF-α, TLR-2, NF-κB, CXCL8, CXCL6 and interferon-induced protein with tetratricopeptide repeats 3 (IFIT3) genes was also significantly higher in the LPS group compared to the 0.05% TTO group. However, the expression of IL-1β, IL-6, TNF-α, TLR-2, CXCL6 and IFIT3 genes was significantly lower in the LPS and 0.05% TTO group compared to the 1 μg/mL LPS group. These results suggest that TTO can inhibit LPS-induced inflammatory cytokines expression in GRECs.

Butyrate Permeation across the Isolated Ovine Reticulum Epithelium

Simple Summary

Short-chain fatty acids are the main source of energy for ruminants. The effective uptake of these substrates from the forestomach is a prerequisite for the health and performance of these animals. Thus far, the mechanisms of uptake have been investigated almost exclusively in the epithelium of the largest forestomach section, the rumen. Previous research suggests that the reticulum is also involved in the uptake of short-chain fatty acids, but the mechanisms involved have not been studied and may differ from those known from the rumen epithelium due to the different milieu in this compartment. To investigate this, ovine reticulum epithelium was mounted in Ussing chambers, and the transport of radiolabeled butyrate (as a representative of short-chain fatty acids) across the tissue was measured with and without the addition of inhibitors of particular transport proteins. Our results show that butyrate can be taken up effectively across the reticulum epithelium via pathways that are energized by the Na⁺/K⁺-ATPase and may involve monocarboxylate transporters, sodium-proton exchangers, and anion channels. However, our results are not completely congruent to those obtained in the rumen epithelium. These modifications could assure the effective uptake of short-chain fatty acids from the reticulum lumen under the particular conditions (p. e. high pH) of this forestomach compartment.

Abstract

We hypothesized that, due to the high pH of this compartment, the reticulum epithelium displays particular features in the transport of short-chain fatty acids (SCFA). Ovine reticulum epithelium was incubated in Ussing chambers using a bicarbonate-free buffer solution containing butyrate (20 mmol L⁻¹). p-hydroxymercuribenzoic acid (pHMB), 5-(N-Ethyl-N-isopropyl)amiloride (EIPA), or ouabain were added to the buffer solution as inhibitors of monocarboxylate transporters, sodium-proton-exchangers, or the Na⁺/K⁺-ATPase, respectively. The short-circuit current (I_sc) and transepithelial conductance (G_t) were monitored continuously while the flux rates of ¹⁴C-labelled butyrate were measured in the mucosal-to-serosal (J_ms^but) or serosal-to-mucosal direction (J_sm^but). Under control conditions, the mean values of I_sc and G_t amounted to 2.54 ± 0.46 µEq cm⁻² h⁻¹ and 6.02 ± 3.3 mS cm⁻², respectively. J_ms^but was 2.1 ± 1.01 µmol cm⁻² h⁻¹ on average and about twice as high as J_sm^but. Incubation with ouabain reduced J_ms^but, while J_sm^but was not affected. The serosal addition of EIPA did not affect J_ms^but but reduced J_sm^but by about 10%. The addition of pHMB to the mucosal or serosal solution reduced J_ms^but but had no effect on J_sm^but. Mucosally applied pHMB provoked a transient increase in the I_sc. The serosal pHMB sharply reduced I_sc. Our results demonstrate that butyrate can be effectively transported across the reticulum epithelium. The mechanisms involved in this absorption differ from those known from the rumen epithelium.

The functional development of the rumen is influenced by weaning and associated with ruminal microbiota in lambs

Rumen development is critical for the development of early lambs. This work aims to evaluate the effects of abrupt weaning at day 21 on rumen fermentation, histomorphological traits and the ruminal microbiota compared with continuous suckling. Twelve pairs of artificially reared full-sib neonatal male Hu lambs were allocated to two groups, one of which was weaned at day 21 (EW group) and the other which was not weaned (CON group). At day 26 and day 49, six lambs from each group were randomly selected and sacrificed to collect ruminal contents and rumen tissue samples. Results showed that weaning influenced the fermentation parameters in the rumen, and altered the microbial community composition on day 49 (p < 0.05). Several genera were associated with rumen fermentation parameters (p < 0.05). Volatile fatty acid (VFA) concentration is the key parameter impacting microbiota composition. Weaning influenced the expression of genes associated with VFA metabolism and regulation of cell proliferation (p < 0.05). In conclusion, weaning significantly influenced the morphological and functional development of the rumen, and bacterial community composition. The microbial community composition was strongly associated with rumen weight and fermentation profiles, but not with morphological development.

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.

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.

Dietary supplements during the cold season increase rumen microbial abundance and improve rumen epithelium development in Tibetan sheep

Livestock on the Qinghai-Tibetan Plateau are faced with extreme harsh winters and are often in negative energy balance during this period. Dietary supplementation can improve growth performance of Tibetan sheep and, consequently, we hypothesized that it would also increase microbial abundance and rumen epithelium development. To test this hypothesis, we examined the effect of feed supplementation during the cold season on rumen microbes, fermentation, epithelium development, and absorptive capability in Tibetan sheep. Eighteen 1-yr-old ewes (BW = 29.4 ± 1.79, kg) were offered oat hay ad libitum for 60 d and divided randomly into three groups: 1) no supplement; control group (CON); 2) urea-molasses lick block supplement (BS); and 3) concentrate feed supplement (CS). The ADG of CS ewes (143.3, g/d) was greater (P < 0.05) than BS ewes (87.9, g/d), which was greater (P < 0.05) than CON ewes (44.5, g/d). Serum concentrations of GH, IGF-1, and IGF-2 in the CS and BS groups were greater than in the CON group (P < 0.05). Greater relative abundance of protozoa, Ruminococcus albus, Fibrobacter succinogenes, Streptococcus bovis, and Ruminobacter amylophilus was observed in the CS and BS groups than in the CON group (P < 0.05), and relative abundances of rumen fungi, Butyrivibrio fibrisolvens, and Prevotella ruminicola in the CS group were greater than in the BS and CON groups (P < 0.05). Ruminal total VFA, ammonia, and microbial protein concentrations in the CS and BS groups were greater than in the CON group (P < 0.05), and in the CS group were greater than in the BS group (P < 0.05). Ruminal papillae width and surface area in the CS and BS groups were greater than in the CON group (P < 0.05), while in the CS group were greater than in the BS group (P < 0.05). The mRNA expressions of IGFBP5, NHE1 (sodium/hydrogen antiporter, isoform 1), DRA (downregulated in adenoma), and Na+/K+-ATPase (sodium/potassium ATPase pump) in ruminal epithelium were greater in the CS and BS groups than in the CON group (P < 0.05), and in the CS group was greater than in the BS group (P < 0.05), while NHE3 (sodium/hydrogen antiporter, isoform 3), MCT1 (monocarboxylate transporter 1), and MCT4 (monocarboxylate transporter 4) mRNA expressions in the CS group were greater than in the BS and CON groups (P < 0.05). It was concluded that supplementing Tibetan sheep during the cold season increases rumen microbial abundance and improves fermentation parameters, rumen epithelium development, and absorptive capability.

A look at the smelly side of physiology: transport of short chain fatty acids

Fermentative organs such as the caecum, the colon, and the rumen have evolved to produce and absorb energy rich short chain fatty acids (SCFA) from otherwise indigestible substrates. Classical models postulate diffusional uptake of the undissociated acid (HSCFA). However, in net terms, a major part of SCFA absorption occurs with uptake of Na⁺ and resembles classical, coupled electroneutral NaCl transport. Considerable evidence suggests that the anion transporting proteins expressed by epithelia of fermentative organs are poorly selective and that their main function may be to transport acetate^-, propionate^-, butyrate^- and HCO₃^- as the physiologically relevant anions. Apical uptake of SCFA thus involves non-saturable diffusion of the undissociated acid (HSCFA), SCFA^-/HCO₃^- exchange via DRA (SLC26A3) and/or SCFA^--H⁺ symport (MCT1, SLC16A1). All mechanisms lead to cytosolic acidification with stimulation of Na⁺/H⁺ exchange via NHE (SLC9A2/3). Basolaterally, Na⁺ leaves via the Na⁺/K⁺-ATPase with recirculation of K⁺. Na⁺ efflux drives the transport of SCFA^- anions through volume-regulated anion channels, such as maxi-anion channels (possibly SLCO2A1), LRRC8, anoctamins, or uncoupled exchangers. When luminal buffering is inadequate, basolateral efflux will increasingly involve SCFA^-/ HCO₃^- exchange (AE1/2, SCL4A1/2), or efflux of SCFA^- with H⁺ (MCT1/4, SLC16A1/3). Furthermore, protons can be basolaterally removed by NHE1 (SCL9A1) or NBCe1 (SLC4A4). The purpose of these transport proteins is to maximize the amount of SCFA transported from the tightly buffered ingesta while minimizing acid transport through the epithelium. As known from the rumen for many decades, a disturbance of these processes is likely to cause severe colonic disease.

Growth performance, rumen fermentation, bacteria composition, and gene expressions involved in intracellular pH regulation of rumen epithelium in finishing Hu lambs differing in residual feed intake phenotype

The objective of this study was to evaluate the effect of residual feed intake (RFI) on rumen function in finishing lambs. A total of 60 male Hu lambs (average initial BW = 25.2 ± 2.5kg) were used and were offered a pelleted high-concentrate diet, of which the forage to concentrate ratio was 25:75. Individual feed intake was recorded over a period of 42 d, then 10 lambs with the lowest RFI and the highest RFI were selected, respectively. The rumen fluid used for fermentation variables and relative abundance of bacteria measurement was obtained on d 10 and 20 after RFI measurement. At the end of this experiment, the selected lambs were slaughtered and rumen epithelium and liver tissues were collected for RNA extraction. Low-RFI lambs had lower ( < 0.01) DMI and greater ( < 0.05) G:F than the high-RFI ones, while the RFI groups did not differ in ADG and BW ( > 0.05). Additionally, RFI was positively ( = 0.57; < 0.01) correlated with DMI and negatively ( = -0.53; < 0.05) correlated with G:F. Total VFA and individual VFA decreased ( < 0.05) over time. The concentrations of total VFA, acetate, valerate, isobutyrate, isovalerate, and rumen pH ( > 0.05) were not affected by RFI classification. Nonetheless, low-RFI group lambs had a greater ( < 0.05) concentration of propionate, a lower ( < 0.05) concentration of butyrate, and a lower ( < 0.05) acetate to propionate ratio compared with the high-RFI group. There was a significant ( < 0.05) effect of RFI on the relative abundance of and . The relative abundance of , , and decreased ( < 0.05) over time in high-RFI group. And the relative abundance of in high-RFI group was greater ( < 0.05) than its low-RFI counterpart. Furthermore, RFI had no effect ( > 0.05) on gene expression associated with intracellular pH regulation (, , , , , , , and ) in rumen epithelium and β-hydroxybutyrate metabolism () in both rumen epithelium and liver tissues. In conclusion, even though low-RFI lambs had lower DMI, however, the number of was lower. Additionally, there was no difference in gene expressions level associated with intracellular pH regulation in rumen epithelium between RFI groups.

Effects of dissolved carbon dioxide on the integrity of the rumen epithelium: An agent in the development of ruminal acidosis

The carbon dioxide released and dissolved in rumen fluid may easily permeate across the epithelial cell membrane. Thus, we hypothesized that CO₂ may act as proton carrier and induce epithelial damage under acidotic conditions. Ovine ruminal epithelia were mounted in Ussing chambers under short-circuit conditions. The serosal buffer solution had a constant pH of 7.4 and was gassed either with 100% oxygen or with carbogen (95% O₂ /5% CO₂ ). The mucosal solution was gassed with either 100% oxygen or 100% carbon dioxide. The mucosal pH was lowered stepwise from 6.6 to 5.0 in the presence or absence of short-chain fatty acids (SCFA). The transepithelial conductance (G_t ) as an indicator of epithelial integrity and the short-circuit current (I_sc ) as an indicator of active electrogenic ion transfer were continuously monitored. At an initial mucosal pH of 6.6, there was no significant difference in G_t between the treatment groups. In the absence of both SCFA and CO₂ , G_t remained constant when the mucosal solution was acidified to pH 5.0. In the presence of SCFA, mucosal acidification induced a significant rise in G_t when the solutions were gassed with oxygen. A small increase in G_t was observed in the mucosal presence of CO₂ . However, no difference in final G_t was observed between SCFA-containing and SCFA-free conditions under carbon dioxide gassing during stepwise mucosal acidification. The SCFA+proton-induced increase in G_t could also be minimized by serosal gassing with carbogen. Because of the SCFA+proton-induced changes in G_t and their attenuation by CO₂ , a protective role for mucosally available carbon dioxide may be assumed. We suggest that this effect may be due to the intraepithelial conversion of carbon dioxide to bicarbonate. However, the serosal presence of CO₂ at a physiological concentration may be sufficient to protect the epithelia from SCFA+proton-induced damage for a certain period of time.

Effects of partial replacement of dietary starch from barley or corn with lactose on ruminal function, short-chain fatty acid absorption, nitrogen utilization, and production performance of dairy cows

In cows fed diets based on corn-alfalfa silage, replacing starch with sugar improves milk production. Although the rate of ruminal fermentation of sugar is more rapid than that of starch, evidence has been found that feeding sugar as a partial replacement for starch does not negatively affect ruminal pH despite increasing diet fermentability. The mechanism(s) for this desirable response are unknown. Our objective was to determine the effects of replacing barley or corn starch with lactose (as dried whey permeate; DWP) on ruminal function, short-chain fatty acid (SCFA) absorption, and nitrogen (N) utilization in dairy cows. Eight lactating cows were used in a replicated 4 × 4 Latin square design with 28-d periods and source of starch (barley vs. corn) and level of DWP (0 vs. 6%, DM basis) as treatment factors. Four cows in 1 Latin square were ruminally cannulated for the measurement of ruminal function, SCFA absorption, and N utilization. Dry matter intake and milk and milk component yields did not differ with diet. The dietary addition of DWP tended to increase ruminal butyrate concentration (13.6 vs. 12.2 mmol/L), and increased the Cl(-)-competitive absorption rates for acetate and propionate. There was no sugar effect on minimum ruminal pH, and the duration and area when ruminal pH was below 5.8. Minimum ruminal pH tended to be lower in cows fed barley compared with those fed corn (5.47 vs. 5.61). The duration when ruminal pH was below pH 5.8 tended to be shorter (186 vs. 235 min/d), whereas the area (pH × min/d) that pH was below 5.8 was smaller (47 vs. 111) on the corn than barley diets. Cows fed the high- compared with the low-sugar diet had lower ruminal NH3-N concentration. Feeding the high-sugar diet tended to increase apparent total-tract digestibility of dry matter and organic matters and increased apparent total-tract digestibility of fat. Apparent total-tract digestibility of N tended to be greater in cows fed barley compared with those fed corn, whereas apparent total-tract digestibility of acid-digestible fiber was greater in cows fed corn compared with those fed barley. In conclusion, partially replacing dietary corn or barley starch with sugar upregulated ruminal acetate and propionate absorption, suggesting that the mechanisms for the attenuation of ruminal acidosis when sugar is fed is partly mediated via increased SCFA absorption.

Both butyrate incubation and hypoxia upregulate genes involved in the ruminal transport of SCFA and their metabolites

Butyrate modulates the differentiation, proliferation and gene expression profiles of various cell types. Ruminal epithelium is exposed to a high intraluminal concentration and inflow of n-butyrate. We aimed to investigate the influence of n-butyrate on the mRNA expression of proteins involved in the transmembranal transfer of n-butyrate metabolites and short-chain fatty acids in ruminal epithelium. N-butyrate-induced changes were compared with the effects of hypoxia because metabolite accumulation after O2 depletion is at least partly comparable to the accumulation of metabolites after n-butyrate exposure. Furthermore, in various tissues, O2 depletion modulates the expression of transport proteins that are also involved in the extrusion of metabolites derived from n-butyrate breakdown in ruminal epithelium. Sheep ruminal epithelia mounted in Ussing chambers were exposed to 50 mM n-butyrate or incubated under hypoxic conditions for 6 h. Electrophysiological measurements showed hypoxia-induced damage in the epithelia. The mRNA expression levels of monocarboxylate transporters (MCT) 1 and 4, anion exchanger (AE) 2, downregulated in adenoma (DRA), putative anion transporter (PAT) 1 and glucose transporter (GLUT) 1 were assessed by RT-qPCR. We also examined the mRNA expression of nuclear factor (NF) κB, cyclooxygenase (COX) 2, hypoxia-inducible factor (HIF) 1α and acyl-CoA oxidase (ACO) to elucidate the possible signalling pathways involved in the modulation of gene expression. The mRNA expression levels of MCT 1, MCT 4, GLUT 1, HIF 1α and COX 2 were upregulated after both n-butyrate exposure and hypoxia. ACO and PAT 1 were upregulated only after n-butyrate incubation. Upregulation of both MCT isoforms and NFκB after n-butyrate incubation could be detected on protein level as well. Our study suggests key roles for MCT 1 and 4 in the adaptation to an increased intracellular load of metabolites, whereas an involvement of PAT 1 in the transport of n-butyrate also seems possible.

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.

Invited review: Role of physically effective fiber and estimation of dietary fiber adequacy in high-producing dairy cattle

Highly fermentable diets require the inclusion of adequate amounts of fiber to reduce the risk of subacute rumen acidosis (SARA). To assess the adequacy of dietary fiber in dairy cattle, the concept of physically effective neutral detergent fiber (peNDF) has received increasing attention because it amalgamates information on both chemical fiber content and particle size (PS) of the feedstuffs. The nutritional effects of dietary PS and peNDF are complex and involve feed intake behavior (absolute intake and sorting behavior), ruminal mat formation, rumination and salivation, and ruminal motility. Other effects include fermentation characteristics, digesta passage, and nutrient intake and absorption. Moreover, peNDF requirements depend on the fermentability of the starch source (i.e., starch type and endosperm structure). To date, the incomplete understanding of these complex interactions has prevented the establishment of peNDF as a routine method to determine dietary fiber adequacy so far. Therefore, this review is intended to analyze the quantitative effects of and interactions among forage PS, peNDF, and diet fermentability with regard to rumen metabolism and prevention of SARA, and aims to give an overview of the latest achievements in the estimation of dietary fiber adequacy in high-producing dairy cattle. Recently developed models that synthesize the effects of both peNDF and fermentable starch on rumen metabolism appear to provide an appropriate basis for estimation of dietary fiber adequacy in high-producing dairy cows. Data suggest that a period lasting more than 5 to 6h/d during which ruminal pH is <5.8 should be avoided to minimize health disturbances due to SARA. The knowledge generated from these modeling approaches recommends that average amounts of 31.2% peNDF inclusive particles >1.18mm (i.e., peNDF(>1.18)) or 18.5% peNDF inclusive particles >8mm (i.e., peNDF(>8)) in the diet (DM basis) are required. However, inclusion of a concentration of peNDF(>8) in the diet beyond 14.9% of diet DM may lower DM intake level. As such, more research is warranted to develop efficient feeding strategies that encourage inclusion of energy-dense diets without the need to increase their content in peNDF above the threshold that leads to lower DM intake. The latter would require strategies that modulate the fermentability characteristics of the diet and promote absorption and metabolic capacity of ruminal epithelia of dairy cows.

Ruminant Nutrition Symposium: Role of fermentation acid absorption in the regulation of ruminal pH

Highly fermentable diets are rapidly converted to organic acids [i.e., short-chain fatty acids (SCFA) and lactic acid] within the rumen. The resulting release of protons can constitute a challenge to the ruminal ecosystem and animal health. Health disturbances, resulting from acidogenic diets, are classified as subacute and acute acidosis based on the degree of ruminal pH depression. Although increased acid production is a nutritionally desired effect of increased concentrate feeding, the accumulation of protons in the rumen is not. Consequently, mechanisms of proton removal and their quantitative importance are of major interest. Saliva buffers (i.e., bicarbonate, phosphate) have long been identified as important mechanisms for ruminal proton removal. An even larger proportion of protons appears to be removed from the rumen by SCFA absorption across the ruminal epithelium, making efficiency of SCFA absorption a key determinant for the individual susceptibility to subacute ruminal acidosis. Proceeding initially from a model of exclusively diffusional absorption of fermentation acids, several protein-dependent mechanisms have been discovered over the last 2 decades. Although the molecular identity of these proteins is mostly uncertain, apical acetate absorption is mediated, to a major degree, via acetate-bicarbonate exchange in addition to another nitrate-sensitive, bicarbonate-independent transport mechanism and lipophilic diffusion. Propionate and butyrate also show partially bicarbonate-dependent transport modes. Basolateral efflux of SCFA and their metabolites has to be mediated primarily by proteins and probably involves the monocarboxylate transporter (MCT1) and anion channels. Although the ruminal epithelium removes a large fraction of protons from the rumen, it also recycles protons to the rumen via apical sodium-proton exchanger, NHE. The latter is stimulated by ruminal SCFA absorption and salivary Na(+) secretion and protects epithelial integrity. Finally, SCFA absorption also accelerates urea transport into the rumen, which via ammonium recycling, may remove protons from rumen to the blood. Ammonium absorption into the blood is also stimulated by luminal SCFA. It is suggested that the interacting transport processes for SCFA, urea, and ammonia represent evolutionary adaptations of ruminants to actively coordinate energy fermentation, protein assimilation, and pH regulation in the rumen.

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.

Bicarbonate-dependent and bicarbonate-independent mechanisms contribute to nondiffusive uptake of acetate in the ruminal epithelium of sheep

The present study investigated the significance of apical transport proteins for ruminal acetate absorption and their interaction with different anions. In anion competition experiments in the washed reticulorumen, chloride disappearance rate (initial concentration, 28 mM) was inhibited by the presence of a short-chain fatty acid mixture (15 or 30 mM of each acetate, propionate, and butyrate). Disappearance rates of acetate and propionate, but not butyrate (initial concentration, 25 mM each) were diminished by 40 or 80 mM chloride. In isolated ovine ruminal epithelia mounted in Ussing chambers, an increase in chloride concentration from 4.5 to 90 mM led to a decrease of apical acetate uptake at a concentration of 0.5 mM. Mucosal nitrate inhibited acetate uptake most potently whereas sulfate had no effect. Decreasing mucosal pH from 7.4 to 6.1 approximately doubled uptake of acetate both at 0.5 and 10 mM, but this doubling was almost abolished when HCO(3)(-) was absent. The stimulated uptake at mucosal pH 6.1 consisted of a bicarbonate-dependent, nitrate-inhibitable part (K(m) = 54 mM) and a bicarbonate-independent component (K(m) = 12 mM) that was also sensitive to nitrate inhibition. Maximal uptake was three times larger for bicarbonate-dependent vs. bicarbonate-independent uptake. Mucosal addition of 200 microM DIDS, 400 microM p-chloromercuribenzene sulfonic acid, 800 microM p-hydroxymercuribenzoic acid, or 100 microM phloretin had no effects on acetate uptake although the latter two inhibited l-lactate uptake. Our data conclusively show a dominant involvement of proteins in apical acetate uptake. Previously described pH effects on acetate absorption originate mainly from modulation of acetate/bicarbonate exchange. Additionally, there is bicarbonate-independent uptake of acetate anions that is protein coupled but not via monocarboxylate cotransporter.

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.

Effect of chloride on pH microclimate and electrogenic Na+ absorption across the rumen epithelium of goat and sheep

Active Na+ absorption across rumen epithelium comprises Na+/H+ exchange and a nonselective cation conductance (NSCC). Luminal chloride is able to stimulate Na+ absorption, which has been attributed to an interaction between Cl-/HCO3- and Na+/H+ exchangers. However, isolated rumen epithelial cells also express a Cl- conductance. We investigated whether Cl- has an additional effect on electrogenic Na+ absorption via NSCC. NSCC was estimated from short-circuit current (Isc) across epithelia of goat and sheep rumen in Ussing chambers. Epithelial surface pH (pHs) was measured with 5-N-hexadecanoyl-aminofluorescence. Membrane potentials were measured with microelelectrodes. Luminal, but not serosal, Cl- stimulated the Ca2+ and Mg2+ sensitive Isc. This effect was independent of the replacing anion (gluconate or acetate) and of the presence of bicarbonate. The mean pHs of rumen epithelium amounted to 7.47 +/- 0.03 in a low-Cl- solution. It was increased by 0.21 pH units when luminal Cl- was increased from 10 to 68 mM. Increasing mucosal pH from 7.5 to 8.0 also increased the Ca2+ and Mg2+ sensitive Isc and transepithelial conductance and reduced the fractional resistance of the apical membrane. Luminal Cl- depolarized the apical membrane of rumen epithelium. 5-Nitro-2-(3-phenylpropylamino)-benzoate reduced the divalent cation sensitive Isc, but only in low-Cl- solutions. The results show that luminal Cl- can increase the microclimate pH via apical Cl-/HCO3- or Cl-/OH- exchangers. Electrogenic Na+ absorption via NSCC increases with pH, explaining part of the Cl- effects on Na+ absorption. The data further show that the Cl- conductance of rumen epithelium must be located at the basolateral membrane.