Quantifying the effects of hydrogen on carbon assimilation in a seafloor microbial community associated with ultramafic rocks

Thermodynamic models predict that H₂ is energetically favorable for seafloor microbial life, but how H₂ affects anabolic processes in seafloor-associated communities is poorly understood. Here, we used quantitative ¹³C DNA stable isotope probing (qSIP) to quantify the effect of H₂ on carbon assimilation by microbial taxa synthesizing ¹³C-labeled DNA that are associated with partially serpentinized peridotite rocks from the equatorial Mid-Atlantic Ridge. The rock-hosted seafloor community was an order of magnitude more diverse compared to the seawater community directly above the rocks. With added H₂, peridotite-associated taxa increased assimilation of ¹³C-bicarbonate and ¹³C-acetate into 16S rRNA genes of operational taxonomic units by 146% (±29%) and 55% (±34%), respectively, which correlated with enrichment of H₂-oxidizing NiFe-hydrogenases encoded in peridotite-associated metagenomes. The effect of H₂ on anabolism was phylogenetically organized, with taxa affiliated with Atribacteria, Nitrospira, and Thaumarchaeota exhibiting the most significant increases in ¹³C-substrate assimilation in the presence of H₂. In SIP incubations with added H₂, an order of magnitude higher number of peridotite rock-associated taxa assimilated ¹³C-bicarbonate, ¹³C-acetate, and ¹³C-formate compared to taxa that were not associated with peridotites. Collectively, these findings indicate that the unique geochemical nature of the peridotite-hosted ecosystem has selected for H₂-metabolizing, rock-associated taxa that can increase anabolism under high H₂ concentrations. Because ultramafic rocks are widespread in slow-, and ultraslow-spreading oceanic lithosphere, continental margins, and subduction zones where H₂ is formed in copious amounts, the link between H₂ and carbon assimilation demonstrated here may be widespread within these geological settings.Subject terms: Microbial ecology, Water microbiology, Microbial biooceanography, Microbiome, Biogeochemistry     

Tracking the fate of digesta <Superscript>13</Superscript>C and <Superscript>15</Superscript>N compositions along the ruminant gastrointestinal tract: Does digestion influence the relationship between diet and faeces?

Faecal stable isotope compositions reflect wildlife diets, if digestive processes along the gastrointestinal tract (GIT) do not alter diet–faeces isotopic relationships in an unpredictable way. We investigated ¹³C and ¹⁵N compositions of digesta along the ruminant GIT, using Saanen dairy goats kept on pure grass hay or browse for >20 days. Isotopic changes occurred in the ventral rumen, and in the small intestine, where digesta had significantly higher δ¹³C and δ¹⁵N (associated with lower C or higher N content, respectively) values relative to other GIT sites. However, effects on isotope fractionation were small (∼1.0‰ for δ¹³C and ∼ 2.0‰ for δ¹⁵N), and were reversed in the hindgut such that faecal isotope compositions did not differ from the foregut. No other substantial isotopic changes occurred across GIT sites, despite the morphophysiological complexity of the ruminant GIT. We found similarly small differences across GIT components of rheem gazelles (Gazella leptoceros) fed a mixture of C₃ lucerne and C₄ grass, although in this case faeces were ¹⁵N-depleted relative to other GIT components. Along with differences in δ¹⁵N between goats fed browse or grass, this result implies a systematic difference in diet–faeces δ¹⁵N relationships, contingent on the botanical composition of ruminant diets. Thus, while our results support faecal δ¹³C as a reliable proxy for wildlife diets, further work on factors influencing faecal ¹⁵N abundance is needed. Finally, we note high levels of isotopic variability between individuals fed the same diets, even accounting for the relatively short duration of the experiments, suggesting an important influence of stochasticity on isotope fractionation.     

Digestive system functioning during simulation of microravity effects on humans by means of immersion

The functioning of the digestive system was investigated in ten volunteers after a seven-day dry immersion (DI). The experimental conditions were found to raise the secretory activities of the stomach, pancreas, and liver and to increase the spectral indices of the gastrointestinal tract’s (GIT) electrical activity against the background of an increased insular secretion and decreased gastrin secretion. The elevated GIT electrical activities and changes in their relationships were determined by an increased gastric secretion and elevated intestine tone in fasting subjects and displayed a close similarity to changes induced by caffeine stimulation and those arising during long-term head-down tilt bed rest (HDTBR) or spaceflight.     

An in vitro investigation of gastrointestinal Na+ uptake mechanisms in freshwater rainbow trout

In vitro gut-sac preparations of all four sections (stomach, anterior, mid, and posterior intestine) of the gastrointestinal tract (GIT) of freshwater rainbow trout, together with radiotracer (²²Na) techniques, were used to study unidirectional Na⁺ uptake rates (UR, mucosal → blood space) and net absorptive fluid transport rates (FTR) under isosmotic conditions (mucosal = serosal osmolality). On an area-specific basis, unidirectional Na⁺ UR was highest in the mid-intestine, but when total gut area was taken into account, the three intestinal sections contributed equally, with very low rates in the stomach. The theoretical capacity for Na⁺ uptake across the whole GIT is sufficient to supply all of the animal’s nutritive requirements for Na⁺. Transport occurs by low affinity systems with apparent K _m values 2–3 orders of magnitude higher than those in the gills, in accord with comparably higher Na⁺ concentrations in chyme versus fresh water. Fluid transport appeared to be Na⁺-dependent, such that treatments which altered unidirectional Na⁺ UR generally altered FTR in a comparable fashion. Pharmacological trials (amiloride, EIPA, phenamil, bafilomycin, furosemide, hydrochlorothiazide) conducted at a mucosal Na⁺ concentration of 50 mmol L^?1 indicated that GIT Na⁺ uptake occurs by a variety of apical mechanisms (NHE, Na⁺ channel/H⁺ ATPase, NCC, NKCC) with relative contributions varying among sections. However, at a mucosal Na⁺ concentration of 10 mmol L^?1, EIPA, phenamil, bafilomycin, and hydrochlorothiazide were no longer effective in inhibiting unidirectional Na⁺ UR or FTR, suggesting the contribution of unidentified mechanisms under low Na⁺ conditions. A preliminary model is presented.     

In vivo 13C NMR analysis of acetate metabolism in Thiobacillus versutus under denitrifying conditions

The disappearance of 2-¹³C-acetate and the subsequent incorporation of label into cellular metabolites were followed in denitrifying cells of Thiobacillus versutus by ¹³C NMR spectroscopy. In cells grown under acetate-limitation, the specific rate of consumption was idependent of the density of the cell suspension. An isotopic steady state was reached within 30 min if sufficient substrate was added to the cell suspension. In cells grown under nitrate-limitation, the consumption of 2-¹³C-acetate proceeded at a significantly lower rate. The decrease and final disappearance of 2-¹³C-acetate were accompanied by incorporation of ¹³C into glutamate, glutamine, and by the release of labeled HCO ₃ ^– and CO₂. The appearance of a broad resonance being the methyl endgroup of poly-3-hydroxybutyrate (PHB) was indicative for PHB mobilization during the incubation. The sequence of label incorporation and the distribution among the various carbon nuclei were consistent with the operation of the tricarboxylic acid cycle.     

Body heat responsive gelation of methylcellulose formulation containing betaine

We examined a methylcellulose (MC) formulation that gels at body temperature for enteral alimentation. Betaine was found to have a lowering effect on the gelation temperature of the MC solution. The thermal gelation temperature of a body heat-responsive (BHR) gelling MC formulation, consisting of 2% MC, 15% glucose, 1.2% sodium citrate, and 3.5% betaine mixture, was approximately 32 °C, indicating that it could gel in response to body heat. Glucose release from the BHR gels was delayed at 37 °C in an in vitro study. In rats, oral administration of BHR gelling MC formulation delayed an increase in blood glucose and appearance of ¹³CO₂ in expired air in a ¹³C-acetate breath test in comparison with the control. These results suggested that the BHR gelling MC formulation was gelled in the stomach and delayed gastric emptying after oral administration and glucose in the gels was absorbed slowly.     

On the distribution and metabolism of Fusarium-toxins along the gastrointestinal tract of endotoxaemic pigs

The aim of this study was to investigate the potential modulatory effect of E. coli lipopolysaccharides (LPS) on residues of deoxynivalenol (DON), de-epoxy-deoxynivalenol (DOM-1), zearalenone (ZEN) and its metabolites α-zearalenol (α-ZEL), β-zearalenol (β-ZEL), zearalanone (ZAN), α-zearalanol (α-ZAL) and β-zearalanol (β-ZAL) after pre- or post-hepatic administration along the gastrointestinal axis. Fifteen barrows were exposed to a naturally mycotoxin contaminated diet (4.59 mg DON/kg feed and 0.22 mg ZEN/kg feed) and equipped with jugular (ju) and portal (po) catheters. On sampling day (day 29), the barrows were infused with LPS or a control fluid (LPS, 7.5 µg/kg body weight; control, 0.9% NaCl) either pre- or post-hepatically, resulting in three infusion groups: CON_ju-CON_po, CON_ju-LPS_po and LPS_ju-CON_po. At 195 min relative to infusion start (210 min post-feeding), pigs were sacrificed and content of stomach and small intestine (proximal, medial and distal part) as well as faeces were collected. In all LPS-infused animals, higher amounts of dry matter were recovered irrespective of LPS entry site suggesting a reduced gastric emptying and a decreased gastrointestinal motility under endotoxaemic conditions. DON metabolism in the gastrointestinal tract (GIT) remained unaltered by treatments and included an increase in the proportion of DOM-1 along the GIT, particularly from distal small intestine to faeces. Variables describing ZEN metabolism suggest a stimulated biliary release of ZEN and its metabolites in LPS-infused groups, particularly in the LPS_ju-CON_po group. In conclusion, the GIT metabolism of ZEN was markedly influenced in endotoxaemic pigs whereby a jugular induction of an acute phase reaction was more effective than portal LPS infusion hinting at a strong hepatic first-pass effect.     

Food transit times in captive leopard seals (Hydrurga leptonyx)

The passage rate of food through the alimentary tract of three captive, female leopard seals (Hydrurga leptonyx) was assessed. Seals were housed during winter in holding pools with access to water to reduce factors affecting digestion. Three different marker types were used; large and small beads and TiO₂. Animals were checked hourly, and sample collections continued for 270 h after dosing. Individual differences in transit and mean retention times were observed, possibly reflecting inter-digestive emptying times of the stomach and small intestine. Age differences and activity levels may also have been a factor. Leopard seals displayed extended food transit times similar to terrestrial carnivores instead of other pinnipeds. This result suggests an adaptation of digestive system to cope with the opportunistic diet and range of prey types consumed.     

Phosphoglucoisomerase-catalyzed interconversion of hexose phosphates

Summary The exchange of protons and deuterons by phosphoglucoisomerase during the single passage conversion of D-[2-¹³C,1-²H]fructose 6-phosphate in H₂O or D-[2-¹³C]fructose 6-phosphate in D₂O to D-[2-¹³C]glucose 6-phosphate, as coupled with the further generation of 6-phospho-D-[2-¹³C]gluconate in the presence of excess glucose-6-phosphate dehydrogenase was investigated by ¹³C NMR spectroscopy of the latter metabolite. In H₂O, the intramolecular deuteron transfer from the C₁ of D-fructose 6-phosphate to the C₂ of D-glucose 6-phosphate amounted to 65%, a value only slightly lower than the 72% intramolecular proton transfer in D₂O. Both percentages, especially the latter one, were lower than those previously recorded during the single passage conversion of D-[1-¹³C,2-²H]glucose 6-phosphate in H₂O or D-[1-¹³C]glucose 6-phosphate in D₂O to D-fructose 6-phosphate and then to D-fructose 1,6-bisphosphate. These differences indicate that the sequence of interactions between the hexose esters and the binding sites of phosphoglucoisomerase is not strictly in mirror image during, respectively, the conversion of the aldose phosphate to ketose phosphate and the opposite process.     

Content of carbohydrates in enteral medium of various parts of digestive tract of the reindeer rangifer tarandus during winter

It has been established that cellulose concentration in dry matter of manifold chymus in female reindeers Rangifer tarandus is increased in comparison with rumen and reticulum, while the reducing sugar concentration is decreased. There is oppositely directed change of the carbohydrate concentration at transit from manifold to the medial part of jejunum—a decrease of cellulose and an increase of sugars. In the distal part of small intestine as well as in large intestine, the cellulose content increases again, whereas the sugar level decreases. The maximal rise of the content of aldoses and ketoses in chymus of the reindeer jejunum proximal part indicates that the high assimilation of reindeer moss polysaccharides is achieved not only due to adaptive alimentary possibilities of rumen, but also owing to intensive hydrolysis of exogenous and mostly endogenous polysaccharides of the secondary microbial origin in small intestine. Comparison of the total amount of reducing sugars in the reindeer digestive tract with the total amount of glucose in blood allows suggesting that alongside with gluconeogenesis the absorption of monosugars from digestive tract plays an essential role in maintenance of the high glycemic level that is especially necessary during winter season to provide the elevated energy demands of the animal organism.     

Taxonomic Identification of Commensal Bacteria Associated with the Mucosa and Digesta throughout the Gastrointestinal Tracts of Preweaned Calves

Bacterial colonization in the gastrointestinal tracts (GIT) of preweaned calves is very important, since it can influence early development and postweaning performance and health. This study investigated the composition of the bacteria along the GIT (rumen, jejunum, ileum, cecum, and colon) of preweaned bull calves (3 weeks old) using pyrosequencing to understand the segregation of bacteria between the mucosal surface and digesta. Phylogenetic analysis revealed that a total of 83 genera belonging to 13 phyla were distributed throughout the GIT of preweaned calves, with the Firmicutes, Bacteroidetes, and Proteobacteria predominating. Quantitative PCR (qPCR) analysis of selected abundant bacterial genera (Prevotella, Bacteroides, Lactobacillus, and Faecalibacterium) revealed that their prevalence was significantly different among the GIT regions and between mucosa- and digesta-associated communities. Rumens contained the most diverse bacterial population, consisting of 47 genera, including 16 rumen-specific genera, followed by the large intestine and then the small intestine. Bacterial species richness was higher at the mucosal surface than in the local digesta, with the exception of the rumen. The majority of bacteria found on the rumen epithelial surface and within the small intestine could not be identified due to a lack of known genus-level information. Thus, future studies will be required to fully characterize the microbiome during the development of the rumens and the mucosal immune systems of newborn calves. This is the first study to analyze in depth the bacterial composition of the GIT microbiome in preweaned calves, which extends previous findings regarding early rumen colonization and bacterial segregation between mucosa- and digesta-associated microbial communities.     

Study of the physicochemical and biological stability of pediocin PA‐1 in the upper gastrointestinal tract conditions using a dynamic in vitro model

Aims: To evaluate the survival of Pediococcus acidilactici UL5 and its ability to produce pediocin PA‐1 during transit in an artificial gastrointestinal tract (GIT). To investigate the physicochemical and biological stability of purified pediocin PA‐1 under GIT conditions. Methods and Results: Skim milk culture of Ped. acidilactici UL5 was fed to a dynamic gastrointestinal (GI) model known as TIM‐1, comprising four compartments connected by computer‐controlled peristaltic valves and simulating the human stomach, duodenum, jejunum and ileum. This strain tolerated a pH of 2·7 in the gastric compartment, while lower pH reduced its viability. Bile salts in the duodenal compartment brought a further 4‐log reduction after 180 min of digestion, while high viable counts (up to 5 × 10⁷ CFU ml^?1 fermented milk) of Ped. acidilactici were found in both the jejunal and ileal compartments. Pediococcus acidilactici recovered from all four compartments was able to produce pediocin at the same level as unstressed cells. The activity of the purified pediocin in the gastric compartment was slightly reduced after 90 min of gastric digestion, while no detectable activity was found in the duodenal, jejunal and ileal compartments during 5 h of digestion. HPLC analysis showed partial degradation of the pediocin peptide in the duodenal compartment and massive breakdown in the jejunal and ileal compartments. Conclusions: Pediococcus acidilactici UL5 showed high resistance to GIT conditions, and its ability to produce pediocin was not affected, suggesting its potential as a probiotic candidate. The physicochemical and biological stability of pediocin was significantly poor under GIT conditions. Significance and Impact of the Study: Pediococcus acidilactici UL5 appears to be a potential probiotic candidate because its capacity to produce pediocin PA‐1 is not affected by the GI conditions as well as the strain shows an acceptable survival rate. Meanwhile, purified pediocin PA‐1 losses activity during GIT transit; microcapsules could be used to deliver it to the target site.     

Partial characterization of the gastrointestinal weight changes produced in the female rat by 16,16-dimethylprostaglandin E2

Oral and subcutaneous administration of 16,16-dimethylprostaglandin E₂ (16,16-dimethyl PGE₂) resulted in an increase in the dry weight of the stomach and small intestine of the female rat. This weight response was rapid, controlled rather than continuously progressing, dose dependent and reversible. The dry weight of the colon also increased but this was not studied in detail.Two-day treatment with 16,16-dimethyl PGE₂ caused an increase in the incorporation of ³H-thymidine into the duodenum, jejunum and colon suggesting an increase in cell number. Incorporation into the stomach and ileum was not changed.The number of goblet cells per crypt was increased by prostaglandin treatment in all parts of the small intestine. Since these are mucus producing cells, the small intestine may have increased in cell number and mucus production.Both anti-secretory and cytoprotective doses of 16,16-dimethyl PGE₂ caused weight increases in the stomach and small intestine. However, the weight gain by itself was not sufficient to protect the stomach or small intestine from necrotic agents after the prostaglandin was discontinued.     

Stable Isotope Labeled n-Alkanes to Assess Digesta Passage Kinetics through the Digestive Tract of Ruminants

We describe the use of carbon stable isotope (¹³C) labeled n-alkanes as a potential internal tracer to assess passage kinetics of ingested nutrients in ruminants. Plant cuticular n-alkanes originating from intrinsically ¹³C labeled ryegrass plants were pulse dosed intraruminally in four rumen-cannulated lactating dairy cows receiving four contrasting ryegrass silage treatments that differed in nitrogen fertilization level (45 or 90 kg nitrogen ha⁻¹) and maturity (early or late). Passage kinetics through the gastrointestinal tract were derived from the δ¹³C (i.e. the ratio ¹³C:¹²C) in apparently undigested fecal material. Isotopic enrichment was observed in a wide range of long-chain n-alkanes (C₂₇–C₃₆) and passage kinetics were determined for the most abundant C₂₉, C₃₁ and C₃₃ n-alkanes, for which a sufficiently high response signal was detected by combustion isotope ratio mass spectrometry. Basal diet treatment and carbon chain length of n-alkanes did not affect fractional passage rates from the rumen (K ₁) among individual n-alkanes (3.71–3.95%/h). Peak concentration time and transit time showed a quantitatively small, significant (p≤0.002) increase with carbon chain length. K ₁ estimates were comparable to those of the ¹³C labeled digestible dry matter fraction (3.38%/h; r = 0.61 to 0.71; p≤0.012). A literature review has shown that n-alkanes are not fermented by microorganisms in the rumen and affirms no preferential depletion of ¹³C versus ¹²C. Our results suggest that ¹³C labeled n-alkanes can be used as nutrient passage tracers and support the reliability of the δ¹³C signature of digestible feed nutrients as a tool to measure nutrient-specific passage kinetics.     

Mixotrophy in the termite gut acetogen,Sporomusa termitida

Cell suspensions of H₂/CO₂-grown Sporomusa termitida catalyzed an H₂-supported synthesis of acetate from CO₂ at rates of about 1 mol acetate x h^-1 x mg protein^-1. Cells pre-grown on methanol, mannitol, lactate, or glycine also displayed H₂-supported acetogenesis from CO₂, although at rates 5–85% that of H₂/CO₂-grown cells. With methanol-grown cell suspensions: the presence of methanol greatly stimulated the rate of H₂-supported conversion of ¹⁴CO₂ to ¹⁴C-acetate (which became labeled mainly in the COOH-group); and like-wise the presence of H₂ stimulated the conversion of ¹⁴CH₃OH+CO₂ to ¹⁴C-acetate (which became labeled mainlyan the CH₃-group). Analogous stimulatory effects were observed for cell suspensions pre-grown on methanol + CO₂+H₂. Furthermore, when H₂ (+CO₂) was included as a growth substrate with either methanol or lactate: both substrates were used simultaneously; there was no diauxie in the growth of cells or in acetate production; and the molar growth yield of S. termitida was close to that predicted from summation of the yields observed when grown with each substrate alone. These data indicated that S. termitida can grow by mixotrophy, i.e. by the simultaneous use of H₂/CO₂ and organic compounds for energy. Results are discussed in light of the ability of H₂/CO₂ acetogens to outprocess methanogens in H₂ consumption in the hindgut fermentation of wood-feeding termites.     

Increased diet viscosity by oat β-glucans decreases the passage rate of liquids in the stomach and affects digesta physicochemical properties in growing pigs

Rheological properties of digesta play a role in digesta passage kinetics through the gastrointestinal tract, in turn affecting nutrient absorption kinetics. Therefore, we studied the effects of diet viscosity on digesta passage and physicochemical properties in pigs. Twenty male growing pigs (35 kg body weight at the start) were assigned to one of five diets with increasing dietary concentrations of β-glucans (BG; from 0 % to 10 %), in exchange for maize starch. After a 17-day adaptation period, pigs were euthanised and the mean retention time (MRT) of digesta solids (TiO₂) and liquids (Cr-EDTA) in the stomach, and proximal and distal half of the small intestine was quantified. In the stomach, the MRT of liquids, but not of solids, increased when dietary BG level increased (6 min per % dietary BG, P = 0.008 and R² = 0.35). Concomitantly, stomach DM content (5 g/kg per % dietary BG, P < 0.001 and R² = 0.53) and apparent digesta viscosity (56 Pa × s at 1/s shear rate per % dietary BG, P = 0.003 and R² = 0.41) decreased. In the proximal half of the small intestine, no effects of dietary BG level were observed. In the distal half of the small intestine, water-binding capacity (WBC) of digesta increased (0.11 g/g digesta DM per % dietary BG, P = 0.028 and R² = 0.24) and starch digestibility decreased (0.3% per % dietary BG, P = 0.034 and R² = 0.23) when dietary BG level increased. In the colon, apparent digesta viscosity at 45/s shear rate increased (0.1 Pa × s per % dietary BG, P = 0.03 and R² = 0.24) in the proximal half of the colon, and digesta WBC increased (0.06 g/g digesta DM per % dietary BG, P = 0.024 and R² = 0.26) in the distal half of the colon when dietary BG level increased. To conclude, increasing dietary BG level caused the MRT of liquids, but not that of solids, to increase in the stomach, resulting in reduced separation of the solid and liquid digesta fractions. This caused dilution of the stomach content and reduction in digesta viscosity when dietary BG levels increased. Effects of dietary BG level on physicochemical properties in the proximal small intestine were absent and may have been due to a low DM content. The WBC of digesta in the distal small intestine and colon increased when dietary BG level increased, as did apparent digesta viscosity in the proximal colon. This likely reflects the concentration of BG in digesta when moving through the gastrointestinal tract.     

Passage kinetics of concentrates in dairy cows measured with carbon stable isotopes

Fractional passage rates form a fundamental element within modern feed evaluation systems for ruminants, but knowledge on feed-specific fractional passage is largely lacking. Commonly applied tracer techniques based on externally applied markers, such as chromium-mordanted neutral detergent fibre (Cr-NDF), have been criticised for behaving differently to feed particles. This study describes the use of the carbon stable isotope ratio (¹³C : ¹²C) as an internal digesta marker to quantify the fractional passage rate of concentrates through the digestive tract of dairy cows. In a crossover study, five dairy cows were fed low (24.6%) and high (52.6%) levels of concentrates (dry matter (DM) basis) and received a pulse-dosed Cr-NDF and ¹³C isotopes. The latter was administered orally by exchanging part of the dietary concentrates of low ¹³C natural abundance with a pulse dose of maize bran-based concentrates of high ¹³C natural abundance. Fractional passage rates from the rumen (K₁) and from the large intestine (K₂) were determined from faecal marker concentrations of Cr-NDF and of ¹³C in the DM (¹³C-DM), NDF (¹³C-NDF) and neutral detergent soluble (¹³C-NDS). No differences in K₁ estimates were found for the two concentrate levels fed but significant differences between markers (P<0.001) were observed. Faecal Cr-NDF excretions gave lower K₁ estimates (0.037–0.039/h) than ¹³C-DM (0.054–0.056/h) and ¹³C-NDF (0.061–0.063/h). The ¹³C-NDS was calculated by the difference of ¹³C in the DM and NDF, and K₁ values (0.039–0.043/h) were comparable to Cr-NDF. Total mean retention time was considerably higher for Cr-NDF (40.9–42.0 h) as compared to ¹³C-DM and ¹³C-NDF (32.0–33.5 h; P<0.001). The accuracy of the curve fits for Cr-NDF and ¹³C-DM and ¹³C-NDF was overall good (mean prediction error of 9.9–13.9%). Fractional passage rate of Cr-NDF was comparable to studies where this marker was assumed to represent the fractional passage of roughages. However, K₁ estimates based on the ¹³C : ¹²C ratio varied considerably from studies based on external markers. Our results suggest that the use of ¹³C isotopes as digesta passage markers can provide feed component-specific K₁ estimates for concentrates and provides new insight into passage kinetics of NDF from technologically treated compound feed.     

Energie-, Kohlenstoff-, Stickstoff- und Aminosäurenbilanzversuch mit wachsenden Cornish- Hähnen

The time course of AA digestion, AA balance (sV AS), and AA absorption (wV AS) was estimated on growing rats (Wistar rats, LW= 124 g) in different sections of the intestinal tract using the combination of ¹⁵N tracer and TiO₂ marker techniques. The animals received once a diet of ¹⁵N labelled wheat and yeast as protein sources supplemented by TiO₂ as a marker. Up to 6 h after feeding the amino acid composition the ¹⁵N excess and the TiO₂ content in the digesta of stomach, small and large intestine were determinated in the relation of amino acids resp. of ¹⁵N labelled amino acids to the marker. In addition the content of amino acids and the ¹⁵N excess of these amino acids were estimated in plasma. From these data the disappearance rates and the relation of exogenous to endogenous amino acids as well as the sV and the wV values of the different amino acids were calculated for the different gut sections.

The following results were obtained:

- The relative disappearance rate for N and TiO₂ marker out of the stomach went approximately parallel but with a delay for TiO₂ of about 30 minutes.

- The AA composition of the stomach content, the small and the large intestine content did not vary in dependence of the time.

- The AA composition of the stomach digesta was nearly identical to that of the diet, while that of the small intestine was between exogenous AA composition (feed) and endogenous AA composition (digesta on protein free feeding). AA composition of the large intestine digesta showed quite big differences (bacterial AA break down and AA synthesis).

- Considering a delay time (small intestine: 1 h, large intestine: 4 h) the exogenous portion of the different AA remained constant in both of these intestinal sections during the whole experimental time.

- The exogenous AA part varied for small intestine digesta between 31 and 69% (mean value: 41%), and for large intestine digesta between 13 and 39% (mean value: 22%).

- The sV AS values in the small intestine (AA balance resp. precaecal digestibility) differed from 61% (threonine) to 86% (proline) with an average of 73.4 ± 7.4%, those for wV AS (AA absorption) from 81% (lysine) to 94% (proline) with an average of 88.1±4.1%. There were significant differences between AA, but they are negligible for practical purposes.

- In the small intestine the estimated values for postprandial absorption of the exogenous AA accounted after 1 h, 2 h, 4 h and 6 h for 21%, 33.7%, 46.5%, and 70.7% of the AA intake respectively, mean absorption rate = 9.1 ± 0.5%/h.

- The AA balance in the whole tract (sV AS) was 75 to 94% (on average 82 ± 5.3%) and the wV AS (balance corrected by the ¹⁵N method) ranged from 92 to 99% (on average 96±1.8%). These values correspond to the faecal AA digestibility in conventional experiments.

- In the large intestine the postileal disappearance of the AA (sV) was on average 9.7% with a maximum value of 26% for glycine, and with a minimum for methionine of ‐2.1% caused by bacterial synthesis of methionine. Te postileal wV in the large intestine amounted to 7.5%.

- The time course of the disappearance rate of the ¹⁵N labelled AA in the small intestine and the appearance rate of these AA in the plasma showed an analogous behavior. Both of them characterize the postprandial absorption.

The following conclusion can be drawn:

The method used (combination of ¹⁵N tracer and TiO₂ marker techniques) enables determining the time course of transit and the variation of exogenous AA: endogenous AA proportion in the different intestinal sections and estimating the faecal and precaecal digestibility of the different AA.

The course of secretion and absorption of the different AA should be specified in further experiments using the more precise analysis of ¹⁵N by GC‐MS resp. GC‐C‐IRMS technique. An apply of this method to farm animals (pigs) seems to be possible.     

Metabolism of prostaglandin F1α in the pulmonary circulation

³H_5,6-Prostaglandin F_1α is largely eliminated from the circulation during a single passage through the pulmonary vascular bed. Its elimination requires cellular uptake. The volume of distribution and the mean transit time of the radioactivity are greater than those of an intravascular marker, blue dextran. The lungs retain 25–30% of the radioactivity, most in the form of a metabolite less polar than PGF_1α itself. The pulmonary venous effluent contains the same metabolite along with some unmetabolized PGF_1α. The metabolite can be distinguished from prostaglandins of the A, B, D, E and F series. It is reduced on reaction with borohydride, but reduction does not yield PGF_1α. Na0H has no effect on the metabolite, a finding that rules out the presence of a β-hydroxy-ketone configuration of the ring structure. The chromatographic behavior of the metabolite and its reaction with borohydride are those expected of 9,11-hydroxy-15-ketoprostanoic acid.