Effect of experimentally increased protein supply to postpartum dairy cows on plasma protein synthesis, rumen tissue proliferation, and immune homeostasis

The effect of experimentally increasing the postpartum protein supply on plasma protein synthesis, rumen tissue proliferation, and immune homeostasis was studied using 8 periparturient Holstein cows in a complete randomized design. At calving, cows were assigned to abomasal infusion of water (CTRL) or casein (CAS) in addition to a lactation diet. Casein infusion was gradually decreased from 696 ± 1 g/d at +2 d relative to calving (DRTC) to 212 ± 10 g/d at +29 DRTC to avoid excessive supply. Synthesis rate of plasma proteins was measured at -14, +4, +15, and +29 DRTC by measuring [C]Phe isotopic enrichment in arterial plasma free Phe, total plasma proteins, and albumin after 3, 5, and 7 h of jugular ring[C]Phe infusion. Plasma volume was determined at +4 and +29 DRTC by dilution of a [I]BSA dose. Synthesis rate of tissue protein in biopsied rumen papillae was determined by measuring [C]Phe isotopic enrichment, and mRNA expression of selected genes was measured by real-time qPCR. Total and differential leukocyte counts were performed and immune responsiveness of monocytes was evaluated by tumor necrosis factor ɑ (TNFɑ) concentration on ex vivo whole blood stimulation with Escherichia coli lipopolysaccharide (LPS) and responsiveness of T-lymphocytes by interferon γ (IFNγ) concentration on stimulation with Staphylococcus aureus enterotoxin β (SEB). Further, ELISA plasma concentrations of IgM, IgA, and IgG were determined. The DRTC affected the majority of investigated parameters as expected. The CAS treatment increased milk protein yield (P = 0.04), and tended to lower TNFɑ (P = 0.06), and lowered IFNγ (P = 0.03) responsiveness per monocyte and lymphocyte, respectively, compared with CTRL. Further, fractional synthesis rate of albumin was greater at +4 DRTC for CAS compared with CTRL but did not differ by +29 DRTC (interaction: P = 0.01). In rumen papillae, synthesis rate of tissue protein was greater for CAS compared with CTRL (P < 0.01) and mRNA expression of genes for cell proliferation tended to be or were greater for CAS compared with CTRL (P ≤ 0.07). In conclusion, increased postpartum protein supply seem to enhance vital body functions as interpreted from increased liver synthesis of albumin, increased rumen papillae proliferation, and stabilized the ex vivo inflammatory responsiveness of leukocytes. Further studies are needed to enlighten the importance of increased postpartum protein supply in periparturient cows.

Impact of silage additives on aerobic stability and characteristics of high-moisture maize during exposure to air, and on fermented liquid feed

AIMS

To (i) measure the aerobic stability- and describe the characteristics, during aeration, of high-moisture maize (HMM) treated with various additives, and (ii) describe the microbial characteristics of fermented liquid feed (FLF) added HMM.

METHODS AND RESULTS

Four treatments were prepared with each of three HMM samples: (i) The HMM as is (CONTROL); and the control added (ii) acids (ACID); (iii) heterofermentative lactic acid bacteria (HETERO); or (iv) homofermentative lactic acid bacteria (HOMO). After ensiling, aerobic stability was measured (Aim 1) and FLF prepared (Aim 2). The ACID treatment improved the aerobic stability of samples 1 and 3 from 9 to 14 h in the CONTROL to 67-115 h. All additives improved aerobic stability of sample 3 from 32 h in the CONTROL to 104-168 h. No proliferation of Enterobacteriacaea was detected during incubation of FLF.

CONCLUSION

The microbial profile during aeration- and impact of additives on the aerobic stability of HMM depended on the characteristics of the samples. No blooming of Enterobacteriaceae was observed in FLF containing c. 20 g HMM 100 g(-1) .

SIGNIFICANCE AND IMPACT OF THE STUDY

The impact of silage additives on aerobic stability of HMM should be tested in samples with varying characteristics. Inclusion of HMM could be a way of improving biosafety of FLF.

Ergot alkaloids from endophyte-infected tall fescue decrease reticuloruminal epithelial blood flow and volatile fatty acid absorption from the washed reticulorumen

An experiment was conducted to determine if ergot alkaloids affect blood flow to the absorptive surface of the rumen. Steers (n=8) were pair-fed alfalfa cubes and received ground endophyte-infected (Neotyphodium coenophialum) tall fescue (Lolium arundinaceum; E+) seed (0.015 mg ergovaline·kg BW(-1)·d(-1)) or endophyte-free tall fescue (E-) seed via the rumen cannula 2x daily for 7 d at thermoneutral (TN; 22°C) and heat stress (HS; 32°C) conditions. On d 8, the rumen was emptied and rinsed. A buffer containing VFA was incubated in the following sequence: control (CON), 15 μg ergovaline·kg BW(-1) (1×EXT) from a tall fescue seed extract, and 45 μg ergovaline·kg BW(-1) (3×EXT). For each buffer treatment there were two 30-min incubations: a 30-min incubation of a treatment buffer with no sampling followed by an incubation of an identical sampling buffer with the addition of Cr-EDTA and deuterium oxide (D2O). Epithelial blood flow was calculated as ruminal clearance of D2O corrected for influx of physiological water and liquid outflow. Feed intake decreased with dosing E+ seed at HS but not at thermoneutral conditions (TN; P<0.02). Dosing E+ seed decreased serum prolactin (P<0.005) at TN. At HS, prolactin decreased in both groups over the 8-d experiment (P<0.0001), but there was no difference in E+ and E- steers (P=0.33). There was a seed treatment×buffer treatment interaction at TN (P=0.038), indicating that E+ seed treatment decreased reticuloruminal epithelial blood flow at TN during the CON incubation, but the two groups of steers were not different during 1×EXT and 3×EXT (P>0.05). Inclusion of the extract in the buffer caused at least a 50% reduction in epithelial blood flow at TN (P=0.004), but there was no difference between 1×EXT and 3×EXT. There was a seed × buffer treatment interaction at HS (P=0.005), indicating that the reduction of blood flow induced by incubating the extract was larger for steers receiving E- seed than E+ seed. Volatile fatty acid flux was reduced during the 1×EXT and 3×EXT treatments (P<0.01). An additional experiment was conducted to determine the effect of time on blood flow and VFA flux because buffer sequence could not be randomized. Time either increased (P=0.05) or did not affect blood flow (P=0.18) or VFA flux (P>0.80), indicating that observed differences are due to the presence of ergot alkaloids in the rumen. A decrease in VFA absorption could contribute to the signs of fescue toxicosis including depressed growth and performance.

Effects of β-hydroxy β-methyl butyrate supplementation to sows in late gestation on absorption and hepatic metabolism of glucose and amino acids during transition

A multicatheter sow model was established to study the effects of dietary β-hydroxy β-methyl butyrate (HMB) supplementation on net portal flux (NPF) and net hepatic flux (NHF) of HMB, glucose, and the AA Ala, Gly, Ile, Leu, Phe, Tyr, and Val. Eight second parity sows were fitted with permanent indwelling catheters in an artery and in the portal, hepatic, and mesenteric veins. Eight hourly sets of blood samples were taken starting 30 min before the morning meal on day -3 and day 3 relative to parturition. Four control (CON) sows were fed a standard lactation diet from day -15 and throughout the experiment, and 4 HMB sows were fed the control diet supplemented with 15 mg Ca(HMB)(2)/kg BW mixed in one third of the morning meal from day -10 until parturition. Net portal flux of HMB was affected by treatment (Trt; P < 0.01) and peaked in the HMB sows at 6.9 mmol/h 30 min after the morning meal and then decreased towards preprandial level (0.0 mmol/h) 3.5 h after the meal, revealing that dietary HMB was rapidly absorbed from the intestine. The NHF of HMB tended to be affected by Trt (P = 0.06) showing a small hepatic uptake of HMB (1.1 mmol/h) in HMB sows. Net portal flux of glucose and all measured AA, except for Gly and Tyr, were affected the Trt × time interaction (P < 0.01). The NPF was positive for all nutrients, indicating absorption from the intestine to the portal blood. Absorption rates appeared to be more stable for HMB than for CON sows. Net hepatic flux of glucose was not affected by Trt. It was negative from 1.5 to 2.5 h after the meal, indicating hepatic uptake, but positive before and after, indicating net hepatic release of glucose. Net hepatic fluxes of AA were negative and were not affected by Trt (P > 0.10), except for Phe (P < 0.05). In conclusion, HMB reduced the variation in net portal flux of glucose and AA during 8 h of blood sampling and suggest that the improved sow productivity observed by others may be due to a more uniform nutrient absorption pattern into portal blood.

Metabolic profiling of plasma from sows before parturition and during lactation using a liquid chromatography-mass spectrometry-based approach

During transition from late gestation to lactation, the sow undergoes large and sudden metabolic changes to adapt from anabolic to catabolic metabolism. Little is known about changes in nutrient uptake and intermediary metabolism of transition sows. This study was undertaken to screen the metabolic profile for qualitative changes in nutrient uptake and metabolism during transition. Four sows were fitted with permanent catheters in artery femoralis (AF), portal vein (PV), and hepatic vein (HV) (sampling sites). Sows were fed a standard lactation diet from 15 d prior to 28 d after parturition. Blood samples were taken 1.5 h after feeding on days -10, -3, 3, and 17 relative to parturition and plasma metabolites were analyzed by a liquid chromatography-mass spectrometry-based approach. Principal components analysis was performed to visualize the metabolic profiles and to screen for intermediary metabolites altered during the transition period. The metabolic profile of sows on day 3 after parturition was distinct from other days. Plasma betaine, Pro, and some unidentified lipid compounds contributed to the separation on day 3; betaine and Pro were lowered by 30% at day 3 compared to day -10 and day -3 (P < 0.001). Plasma choline, Pro, creatine, and unidentified lipid compounds contributed to the separation due to sampling sites. Plasma choline was lowest in HV, intermediate in AF, and highest in PV (P < 0.001) plasma, indicating net absorption from the gastrointestinal tract (PV vs. AF) and liver metabolism (HV vs. PV). The majority of unidentified metabolites found using the loadings plots that were affected by day or sampling site or both were revealed as lipid compounds, that is, bile acid, cholesterol, glycerol, phosphatidyl, sphingomyelin, or acylglycerol derivatives. In conclusion, the intermediary metabolism of sows, especially for fat, changed during transition, and a deeper understanding and detection of involved metabolites are needed to optimize sow feeding during transition.

Effect of abomasal infusion of oligofructose on portal-drained visceral ammonia and urea-nitrogen fluxes in lactating Holstein cows

The effects of abomasal infusion of oligofructose in lactating dairy cows on the relationship between hindgut fermentation and N metabolism, and its effects on NH(3) absorption and transfer of blood urea-N across the portal-drained viscera versus ruminal epithelia were investigated. Nine lactating Holstein cows fitted with ruminal cannulas and permanent indwelling catheters in major splanchnic blood vessels were used in an unbalanced crossover design with 14-d periods. Treatments were continuous abomasal infusion of water or 1,500 g/d of oligofructose. The same basal diet was fed with both treatments. Eight sample sets of arterial, portal, hepatic, and ruminal vein blood, ruminal fluid, and urine were obtained at 0.5h before the morning feeding and at 0.5, 1.5, 2.5, 3.5, 4.5, 5.5, and 6.5 h after feeding. It was hypothesized that an increased supply of fermentable substrate to the hindgut would increase the uptake of urea-N from blood to the hindgut at the expense of urea-N uptake to the forestomach. The study showed that abomasal oligofructose infusion decreased the total amount of urea-N transferred from the blood to the gut, NH(3) absorption, and arterial blood urea-N concentration. Subsequently, hepatic NH(3) uptake and urea-N production also decreased with oligofructose infusion. Additionally, urea-N concentration in milk and urinary N excretion decreased with oligofructose treatment. The oligofructose infusion did not affect ruminal NH(3) concentrations or any other ruminal variables, nor did it affect ruminal venous - arterial concentration differences for urea-N and NH(3). The oligofructose treatment did not affect milk yield, but did decrease apparent digestibility of OM, N, and starch. Nitrogen excreted in the feces was greater with the oligofructose infusion. In conclusion, the present data suggest that increased hindgut fermentation did not upregulate urea-N transfer to the hindgut at the expense of urea-N uptake by the rumen, and the observed reduction in arterial blood urea-N concentration appeared not to be due to increased urea-N transport, but rather could be explained by reduced NH(3) input to hepatic urea-N synthesis caused by increased sequestration of NH(3) in the hindgut and excretion in feces. Increasing the hindgut fermentation in lactating dairy cows by abomasal infusion of 1,500 g/d of oligofructose shifted some N excretion from the urine to feces and possibly reduced manure NH(3) volatilization without impairing rumen fermentation.

Metabolic effects of feeding ethanol or propanol to postpartum transition Holstein cows

Eight lactating Holstein cows implanted with a ruminal cannula and permanent indwelling catheters in major splanchnic blood vessels were used to investigate metabolism of propanol and ethanol in the postpartum transition period. Cows were randomly allocated to 1 of 4 treatments in a randomized design with a 2 by 2 factorial arrangement of treatments. Factor 1 was 2.6g of calcium carbonate/kg of dry matter (DM) versus 1.5 g of 2-hydroxy-4-(methylthio)-butanoic acid isopropyl ester/kg of DM. Factor 2 was supplementation with 14 g of propanol/kg of DM (propanol treatment; PT) versus 14 g of ethanol/kg of DM (ethanol treatment; ET). Only factor 2 data are presented in the present paper. Treatments were administered in silage-based total mixed rations and cows were fed the experimental total mixed ration from the day of parturition. Daily rations were fed in 3 equally sized portions at 8-h intervals. Eight hourly sets of ruminal fluid, arterial, and hepatic portal and hepatic vein samples were collected at day -15 ± 5, 4, 15, and 29 relative to parturition. Dry matter intake and milk yield increased with days in milk (DIM), but were not affected by treatment. From prepartum to 4 DIM ruminal concentrations of propanol and ethanol increased with PT and ET, respectively. Postpartum, alcohol intake increased 49% in PT and 34% in ET from 4 to 29 d in milk, respectively. Ruminal concentrations of the alcohols remained unaffected by DIM. Treatments did not affect total ruminal volatile fatty acid concentrations, but the molar proportion of acetate increased in ET and the molar proportion of propionate increased in PT compared with the contrasting treatment. Propanol treatment decreased milk fat content at 15 to 29 DIM compared with ET. The net portal release of propanol and ethanol increased with increasing ruminal concentration of the respective alcohol. The portal release of alcohol accounted for 43 to 85% of ingested propanol and 36 to 57% of ingested ethanol. Hepatic uptake of propanol and ethanol equaled the net portal flux and no effect of treatment was detected for net splanchnic release of propanol and ethanol. In conclusion, ruminal metabolism is a major component of alcohol metabolism in dairy cows. The postpartum transition dairy cow has sufficient metabolic capacity to cope with high dietary concentrations of primary alcohols even when alcohol intake is abruptly increased at the day of calving. Alcohol intake affects milk fat content and alcohol composition of silage might be important to improve predictions of milk composition.

Short communication: Effects of dietary nitrogen concentration on messenger RNA expression and protein abundance of urea transporter-B and aquaporins in ruminal papillae from lactating Holstein cows

To test the hypothesis that dietary N concentrations affect gut epithelial urea transport by modifying the expression of urea transporter B (UT-B) and aquaporins (AQP), the mRNA expression and protein abundance of UT-B and AQP3, AQP7, AQP8, and AQP10 were investigated in ruminal papillae from 9 lactating dairy cows. Ruminal papillae were harvested from cows fed low N (12.9% crude protein) and high N (17.1% crude protein) diets in a crossover design with 21-d periods. The mRNA expression was determined by real-time reverse transcription-PCR and protein abundance by immunoblotting. The mRNA expression of UT-B was not affected by dietary treatment, whereas mRNA expression of AQP3, 7, and 10 were greater in the high N compared with the low N fed cows. Using peptide-derived rabbit antibodies to cow AQP3, 7, and 8, immunoblotting revealed bands of approximately 27, 27, and 24 kDa in ruminal papillae, respectively. A peptide-derived chicken antibody to cow UT-B detected a band of approximately 30 to 32 kDa in ruminal papillae. The abundance of UT-B and AQP3 and 7 were not affected by dietary treatment. In contrast, the abundance of AQP8 was greater in high N compared with low N diets. In conclusion, AQP3, 7, and 8 were found to be expressed in bovine rumen papillae. None of the investigated transcripts or proteins correlated to the increased rumen epithelial urea permeability observed with low dietary N concentration.

Technical note: ruminal cannulation technique in young Holstein calves: effects of cannulation on feed intake, body weight gain, and ruminal development at six weeks of age

Ruminal cannulation techniques are frequently used to study fermentation in the ruminant forestomach. Unsatisfactory results with the traditionally applied procedure for cannulation of young calves stimulated the development of a simpler and more robust procedure; this procedure was tested for effects on performance traits and gross anatomy of the gastrointestinal tract compared with a control group not undergoing surgery. Five calves were ruminally cannulated at approximately 10 d of age and 5 matching calves were used as controls. All calves were fed milk replacer and a diet based on clover grass silage and sodium hydroxide-treated wheat. Ruminal fluid was collected from cannulated calves once weekly for 3 consecutive weeks. All calves were euthanized at 43+/-3 d of age. No apparent adverse effects of cannulation were observed. Feed intake, BW gain, and gross anatomy of the gastrointestinal tract were not affected by cannulation. Minimum ruminal pH increased with sampling week, but average ruminal pH, total volatile fatty acids concentration, and volatile fatty acids proportions were not affected by sampling week. In conclusion, the implemented surgical technique was found to have no major effect on apparent animal health and performance traits, and the cannula proved useful for multiple samplings of ruminal contents in young calves.

Ruminal and intermediary metabolism of propylene glycol in lactating Holstein cows

Four lactating Holstein cows fitted with ruminal cannulas and permanent indwelling catheters in the mesenteric artery, mesenteric vein, hepatic portal vein, and hepatic vein were used in a cross-over design to study the metabolism of propylene glycol (PG). Each cow received 2 treatments: control (no infusion) and infusion of 650 g of PG into the rumen at the time of the morning feeding. Propylene glycol was infused on the day of sampling only. Samples of arterial, portal, and hepatic blood as well as ruminal fluid were obtained at 0.5 h before feeding and at 0.5, 1.5, 2.5, 3.5, 5, 7, 9, and 11 h after feeding. Infusion of PG did not affect ruminal pH or the total concentration of ruminal volatile fatty acids, but did decrease the molar proportion of ruminal acetate. The ruminal concentrations of PG, propanol, and propanal as well as the molar proportion of propionate increased with PG infusion. The plasma concentrations of PG, ethanol, propanol, propanal, glucose, L-lactate, propionate, and insulin increased with PG and the plasma concentrations of acetate and beta-hydroxybutyrate decreased. The net portal flux of PG, propanol, and propanal increased with PG. The hepatic uptake of PG was equivalent to 19% of the intraruminal dose. When cows were dosed with PG, the hepatic extraction of PG was between 0 and 10% depending on the plasma concentration of PG, explaining the slow decrease in arterial PG. The increased net hepatic flux of L-lactate with PG could account for the entire hepatic uptake of PG, which suggests that the primary hepatic pathway for PG is oxidation to l-lactate. The hepatic uptake of propanol increased with PG, but no effects of PG on the net hepatic and net splanchnic flux of glucose were observed. Despite no effect of PG on net portal flux and net hepatic flux of propionate, the net splanchnic flux of propionate increased and the data suggest that propionate produced from hepatic metabolism of propanol is partly released to the blood. The data suggest that PG affects metabolism of the cows by 2 modes of action: 1) increased supply of l-lactate and propionate to gluconeogenesis and 2) insulin resistance of peripheral tissues induced by increased concentrations of PG and propanol as well as a decreased ratio of ketogenic to glucogenic metabolites in arterial blood plasma.

Effect of Milk Allowance on Concentrate Intake, Ruminal Environment, and Ruminal Development in Milk-Fed Holstein Calves

The aim of the present experiment was to test the hypothesis that a barley-based concentrate would induce an acidic ruminal environment in young calves and that increased milk allowance would alleviate this condition. Eight Holstein calves ruminally cannulated at d 7 +/- 1 of age were used to study the effect of variation in barley-based starter concentrate intake induced by 4 different milk allowances (3.10, 4.84, 6.60, and 8.34 kg of milk replacer/d; 123 g of dry matter/kg of milk) on the ruminal environment, blood variables, and fore-stomach development from wk 2 to 5 of age. Twelve ruminal fluid samples were collected during a weekly 24-h sampling in 4 consecutive weeks. Blood samples were collected by venipuncture between 1200 and 1300 h on ruminal sampling days. Rumen papillae development and visceral organ mass were recorded at slaughter. A linear treatment x week effect was observed for concentrate intake, with the calves fed the lowest milk allowance having the fastest increase in concentrate intake whereby these calves reached the same ME intake in wk 5 compared with calves with the highest milk allowance. Effects on ruminal variables were dominated by week of sampling, with minor differences among treatments. Ruminal pH was below 5.5 for 5 to 13 h/d and all calves with concentrate intake above 20 g of dry matter/d were observed to have a daily ruminal pH minimum at pH 5.5 or lower. The ruminal concentration of total volatile fatty acids (VFA) increased from 71 to 133 +/- 9 mmol/L in wk 2 to 5 and was characterized by a relatively high molar proportion of propionate, increasing from 34 to 40 mol/100 mol of VFA in wk 2 to 5. In addition, the presence of ethanol and propanol as well as numerous VFA esters points to a ruminal environment with a relatively high hydrogen pressure. Plasma glucose and insulin responded to the highest milk allowance in wk 2 to 4. Plasma VFA and ketone bodies increased with the lowest milk allowance in wk 4 to 5. At slaughter, empty wet weights of the rumen + reticulum and omasum as well as mass of digesta in these compartments were found to decrease linearly and perirenal fat was found to increase linearly with milk allowance, indicating that the milk allowance changed the body composition of the calves. Lengths of ruminal papillae in the atrium and ventral ruminal sac were not affected by treatment. We concluded that the ruminal environment of young calves fed a barley-based starter concentrate was characterized by a low ruminal pH and high VFA concentration regardless of the milk allowance.

Splanchnic metabolism of volatile fatty acids in the dairy cow

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

Metabolism of Silage Alcohols in Lactating Dairy Cows

Dairy cows fed silage are subjected to various alcohols and low molecular weight esters. Four lactating Holstein cows fitted with ruminal cannulas and permanent indwelling catheters in the hepatic portal vein, hepatic vein, mesenteric vein, and mesenteric artery were used to study the absorption of alcohols into portal blood and the metabolism of feed alcohols in the rumen and splanchnic tissues. The cows were allocated to 4 experimental treatments in a Latin square design. All treatments were formulated as total mixed rations with the same overall nutrient composition, differing by the source of corn silage. Treatments were a control silage and 3 qualities of problematic corn silage (silage with Fusarium toxin, Penicillium-infected silage, and silage with a high propanol content). Feeding was followed by a decreasing ruminal pH, as well as decreasing molar proportions of ruminal acetate and isobutyrate. The ruminal concentrations of total VFA, ethanol, propanol, 2-butanol, ethyl acetate, propyl acetate, glucose, and L-lactate, and molar proportions of propionate, butyrate, isovalerate, valerate, and caproate increased after feeding. Treatments affected ruminal concentrations of propanol, propyl acetate, and butyrate and a strong correlation was observed between ruminal propyl acetate and the molar proportion of butyrate (r = -0.79). Arterial concentrations of ethanol, propanol, propanal, acetone (sum of acetone and acetoacetate), 3-hydroxybutyrate, L-lactate, glutamate, and glutamine increased, and the arterial concentration of glucose decreased after feeding, but no effects of treatment were observed for arterial variables. The postprandial increase in arterial ethanol was maintained for 5 h. The net portal release of ethanol tended to decrease with the treatment with the lowest ethanol content, and the net splanchnic release of ethanol increased after feeding, but overall, the net splanchnic flux of ethanol was not different from zero, in agreement with the liver being the major organ for alcohol metabolism. The net portal flux and net hepatic flux of propanol were affected by treatment. All dietary ethanol and propanol were accounted for by absorption of the respective alcohol into the portal blood. The hepatic extraction ratios of ethanol and propanol were, on average, 63 to 66%, and no indications of saturation of hepatic alcohol metabolism were observed at any time. We concluded that typical amounts of alcohols in corn silage do not interfere with splanchnic metabolism of any of the measured variables and do not saturate hepatic pathways for alcohol metabolism. However, even low concentrations of alcohols in feed might affect ruminal metabolism and are followed by hours of elevated peripheral blood concentrations of alcohols.

Effect of increasing ruminal butyrate absorption on splanchnic metabolism of volatile fatty acids absorbed from the washed reticulorumen of steers

Four steers fitted with a ruminal cannula and chronic indwelling catheters in the mesenteric artery, mesenteric vein, hepatic portal vein, hepatic vein, and the right ruminal vein were used to study the absorption and metabolism of VFA from bicarbonate buffers incubated in the temporarily emptied and washed reticulorumen. Portal and hepatic vein blood flows were determined by infusion of p-aminohippurate into the mesenteric vein, and portal VFA fluxes were calibrated by infusion of isovalerate into the ruminal vein. The steers were subjected to four experimental treatments in a Latin square design with four periods within 1 d. The treatments were Control (bicarbonate buffer) and VFA buffers containing 4, 12, or 36 mmol butyrate/kg of buffer, respectively. The acetate content of the buffers was decreased with increasing butyrate to balance the acidity. The butyrate absorption from the rumen was 39, 111, and 300 +/- 4 mmol/h for the three VFA buffers, respectively. The ruminal absorption rates of propionate (260 +/- 12 mmol/h), isobutyrate (11.4 +/- 0.7 mmol/h), and valerate (17.3 +/- 0.7 mmol/h) were not affected by VFA buffers. The portal recovery of butyrate and valerate absorbed from the rumen increased (P < 0.01) with increasing butyrate absorption and reached 52 to 54 +/- 4% with the greatest butyrate absorption. The liver responded to the increased butyrate absorption with a decreasing fractional extraction of propionate and butyrate, and with the greatest butyrate absorption, the splanchnic flux was 22 +/- 1% and 18 +/- 1% of the absorbed propionate and butyrate, respectively. The increased propionate and butyrate release to peripheral tissues was followed by increased (P < 0.05) arterial concentrations of propionate (0.08 +/- 0.01 mmol/kg) and butyrate (0.07 +/- 0.01 mmol/kg). Arterial insulin concentration increased (P = 0.01) with incubation of VFA buffers compared with Control and was numerically greatest with the greatest level of butyrate absorption. We conclude that the capacity to metabolize butyrate by the ruminal epithelium and liver is limited. If butyrate absorption exceeds the metabolic capacity, it affects rumen epithelial and hepatic nutrient metabolism and affects the nutrient supply of peripheral tissues.

Effects of adding valerate, caproate, and heptanoate to ruminal buffers on splanchnic metabolism in steers under washed-rumen conditions1

Four steers fitted with a ruminal cannula and chronic indwelling catheters in the mesenteric artery, mesenteric vein, hepatic portal vein, hepatic vein, and the right ruminal vein were used to study VFA absorption from bicarbonate buffers incubated in the washed reticulorumen, and metabolism by splanchnic tissues. Portal and hepatic vein blood flows were determined by infusion of p-aminohippurate into the mesenteric vein. The steers were subjected to four experimental treatments in a Latin square design. The treatments were Control (ruminal bicarbonate buffer with [mmol/kg]: acetate = 72; propionate = 30; isobutyrate = 2.1; butyrate = 12; valerate = 1.2; caproate = 0; and heptanoate = 0); Val (same as control except for valerate = 8 mmol/kg); Cap (same as control except for caproate = 3.5 mmol/kg); and Hep (same as control except for heptanoate = 3 mmol/kg). All buffers were incubated for 90 min in the rumen, and ruminal VFA absorption rates were maintained by continuous intraruminal infusion of VFA. The arterial concentrations of valerate and heptanoate showed a small increase (< or = 1 micromol/L; P < 0.05) with inclusion of the respective acid in the ruminal buffer, but no change (P = 0.57) in arterial concentration of caproate was detected. Valerate increased (P < 0.05) the net portal flux of butyrate and valerate, as well as the net splanchnic flux of propionate, butyrate, and valerate. With Cap and Hep, the net portal flux of caproate and heptanoate accounted for 54 and 45% of ruminal disappearance rates, respectively, indicating that these acids were extensively metabolized by the ruminal epithelium. Caproate was ketogenic both in the ruminal epithelium and in the liver, and Cap increased (P < 0.05) the arterial concentration, ruminal vein minus arterial concentration difference, net hepatic flux, and net splanchnic flux of 3-hydroxybutyrate. The net hepatic flux of glucose decreased (P = 0.02) with Cap and Hep compared with Control and Val; however, no effect (P = 0.14) on the net splanchnic flux of glucose could be detected. We conclude that the strong biological activity of valerate, caproate, and heptanoate warrant increased emphasis on monitoring their ruminal presence and their potential systemic effects on ruminant metabolism.

An evaluation of parameters for the detection of subclinical rumen acidosis in dairy herds

An observational study was conducted in six Danish dairy herds. A specially designed stomach tube was compared to the rumenocentesis technique as part of the monitoring of rumen pH. In contrast to a previous study, the use of the stomach tube appeared to reduce saliva contamination. However, correlation with the rumenocentesis technique was poor ( r = 0.33; p = 0.019) and a linear model could only partly explain variations between either results. The presence of subclinical rumen acidosis (SRA) was evidenced in one herd only, as judged by results obtained by the rumenocentesis technique. The present study revealed some limitations of the rumenocentesis technique in small or medium-sized herds due to difficulties in selecting sufficient numbers of cows in the respective groups at risk. The finding of two apparently clinical normal cows with rumen pH values below 5.0 leads to the consideration that such fluctuations may be temporary and at least does not give rise to clinical symptoms. However, the long-term effect of such fluctuations is not known. In general, primiparous cows seemed more prone to low ruminal pH values (< 6.0), higher ruminal concentrations of short-chain fatty acids, and possibly to metabolic acidosis, than were multiparous cows. Ruminal propionate was the most precise predictor of rumen pH, whereas milk fat percentage varied greatly between lactational groups. Blood lactate dehydrogenase (LDH), beta-hydroxybutyrate (BHB) and fructosamine as well as urine phosphorus excretion and renal net acid-base excretion (NABE) were related to ruminal acid load, but were not predictive of rumen pH. Monitoring of dairy herds for SRA should be performed routinely and employ several diagnostic tools (rumenocentesis, renal NABE determination) as well as specific knowledge of herd management and feeding routines.

Absorption and metabolism of alpha-ketoglutarate in growing pigs

The portal appearance of enteral alpha-ketoglutarate (AKG) and the effect of enteral or parenteral AKG on portal net appearance of glucose, short-chain fatty acids, alanine, aspartate, glutamate, glutamine, proline and insulin were investigated in three growing pigs. During the experimental samplings the pigs were fed hourly with a standard feed mix with 5% glucose (control), 5% AKG (enteral) or no feed additive but continuously infused with AKG into the mesenteric vein in an amount equivalent to 5% of feed intake (parenteral). The arterial plasma concentration of AKG increased (p < 0.05) following both enteral (from 16+/-2 to 22+/-3 micromol/l) and parenteral (from 16+/-2 to 425+/-27 micromol/l) administration of AKG. With the enteral treatment 4+/-1% of the AKG could be accounted for in the portal vein, however, with the parenteral treatment 86+/-5% could be accounted for in the portal vein. The arterial plasma concentration of proline increased (p < 0.05) with the enteral treatment (365 +/- 3 to 443 +/- 39 micromol/l), but was not affected by the parenteral treatment (p > 0.10). The plasma concentration glutamine decreased (p < 0.05) with the parenteral treatment only. The portal net appearance of proline showed a numerical increase with the enteral treatment but no other affects on arterial concentrations or portal net appearance were found. A small accompanying study showed that only small amounts of enteral AKG was present in the small intestine. It was therefore concluded that enteral AKG has a low availability to peripheral tissues either because it is absorbed and metabolized in the stomach and duodenum or because it is metabolized by microbes in the stomach. The study showed that AKG is metabolized differently following enteral and parenteral application in growing pigs.

Splanchnic metabolism of volatile fatty acids absorbed from the washed reticulorumen of steers1

Six steers fitted with a ruminal cannula and chronic indwelling catheters in the mesenteric artery, mesenteric vein, hepatic portal vein, hepatic vein, as well as in the right ruminal vein were used to study metabolism of VFA absorbed from buffers in the emptied and washed reticulorumen. [2-(13)C]Acetate was infused into a jugular vein to study portal-drained visceral (PDV) uptake of arterial acetate, hepatic unidirectional uptake of acetate, and whole-body irreversible loss rate (ILR). Isobutyrate was infused into the right ruminal vein to calibrate VFA fluxes measured in the portal vein. On sampling days, the rumen was emptied and incubated in sequence with a 0-buffer (bicarbonate buffer without VFA), a VFA-buffer plus continuous intraruminal infusion of VFA, and finally another 0-buffer. Ruminal VFA absorption was determined as VFA uptake from the VFA-buffer and metabolic effects determined as the difference between metabolite fluxes with VFA-buffer and 0-buffers. Steady absorption rates of VFA were maintained during VFA-buffer incubations (4 h; 592+/-16, 257+/-5, 127+/-2, 17+/-<1, 20+/-<1 mmol/h, respectively, of acetate, propionate, butyrate, isovalerate, and valerate). The portal flux of acetate corrected for PDV uptake of arterial acetate accounted for 105+/-3% of the acetate absorption from the rumen, and the net portal flux of propionate accounted for 91+/-2% of propionate absorption. Considerably less butyrate (27+/-3%) and valerate (30+/-3%) could be accounted for in the portal vein. The sum of portal VFA and 3-hydroxybutyrate as well as lactate represented 99+/-3% of total VFA acetyl units and 103+/-2% of VFA propionyl units. Estimates are maximum because no accounting was made for lactate derived from glycolysis in the PDV. The net splanchnic flux of VFA, lactate, 3-hydroxybutyrate, and glucose accounted for 64+/-2% of VFA acetyl units and 34+/-5% of VFA propionyl units. Results indicate that there is a low "first-pass" uptake of acetate and propionate in the ruminal epithelium of cattle, whereas butyrate and valerate are extensively metabolized, though seemingly not oxidized to carbon dioxide in the epithelium but repackaged into acetate, 3-hydroxybutyrate, and perhaps other metabolites. When PDV "second-pass" uptake of arterial nutrients is accounted for, PDV fluxes of VFA, lactate, and 3-hydroxybutyrate represent VFA production in the gastrointestinal tract and thereby VFA availability to the ruminant animal.

Energy intake and milk production in mink (Mustela vison)--effect of litter size

Energy intake and milk production were measured in 12 mink dams raising litters of 3, 6 and 9 kits one to four weeks post partum by means of balance experiments and measurements of milk intake of the kits by the water isotope dilution technique. The dams were fed ad libitum on a conventional wet mink diet (DM: 323 g/kg; CP: 173 g/kg; ME: 4.4 MJ/kg). Milk samples collected from dams with corresponding litter sizes and lactation weeks, and body composition of kits nursed by these dams, were analysed for content of DM, ash, N and fat. The ME and drinking water consumption were higher in dams nursing 9 kits than in dams nursing 3 kits. The N and water balances as well as the live weight of dams were not affected by litter size. Daily milk production was higher in dams nursing 9 kits than in dams nursing 3 kits. The DM, N and fat content of the milk increased during lactation, but were not affected by litter size. Individual kit live weight was higher in litters of 3 than in litters of 6 and 9 kits four weeks post partum. The DM and fat content of the kits were lowest in kits from litters of 9 kits, whereas these kits had the highest protein content. Daily ME for maintenance of kits and the efficiency of utilisation of ME in milk for body gain were estimated to 356 kJ/kg0.75, kp approximately 0.53 and kf approximately 0.71, respectively. In conclusion, daily milk production increased with increasing litter size, but not in proportion to the number of kits, indicating that milk production limits the growth rate of the young. In the fourth week of lactation, milk production was not different between dams nursing 6 or 9 kits, indicating a maximum capacity.

Metabolism of propionate and 1,2-propanediol absorbed from the washed reticulorumen of lactating cows

To investigate the metabolism of 1,2-propanediol (PPD) in lactating cows independently of normal rumen microbial metabolism, three ruminally cannulated lactating Holstein cows were subjected to three experimental infusion protocols under washed reticulo-ruminal conditions in a Latin square design. Reticulo-ruminal absorption rates were maintained for 420 min by continuous intraruminal infusion of VFA and PPD. With the control treatment, 1,246 +/- 39 mmol/ h of acetate and 213 +/- 5 mmol/h of butyrate were absorbed from the reticulorumen. With the propionate treatment, 1,148 +/- 39 mmo/h of acetate, 730 +/- 23 mmol/h of propionate and 196 +/- 5 mmol/h of butyrate were absorbed from the reticulorumen. With PPD treatment, 1,264 +/- 39 mmol/h of acetate, 220 +/- 5 mmol/h of butyrate and 721 +/- 17 mmol/h of PPD were absorbed from the reticulorumen. Glucose irreversible loss rate (ILR), as well as the relative enrichment of plasma lactate and alanine, were determined by primed continuous infusion of [U-13C]glucose in a jugular vein. Treatments did not affect (P > 0.10) the plasma concentrations of glucose (4.2 +/- 0.1 mmoVL), alanine (0.14 +/- 0.01 mmol/L), or insulin (80 +/- 25 pmol/L). The plasma concentration of lactate was higher (P < 0.05) with both propionate (0.84 +/- 5 mmol/L) and PPD treatment (0.81 +/- 5 mmol/ L) compared with the control treatment (0.29 +/- 0.5 mmol/L). The plasma concentration of pyruvate was higher (P < 0.05) with the propionate treatment (0.09 +/- 0.01 mmol/L) compared with the control treatment (0.03 +/- 0.01 mmol/L). The plasma concentration of 3-hydroxybutyrate was lower (P < 0.05) with the propionate treatment (0.15 +/- 0.03 mmol/L) compared with the control treatment (0.40 +/- 0.03). With the PPD treatment, the plasma concentrations of pyruvate and 3-hydroxybutyrate were in between the other treatments and tended (P < 0.10) to be different from both. The plasma concentration of PPD increased throughout the infusion period with the PPD treatment and reached a concentration of 4.9 +/- 0.6 mmol/L at 420 min. The ILR of glucose was not affected (P > 0.10) by treatments (441 +/- 35 mmol/h). The relative 13C enrichment of plasma lactate compared with that of glucose decreased (P < 0.05) with the PPD treatment compared with the control treatment (44 to 21 +/- 3%). It was concluded that PPD has a low rate of metabolism in cows without a normal functioning rumen, although about 10% of the absorbed PPD was metabolized into lactate.

Rumen microbial sequestration of [2-(13)C]acetate in cattle

To investigate the impact of rumen microbial sequestration of VFA carbon on estimates of acetate availability based on intraruminal infusion of [2-(13)C] acetate, three nonlactating or low-yielding dairy cows were continuously intraruminally infused with [2-(13)C]acetate for 26 h. The 13C content of ruminal VFA, duodenal carbon, and fatty acids (FA) and AA isolated from liquid-associated ruminal microbes and duodenal DM was measured by an isotope ratio mass spectrometer interfaced to an elemental analyzer or a gas-liquid chromatograph. The ruminal gross production of acetate was 38 +/- 4 mol/d and could account for about 38% of the DE intake. Of the intraruminally infused 13C in [2-(13)C]acetate, 7.6 +/- 0.9% was recovered at the duodenum. The 13C content of ruminal propionate, butyrate, and valerate increased (P < 0.05) with intraruminal infusion of [2-(13)C]acetate. It was estimated that about 28% of the 13C intraruminally infused in [2-(13)C]acetate could be accounted for by duodenal 13C flow and absorption of non-acetate VFA. A number of FA isolated from liquid-associated ruminal microbes (C6, C12, C14, anteiso C15, and iso C15) were enriched with 13C (P < 0.05) at a level comparable to the enrichment of ruminal butyrate. Any absorption of these FA from the rumen would further contribute to non-acetate 13C uptake. A maximum of 72% of the ruminal gross production of acetate represented acetate absorption from the rumen in the present study. Consequently, previously used models using intraruminal isotope dilution techniques seem not to be appropriate for measuring acetate availability in ruminants. The number of metabolites exchanging carbon with acetate was found to be so high that assessments of the entire range of inter conversions seem to be practically impossible. Portal absorption studies are discussed as an alternative method of estimating VFA availability to the metabolism in ruminants.

Comparison of non-tracer and tracer methods for determination of volatile fatty acid production rate in the rumen of sheep fed on two levels of intake

The aim of the present work was to estimate volatile fatty acid (VFA) production rate in the rumen of sheep fed two levels of intake using both a tracer (TM; by isotope dilution) and a non-tracer method (NTM; by supplementary infusion) in steady-state conditions. Six wethers received a diet containing 700 g lucerne hay and 300 g ground maize/kg in eight equal meals at 3 h intervals per d. The diet (9.8 MJ metabolizable energy (ME)/kg DM) was offered at 90 % ad libitum consumption (high intake, HI) or 45 % ad libitum consumption (low intake, LI) in a crossover design. Each sheep received five intrarumen VFA solutions infused continuously for 24 h at rates of 250 ml and 165 ml/h for the HI and LI respectively. The first infusion, considered as a control treatment (Con), consisted of a solution of [1-(13)C]propionate (7 mmol/d). The four other solutions were isoenergetic (1.9 MJ ME/kg DM intake) mixtures of unlabelled propionate (C(3)) and butyrate (C(4)) at different levels: 0.90 mol C(4)/kg DM intake; 0.60 mol C(3)/kg DM intake; 0.30 mol C(3)/kg DM intake; 1.35 mol C(3)/kg DM intake. The VFA infusions did not affect rumen fermentation of the basal diet (pH, osmotic pressure, protozoa numbers), and comparable DM digestibility of the diet among the different treatments was observed. Both estimation methods demonstrated a similar increase (1.7-fold) in the rumen VFA production rate of sheep fed at intakes varying between 0.9 to 1.7 times maintenance. Irrespective of the intake level, the rumen production rate of individual VFA was on average 1.5-fold higher when estimated by the TM compared with the NTM. Rumen VFA production rates estimated by the NTM and TM represented 80 % and 120 % ME intake respectively. The difference between NTM and TM estimates seems likely to be caused mainly by overestimation of the VFA production rates by the TM.

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

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

Portal-drained visceral metabolism of 3-hydroxybutyrate in sheep

The present experiment was conducted to study the impact of portal-drained visceral (PDV) metabolism of arterial 3-OH-butyrate on estimates of the portal recovery of intraruminally infused butyrate. Three multicatheterized and rumen-fistulated Leicester ewes were subjected to three intraruminal infusion protocols in a Latin square design: control (C; water), butyrate (B; 20 mmol x h(-1)), and butyrate (20 mmol x h(-1)) + propionate (40 mmol x h(-1)) (BP). During the experiments, the sheep were infused with 1,2,3,4-13C4-D-3-OH-butyrate in a mesenteric vein. Portal recoveries of intraruminally infused butyrate and propionate were obtained by comparing Treatments B and BP, respectively, with Treatment C. The portal net appearance of butyrate and the portal net appearance of butyrate + 3-OH-butyrate accounted for 20 +/- 2% and 48 +/- 14% of intraruminally infused butyrate, respectively. Metabolism by the PDV tissues accounted for 32 to 44% of the whole-body irreversible loss rate of 3-OH-butyrate (12.0 to 24.7 +/- 0.5 mmol x h(-1)). The portal net appearance of butyrate plus the unidirectional PDV output of 3-OH-butyrate accounted for 62 +/- 5% of the intraruminally infused butyrate, and this estimate was comparable to the portal recovery of intraruminally infused propionate (62 +/- 7%). The results from the present study show that the extent of epithelial butyrate oxidation is overestimated and the portal recovery of butyrate carbon underestimated if only portal net appearance rates of butyrate and 3-OH-butyrate are considered.

Effect of composition of ruminally-infused short-chain fatty acids on net fluxes of nutrients across portal-drained viscera in underfed ewes

Four ewes, each fitted with a rumen cannula and with catheters in the mesenteric artery and portal and mesenteric veins, received continuous intrarumen infusions of water or of short-chain fatty acids (SCFA). SCFA infusions were isoenergetic (83 kJ/h) and provided rumen molar proportions (acetate:propionate:butyrate) of 70:20:10, 50:40:10 or 50:20:30. The rumen SCFA production rate with the basal diet was 90.0, 23.1 and 8.8 mmol/h for acetate, propionate and butyrate respectively. Portal net fluxes indicated that 74, 67 and 22-30% of infused acetate, propionate and butyrate respectively, reached the portal vein. Portal net release of beta-hydroxybutyrate increased with SCFA infusions, irrespective of the amount of butyrate infused. Portal net release of lactate decreased with high-butyrate infusion. Portal net uptake of glucose increased with the SCFA infusions. In ewes infused with water, a portal net uptake of total amino acids (AA) was observed. SCFA infusions decreased the uptake of nonessential AA (glutamate, glycine, but not glutamine) and increased the net release of tyrosine and essential AA (isoleucine, leucine). Portal net fluxes of AA were similar with both high-acetate and high-propionate infusions. Lower net uptake of glutamine and net release of most essential AA and some nonessential AA were observed with the high-butyrate infusion. Energetic summation of portal net release was not significantly different between the three SCFA infusions, although it tended to be lower with high-butyrate infusion. This may be related to the higher trophic effect of butyrate on the digestive mucosa.

Net portal appearance of volatile fatty acids in sheep intraruminally infused with mixtures of acetate, propionate, isobutyrate, butyrate, and valerate

The net portal appearance of volatile fatty acids (VFA) was investigated in four ruminally fistulated and multicatheterized sheep. During the experiments, the sheep were fed once every hour for 14 h and intraruminally infused with mixtures of VFA for the 12 h commencing 2 h after the initiation of the hourly feeding protocol. Paired arterial and portal blood samples were obtained hourly during the last 6 h of the experiments. In the control treatment (1), only water was infused intraruminally. In Treatments 2 through 4, the intraruminal infusion rates of propionate (40 mmol/h), isobutyrate (5 mmol/h), and valerate (5 mmol/h) were unchanged. In Treatments 2, 3, and 4, the acetate infusion rate was 100, 60, and 20 mmol/h, respectively, and the butyrate infusion rate was 10, 30, and 50 mmol/h, respectively. Thus, the infusion rate of VFA carbon was constant across Treatments 2 through 4. Portal recovery estimated from the increased net portal appearance in Treatments 2 through 4 compared to the control treatment was 85% for propionate and 60% for isobutyrate, and these recoveries were unaffected by treatment. The portal recovery of butyrate increased (from 21 to 32%) with increasing infusion rate of butyrate and decreasing infusion rate of acetate, as did the portal recovery of valerate (from 14 to 31%). The portal recovery of acetate was 55%, when measured as net portal appearance. Thus, it seems that the capacity for beta-oxidation in ruminal epithelium is limited, which would explain the increasing portal recovery of butyrate and valerate with increasing infusion rate of butyrate, when infusion rate of VFA carbon is unchanged.

Absorption and metabolism of short-chain fatty acids in ruminants

Short-chain fatty acids (SCFA), viz. acetate, propionate and butyrate are quantitatively important substrates in ruminant energy metabolism. In the reviewed literature, 16 44% of ME intake was recovered as portal appearance of SCFA. This is considerably lower than expected when related to the estimated intragastric flux of SCFA. The discrepancy is caused by portal drained viscera metabolism of arterially abundant metabolites e.g., acetate and the metabolism of acetate and butyrate to acetoacetate and D-3-hydroxybutyrate in the absorptive epithelia. Even though considerable variations between experiments on acetate and propionate appearance are found, there seems to be a great deal of evidence that the proportion of gastrointestinally produced acetate and propionate absorbed to the portal blood is 50-75%. The portal recovery of butyrate has been found to be between 10 and 36% dependent on intraruminal infusion rate. It is concluded that major parts of acetate and propionate are directly absorbed to the portal vein. The true absorption rate of acetate can only be estimated by taking the portal drained viscera metabolism of arterial acetate into account. Butyrate is generally found to have a low recovery in the portal vein, but the production of D-3-hydroxybutyrate seems to be underestimated in major parts of the literature. It is therefore necessary to measure portal appearance as well as portal drained viscera metabolism to assess the quantitative as well as the qualitative contribution of SCFA and SCFA metabolites to whole animal metabolism.

Measurement of the total concentration of functional Na+, K(+)-pumps in rumen epithelium

Using the technique of vanadate-facilitated [3H]ouabain binding we have developed a simple and reliable assay for measuring the concentration of [3H]ouabain binding sites in small fresh or frozen biopsies of rumen epithelium papillae. In bovine and ovine rumen epithelium obtained from the cranio-ventral rumen sac the concentration of [3H]ouabain binding sites was 1.6-4.9 nmol g dry wt-1 (n = 32) and 3.7-5.2 nmol g dry wt-1 (n = 6), respectively. When incubated in oxygenated Krebs-Ringer bicarbonate buffer fresh biopsies of rumen epithelium maintained a high K+ and low Na+ content for at least 6 h. Na+ loading of the biopsies induced about 20-fold increase of the Na+, K(+)-pump activity based on measurement of ouabain suppressible net [86Rb+] influx. The ouabain suppressible net influx of [86Rb+] measured in Na+ loaded biopsies showed a close correlation to the [3H]ouabain binding capacity (r = 0.80, P < 0.01) and corresponded to 47 +/- 2% (n = 9) of the theoretical maximum flux rate. The ouabain suppressible net influx of K+ and [86Rb+] were linearly related (r = 0.73; P < 0.001). The net Na+ efflux was 1.21 times the net K+ influx. It is concluded that rumen epithelium has a large capacity for active Na+/K+ transport and that there is agreement between the concentration of [3H]ouabain binding sites in the epithelium and the ouabain suppressible rate of net [86Rb+] influx in Na+ loaded biopsies in spite of some uncertainty about the maximum turnover number of the Na+, K(+)-pump in rumen epithelium.

Measurement of 13C enrichment of plasma lactate by gas chromatography/isotope ratio mass spectrometry

An application of a gas chromatography/isotope ratio mass spectrometry (GC/IRMS) method for stable carbon isotope analysis of blood plasma lactic acid is presented. The method involves a simple extraction procedure followed by derivatization with diazomethane. It is shown that derivatization is by single methylation, thus minimizing the dilution of the derivative's 13C content, yet still ensuring good chromatographic behaviour on a polar capillary column. This ensures a high sensitivity of the isotopic analysis. Repeatability, expressed by the coefficient of variation, varied from 0.3% to 19%, depending on sample enrichment. Reproducibility was 2.3% over a 10 day period. The detection limit, defined as 2SD, was about 0.0004 atom % excess (APE), equivalent to 0.001 mol % excess, when based on a measured precision of about 0.2/1000 in delta notation. A comparison is made between enrichments obtained using a calibration curve and those obtained using a correction for the added methyl carbon. The two methods agreed well, with a relative difference (delta APE/APE x 100%) of less than 0.5% for samples enriched with between 0.004 and 1.28 APE. It is concluded that the method provides simple and precise isotope analysis of picomole quantities of blood lactate.