An energy-rich diet causes rumen papillae proliferation associated with more IGF type 1 receptors and increased plasma IGF-1 concentrations in young goats

Modulation of vH+-ATPase is part of the functional adaptation of sheep rumen epithelium to high-energy diet

Ruminal vacuolar H(+)-ATPase (vH(+)-ATPase) activity is regulated by metabolic signals. Thus, we tested whether its localization, expression, and activity were changed by different feeding. Young male sheep (n = 12) were either fed hay ad libitum (h) or hay ad libitum plus additional concentrate (h/c) for 2 wk. The vH(+)-ATPase B subunit signal was predominantly found in the cell membrane and cytosol of rumen epithelial cells (REC) with basal/parabasal phenotype. The elevated number (threefold) of these cells in rumen mucosa of h/c-fed sheep reflects a high proliferative capacity and, explains the 2.3-fold increase of the total number of vH(+)-ATPase-expressing REC. However, in accordance with a 58% reduction of the vH(+)-ATPase B subunit mRNA expression in h/c-fed sheep, its protein amount per single REC was decreased. Using the fluorescent probe BCECF and selective inhibitors (foliomycin, amiloride), the contribution of vH(+)-ATPase and Na(+)/H(+) exchanger to intracellular pH (pH(i)) regulation was investigated. REC isolated from h/c-fed sheep keep their pH(i) at a significantly higher level (6.91 ± 0.03 vs. 6.74 ± 0.05 in h-fed sheep). Foliomycin or amiloride decreased pH(i) by 0.16 ± 0.02 and 0.57 ± 0.04 pH units when applied to REC from h-fed sheep, but the effects were markedly reduced (-88 and -33%) after concentrate feeding. Nevertheless, we found that REC proliferation rate and [cAMP](i) were reduced after foliomycin-induced vH(+)-ATPase inhibition. Our results provide the first evidence for a role of vH(+)-ATPase in regulation of REC proliferation, most probably by linking metabolically induced pH(i) changes to signaling pathways regulating this process.

Microbial butyrate and its role for barrier function in the gastrointestinal tract

Butyrate production in the large intestine and ruminant forestomach depends on bacterial butyryl-CoA/acetate-CoA transferase activity and is highest when fermentable fiber and nonstructural carbohydrates are balanced. Gastrointestinal epithelia seem to use butyrate and butyrate-induced endocrine signals to adapt proliferation, apoptosis, and differentiation to the growth of the bacterial community. Butyrate has a potential clinical application in the treatment of inflammatory bowel disease (IBD; ulcerative colitis). Via inhibited release of tumor necrosis factor α and interleukin 13 and inhibition of histone deacetylase, butyrate may contribute to the restoration of the tight junction barrier in IBD by affecting the expression of claudin-2, occludin, cingulin, and zonula occludens poteins (ZO-1, ZO-2). Further evaluation of the molecular events that link butyrate to an improved tight junction structure will allow for the elucidation of the cofactors affecting the reliability of butyrate as a clinical treatment tool.

Cattle genomics and its implications for future nutritional strategies for dairy cattle

The recently sequenced cattle (Bos taurus) genome unraveled the unique genomic features of the species and provided the molecular basis for applying a systemic approach to systematically link genomic information to metabolic traits. Comparative analysis has identified a variety of evolutionary adaptive features in the cattle genome, such as an expansion of the gene families related to the rumen function, large number of chromosomal rearrangements affecting regulation of genes for lactation, and chromosomal rearrangements that are associated with segmental duplications and copy number variations. Metabolic reconstruction of the cattle genome has revealed that core metabolic pathways are highly conserved among mammals although five metabolic genes are deleted or highly diverged and seven metabolic genes are present in duplicate in the cattle genome compared to their human counter parts. The evolutionary loss and gain of metabolic genes in the cattle genome may reflect metabolic adaptations of cattle. Metabolic reconstruction also provides a platform for better understanding of metabolic regulation in cattle and ruminants. A substantial body of transcriptomics data from dairy and beef cattle under different nutritional management and across different stages of growth and lactation are already available and will aid in linking the genome with metabolism and nutritional physiology of cattle. Application of cattle genomics has great potential for future development of nutritional strategies to improve efficiency and sustainability of beef and milk production. One of the biggest challenges is to integrate genomic and phenotypic data and interpret them in a biological and practical platform. Systems biology, a holistic and systemic approach, will be very useful in overcoming this challenge.

Effect of diet on preweaning performance of Sahiwal calves

The objective was to study the growth potential of Sahiwal calves given milk or milk replacer with or without concentrates. For this purpose, forty-eight Sahiwal calves were divided into four groups of 12 animals each with equal sex ratio. In each group, the calves were offered either milk or a milk replacer (MR) at a rate of 10% of their body weight adjusted weekly. In addition to this, calves were fed either a starter ration plus Egyptian clover hay (SR + H) or hay only (H) until the end of trial. The milk or MR was withdrawn gradually from day 56 until animals were weaned completely by day 84. Calves offered milk grew faster than those offered MR (357 ± 9 vs. 162 ± 9 g/day; p < 0.05) and displayed higher weaning weights (51.6 ± 0.8 vs. 35.2 ± 0.8 kg; p < 0.05). The calves offered SR + H grew faster (311 ± 9 vs. 208 ± 9 g/day; p < 0.05) and displayed higher weaning weights (48.7 ± 0.8 vs. 38.1 ± 0.8 kg; p < 0.05) than those fed H alone. Calves offered milk plus SR + H showed the highest growth rate and weaning weights (401 ± 13 g/day and 56.3 ± 1 kg, respectively). The lowest growth rate and weaning weights were observed in calves offered MR and H only (115 ± 13 g/day and 30.3 ± 1 kg, respectively). Calves offered the MR had higher number of scour days than those offered milk (13.5 vs. 3.3). The feeding of whole milk in combination with the starter ration and hay resulted in superior growth rates, higher weaning weights, and healthier calves than the other feeding regimens.

Identification of differentially expressed proteins in ruminal epithelium in response to a concentrate-supplemented diet

Ruminal epithelium adapts to dietary change with well-coordinated alterations in metabolism, proliferation, and permeability. To further understand the molecular events controlling diet effects, the aim of this study was to evaluate protein expression patterns of ruminal epithelium in response to various feeding regimes. Sheep were fed with a concentrate-supplemented diet for up to 6 wk. The control group received hay only. Proteome analysis with differential in gel electrophoresis technology revealed that, after 2 days, 60 proteins were significantly modulated in ruminal epithelium in a comparison between hay-fed and concentrate-fed sheep (P < 0.05). Forty proteins were upregulated and 20 proteins were downregulated in response to concentrate diet. After 6 wk of this diet, only 14 proteins were differentially expressed. Among these, 11 proteins were upregulated and 3 downregulated. To identify proteins that were modulated by dietary change, two-dimensional electrophoresis was coupled with liquid chromatography electrospray ionization mass spectrometry. The differential expression of selected proteins, such as esterase D, annexin 5, peroxiredoxin 6, carbonic anhydrase I, and actin-related protein 3, was verified by immunoblotting and/or mRNA analysis. The identified proteins were mainly associated with functions related to cellular stress, metabolism, and differentiation. These results suggest new candidate proteins that may contribute to a better understanding of the signaling pathways and mechanisms that mediate rumen epithelial adaptation to high-concentrate diet.

Bovine rumen epithelium undergoes rapid structural adaptations during grain-induced subacute ruminal acidosis

Alterations in rumen epithelial structure and function during grain-induced subacute ruminal acidosis (SARA) are largely undescribed. In this study, four mature nonlactating dairy cattle were transitioned from a high-forage diet (HF; 0% grain) to a high-grain diet (HG; 65% grain). After feeding the HG diet for 3 wk, the cattle were transitioned back to the original HF diet, which was fed for an additional 3 wk. Continuous ruminal pH was measured on a weekly basis, and rumen papillae were biopsied during the baseline and at the first and final week of each diet. The mean, minimum, and maximum daily ruminal pH were depressed ( P < 0.01) in the HG period compared with the HF period. During the HG period, SARA was diagnosed only during week 1, indicating ruminal adaptation to the HG diet. Microscopic examination of the papillae revealed a reduction ( P < 0.01) in the stratum basale, spinosum, and granulosum layers, as well as total depth of the epithelium during the HG period. The highest ( P < 0.05) papillae lesion scores were noted during week 1 when SARA occurred. Biopsied papillae exhibited a decline in cellular junctions, extensive sloughing of the stratum corneum, and the appearance of undifferentiated cells near the stratum corneum. Differential mRNA expression of candidate genes, including desmoglein 1 and IGF binding proteins 3, 5, and 6, was detected between diets using qRT-PCR. These results suggest that the structural integrity of the rumen epithelium is compromised during grain feeding and is associated with the differential expression of genes involved in epithelial growth and structure.

Invited review: effects of milk ration on solid feed intake, weaning, and performance in dairy heifers

A feeding regimen that allows a smooth transition from milk to solid feed is vital for successful heifer-rearing programs. In the past, research efforts have focused on the development of feeding methods that allow early weaning, perhaps because the risk of disease is highest during the milk feeding stage. To encourage early intake of calf starter, conventional feeding programs have limited the supply of milk (often to 10% of BW at birth). However, dairy calves provided free access to milk will typically consume more than twice this amount. We critically review the available literature examining the relationship between milk feeding method, solid feed consumption, and rumen development in young dairy calves and identify areas where new work is required. We conclude that milk-fed dairy calves can safely ingest milk at approximately 20% of body weight (BW)/d, and greater milk consumption supports greater BW gain, improved feed efficiency, reduced incidence of disease, and greater opportunity to express natural behaviors, which in combination suggest improved welfare. Method of weaning greatly influences feed consumption, rumen development, and growth check in calves provided higher amounts of milk. Gradual weaning encourages starter intake during the preweaning period, and both weaning age and duration of weaning influence this consumption. Increased solid feed consumption during the weaning process contributes to rumen development, permitting higher starter intake and BW gain after weaning. Growth factors in milk may also enhance the growth and maturation of the gastrointestinal tract, but more research is required to understand the role of these factors. Greater nutrient supply through increased amount of milk appears to improve immune function and long-term performance of heifer calves; for example, reducing the age at first breeding and increasing first-lactation milk yield, but more research is needed to confirm these effects.

Rumen fermentation and histology in light lambs as affected by forage supply and lactation length

This study determined whether the rumen fermentation and histology traits may reflect the feeding strategy in light lambs (22-24 kg). Thirty-two single Rasa Aragonesa lambs were assigned to one of four treatments in a 2×2 factorial design. The factors were the inclusion of forage in the diet (alfalfa grazing vs. concentrate-fed indoors) and lactation length (weaning at 13 kg vs. suckling until slaughter). A multivariate canonical analysis discriminated individuals among feeding strategies. The main function differentiated weaned concentrate-fed lambs from the rest according to dorsal sac papillae height, ventral sac muscular layer thickness and the proportion of rumen valerate. The second function differentiated suckling concentrate-fed lambs from the rest according to plasma urea levels. Lactation length played an important role on rumen histology and protein utilization, especially in concentrate-fed lambs. Alfalfa grazing light lambs had similar rumen morphometric measures and fermentation characteristics, regardless of milk access.

Review: Effects of feeding sugars on productivity of lactating dairy cows

Oba, M. 2011. Review: Effects of feeding sugars on productivity of lactating dairy cows. Can. J. Anim. Sci. 91: 37–46. Sugars are water-soluble carbohydrates that are readily available in the rumen. Although sugars ferment faster than starch or fibre in the rumen, the rates of disaccharide hydrolysis and monosaccharide fermentation vary greatly depending on the type of sugar and rumen environment. Despite rapid fermentation in the rumen and their potential to provide greater fermentable energy to enhance microbial protein production, feeding sugars in place of dietary starch sources may not decrease rumen pH or improve N utilization efficiency and milk protein production in dairy cows. However, feeding high-sugar diets often increases dry matter intake, butyrate concentration in the rumen, and milk fat yield. These nutritional characteristics of sugars may allow us to use high-sugar feedstuffs as an alternative energy source for lactating dairy cows to increase dietary energy density with reduced risk of rumen acidosis, but there is little evidence in the literature to indicate that the synchrony of rumen fermentation would be enhanced by feeding high-sugar diets with high soluble protein. Greater butyrate production from feeding high-sugar diets is expected to enhance proliferation of gut tissues, but its physiological mechanisms and effects of butyrate metabolism on overall productivity of dairy cows warrant further investigations.

Rumen epithelial adaptation to high-grain diets involves the coordinated regulation of genes involved in cholesterol homeostasis

The molecular mechanisms underlying rumen epithelial adaption to high-grain (HG) diets are unknown. To gain insight into the metabolic mechanisms governing epithelial adaptation, mature nonlactating dairy cattle (n = 4) were transitioned from a high-forage diet (HF, 0% grain) to an HG diet (65% grain). After the cattle were fed the HG diet for 3 wk, they returned to the original HF diet, which they were fed for an additional 3 wk. Continuous ruminal pH, ruminal short chain fatty acids, and plasma β-hydroxybutyrate were measured on a weekly basis, and rumen papillae were biopsied from the ventral sac to assess alterations in mRNA expression profiles. The subacute form of ruminal acidosis was diagnosed during the first week of the HG period (4.6 ± 1.6 h/day <pH 5.6), but not during weeks 2 and 3, thereby indicating ruminal adaption to the HG diet. Changes in the mRNA expression profile of rumen papillae were initially examined using Bovine Affymetrix microarrays; a total of 521 differentially expressed genes (false discovery rate P < 0.08) were uncovered from the first to third week of the HG period. Ingenuity Pathway Analysis of microarray results revealed that enzymes involved in cholesterol synthesis were coordinately downregulated from the first to third week of the HG period. In addition, the LXR/RXR activation pathway was significant and included several genes involved in intracellular cholesterol homeostasis. The differential expression signature of eight genes representing the key regulatory points of cholesterol homeostasis was confirmed by quantitative real-time PCR. Based upon our pathway and network results we propose a model to explain cellular events during rumen epithelial adaptation to HG diets and thus provide molecular targets that may be useful in the treatment and prevention of ruminal acidosis.

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.

Physiology, regulation and multifunctional activity of the gut wall: a rationale for multicompartmental modelling

A rationale is given for a modelling approach to identify the mechanisms involved in the functioning and metabolic activity of tissues in the wall of the gastrointestinal tract. Maintenance and productive functions are discussed and related to the distinct compartments of the gastrointestinal tract and the metabolic costs involved. Functions identified are: tissue turnover; tissue proliferation; ion transport; nutrient transport; secretions of digestive enzymes, mucus and immunoglobulins; production of immune cells. The major nutrients involved include glucose, amino acids and volatile fatty acids.In vivomeasurements of net portal fluxes of these nutrients in pigs and ruminants are evaluated to illustrate the complexity of physiology and metabolic activity of the gastrointestinal tract. Experimental evidence indicates that high, but variable and specific, nutrient costs are involved in the functioning of the gastrointestinal tract.

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.

Effects of mild heat stress and grain challenge on acid-base balance and rumen tissue histology in lambs1

The effect of heat stress (HS) and grain challenge (GC) on acid-base balance and rumen tissue histology in lambs was investigated using 24 yearling wether lambs (58 +/- 4.5 kg of BW) in a 2 x 2 factorial experiment with repeated measures for day (10, 14, and 17) of sampling. The factors were temperature [thermoneutral zone (TN) vs. HS] and diet (control vs. GC). Lambs were blocked by BW and assigned to 1 of 4 treatments in temperature-controlled rooms: 1) TN (temperature = 18 to 20 degrees C; relative humidity = 30%; 2) TN + GC; 3) HS (temperature = 35 degrees C for 9 h/d, 20 degrees C for 15 h/d; relative humidity = 40%); and 4) HS + GC. Venous blood samples were collected at 1800 on the first day of GC (d 10), in the middle of GC (d 14), and at the end of the trial (d 17) by jugular venipuncture and analyzed for pH, gases, hematocrit, plasma ions, and total protein. After all measurements in live animals were taken on d 17, lambs were slaughtered, and tissue samples were obtained from the ventral sac of the rumen for histological assessment. Except for the concentration of plasma glucose (P = 0.04) and total protein (P < 0.01), there were no (P > 0.05) diet x temperature interactions. With HS, the concentration of Na+ and Cl- in the control group decreased at d 14 and then increased by d 17, and respiration rates in the control group decreased linearly (P < 0.05). Compared with the control group, respiration rates and the concentration of Cl- in the GC lambs increased linearly over time, whereas the concentration of Na+ decreased linearly (P < 0.05) across time. Under HS, the partial pressure of carbon dioxide, total carbon dioxide, the partial pressure of oxygen and oxygen saturation, and the concentration of Mg2+, glucose, and HCO3- showed quadratic (P < 0.05) responses with time. In both treatments, DMI, base excess of extracellular fluid, base excess of blood, and standard bicarbonate increased linearly (P < 0.05), and hematocrit, plasma protein, Ca2+, anion gap, and plasma strong ion difference decreased linearly (P < 0.05) across day. Compared with the control group, the GC group had decreased papillae count in the ruminal ventral sac (1.3 vs. 1.5; P < 0.05). These results suggest that under HS the acidifying effects of GC on acid-base balance in lambs were counteracted in the short-term through respiratory adaptation.

Intraruminal infusion of n-butyric acid induces an increase of ruminal papillae size independent of IGF-1 system in castrated bulls

The objective of this study was to explore morphological alterations of rumen papillae induced by n-butyric acid in relation to the insulin-like growth factor (IGF) system in adult castrated bulls. Three animals fitted with rumen cannula were fed twice daily at a low and high nutritional level (LL and HL), i.e., at 1.1 x maintenance (M) and 1.6 x M, respectively. Diets contained artificial dried grass and concentrate (74:26 and 52:48). Bulls received no (B0) or daily intraruminal infusions of 500 g n-butyric acid (B500) over 14 d. The infusion started 1 h after the morning feeding (9:00) and lasted for 3.5 h. Thus, four treatments (BOLL, B500LL, BOHL, and B500HL) were compared. Blood and rumen mucosa samples from the atrium ruminis were taken at the last day of each period. Length, width and surface of rumen papillae were greater (p < 0.001) in BOHL than in BOLL. Treatment with n-butyric acid resulted in an increase of the papillae surface of 20-40% (p = 0.047) for both nutritional levels as compared to periods without n-butyric acid treatments. The higher nutritional level and intraruminal n-butyric acid infusion induced epithelial cell death. The percentage of proliferative cells was doubled by n-butyric acid treatment. The mRNA of IGF-1 and IGF type 1 receptor (IGF-1R), as well as IGF-1R binding capacity were unaffected by butyric acid treatments. The abundance of IGF-1 mRNA tended to be lower (p = 0.1) and IGF-1R abundance was lower (p = 0.03) in response to the HL. The plasma IGF-1 concentration was lower with butyric acid treatment (p < 0.01), but was unaffected by the nutritional level. In conclusion, under described experimental preconditions of daily short-time intraruminal n-butyric acid infusion alterations of rumen papillae morphology is not mediated by ruminal IGF type 1 receptor and by local IGF-1 expression in papillae in castrated bulls.

Adrenoceptor heterogeneity in the ruminal epithelium of sheep

The pre-gastric rumen of sheep plays a crucial role in the fermentation of nutrients and in the absorption of nutrients and minerals. Adrenaline has been shown previously to increase ruminal absorption of glucose and water. The present study was intended to elucidate whether ruminal ion transport is also altered by adrenaline. In Ussing chambers, changes of I(sc) were recorded in isolated ovine ruminal epithelia after the serosal additions of adrenoceptor agonists or antagonists. I(sc) increased after the addition of adrenaline (10(-4) M) or clonidine (alpha2-agonist, 10(-4) M), but decreased after the addition of isoproterenol (beta-agonist, 10(-4) M) or terbutaline (beta2-agonist, 10(-5) M). The effect of adrenaline on I(sc) was augmented by the adrenoceptor antagonists prazosin (alpha(1), 10(-4) M) and bupranolol (beta, 10(-6) M), but inversed by yohimbine (alpha(2), 10(-5) M). Adrenaline induced an increase in Na+ net flux across the epithelium that was larger than the increase in equivalent current flow. It is concluded that adrenaline differentially regulates ion transport across the ruminal epithelium via alpha1-, alpha2-, and beta2-receptors. The main effect is a stimulation of electroneutral and electrogenic Na+ absorption. This stimulated Na+ absorption might be causative of increased water absorption from the rumen as described previously.

Functional organization of the bovine rumen epithelium

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

Sodium Transport Across the Isolated Epithelium of Sheep Omasum is Influenced by Luminal Ammonia

Ammonia is a physiological fermentation product in the forestomachs and is absorbed from the rumen and omasum. Cellular uptake of ammonia affects the intracellular pH of polar and non-polar cells. The effect of the uptake on the pH of the cytosol depends on the predominant form of ammonia. NH(3) uptake and its intracellular protonation tend to alkalinize the cytoplasm, whereas the uptake of NH(4)(+) acidifies the cytoplasm by reversing this reaction. Consequently, the absorption of ammonia across the omasal epithelium could cause a change of the intracellular pH and pH-dependent transport mechanisms like Na/H exchange. Because no information is available about the form of ammonia absorbed in the omasum and, hence, possible modulation of Na transport by ammonia, the effect of increasing luminal ammonia concentrations (0, 5, 15 and 30 mmol/l) on Na transport were studied. In epithelia of hay-fed animals, ammonia linearly inhibited Na transport in a dose-dependent manner, at a luminal pH of 7.40, but not at a pH of 6.40. Ammonia did not influence Na transport in epithelia of concentrate-fed animals. Because luminal ammonia did not consistently change the short circuit current or tissue conductance absorption of ammonia as NH(4)(+) appears to be unlikely. The predominant form of ammonia absorbed in the omasum is probably NH(3), which is protonated in the cytosol. The reduced availability of protons may be the cause of inhibition of Na transport via Na/H exchange.