Irreversible Loss Rate Research Articles

Compartmental analysis of magnesium kinetics in Mg-sufficient and Mg-deficient rats

In this study, we determined magnesium kinetic values in normal rats using stable-isotope techniques. Additionally, we calculated the mass of the exchangeable pools of Mg in Mg-deficient rats to determine whether it can be used as a marker of Mg status. Rats were fed either a control diet (1,000 mg Mg/kg) or a Mg-deficient diet (60 mg Mg/kg). After 2 weeks on the experimental diets, each rat received an intravenous injection of 26Mg. The plasma Mg disappearance curve over the next 7 days was used to measure the mass and fractional transport rate of 3 rapidly exchanging Mg metabolic pools. In control rats, the mass of pool 1 (1.37 mg) was half that of pool 2 (2.46 mg), and pool 3 (47.7 mg) accounted for greater than 90% of exchangeable Mg. In Mg-deficient rats, we observed a significant decrease in the size of the 3 exchangeable pools of Mg (0.36, 0.72, and 20.2 mg, respectively) relative to the control rats. Furthermore, the fractional transport rate of Mg from pool 1 to pool 3 in Mg-deficient rats was 3 times the rate in the control rats, and the rate of irreversible loss from pool 1 was lower in Mg-deficient rats. In summary, this study allows us to establish Mg kinetic data in Mg-sufficient and Mg-deficient rats. The present experiment supports the conclusion that the isotopic test identifies animals with severe Mg deficiency.

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.

Influence of hepatic ammonia removal on ureagenesis, amino acid utilization and energy metabolism in the ovine liver.

The mass transfers of O2, glucose, NH3, urea and amino acids across the portal-drained viscera (PDV) and the liver were quantified, by arterio-venous techniques, during the last 4 h of a 100 h infusion of 0 (basal), 150 or 400 mumol NH4HCO3/min into the mesenteric vein of three sheep given 800 g grass pellets/d and arranged in a 3 x 3 Latin-square design. Urea irreversible loss rate (ILR) was also determined by continuous infusion of [14C]urea over the last 52 h of each experimental period. PDV and liver movements of glucose, O2 and amino acids were unaltered by NH4HCO3 administration, although there was an increase in PDV absorption of non-essential amino acids (P = 0.037) and a trend for higher liver O2 consumption and portal appearance of total amino acid-N, glucogenic and non-essential amino acids at the highest level of infusion. PDV extraction of urea-N (P = 0.015) and liver removal of NH3 (P < 0.001), release of urea-N (P = 0.002) and urea ILR (P = 0.001) were all increased by NH4HCO3 infusion. Hepatic urea-N release (y) and NH3 extraction (x) were linearly related (R2 0.89), with the slope of the regression not different from unity, both for estimations based on liver mass transfers (1.16; SE 0.144; P(b) not equal to 1 = 0.31) and [14C]urea (0.97; SE 0.123; P(b) not equal to 1 = 0.84). The study indicates that a sustained 1.5 or 2.4-fold increase in the basal NH3 supply to the liver did not impair glucose or amino acid supply to non-splanchnic tissues; nor were additional N inputs to the ornithine cycle necessary to convert excess NH3 to urea. Half of the extra NH3 removed by the liver was, apparently, utilized by periportal glutamate dehydrogenase and aspartate aminotransferase for sequential glutamate and aspartate synthesis and converted to urea as the 2-amino moiety of aspartate.

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 intra-ruminal urea infusions and changing digestible organic matter intake on nitrogen kinetics in sheep fed rice straw

Sixteen sheep were continuously fed diets based on straw, straw and barley grain, or NaOH-treated straw, each with adequate minerals and with or without additional urea (7·3 g N/day for the straw and straw and barley diets; 14·4 g N/day for the diet containing NaOH-treated straw) infused intra-ruminally. [14C] tracers were used to measure the rate of irreversible loss (i.e. synthesis) of plasma urea and its transfer to the rumen and to the remainder of the gastrointestinal tract in 4–6 sheep per treatment. [15N] tracers were also used to make these measurements in four sheep fed the straw, straw and barley and straw and barley with intra-ruminal urea infusions. Inclusion of the barley grain in the diet increased digestible organic matter intake by c. 70%. Replacing the straw with a greater amount of NaOH-treated straw increased digestible organic matter intake by 43% in the absence of additional urea, and by 65% when urea was infused intra-ruminally. Rumen ammonia concentrations were 37 and 31 mg N/l when straw or straw and barley diets respectively were provided and were reduced to 3 mg N/l when treated straw was given to the sheep. These concentrations exceeded 223 mg N/l when urea was infused intra-ruminally. Rate of irreversible loss of plasma urea measured using [14C]urea ranged from 3·7 g N/day in sheep given the treated straw diet to 16·8 g N/day when urea was infused intra-ruminally, while this rate was correlated with plasma urea concentration. In the three diets where it was measured, rate of irreversible loss of [15N]urea was on average 0·73 of the rate of irreversible loss of [14C]urea. Measurements made with 14C and 15N tracers of plasma urea transfer to the rumen were similar. This transfer measured with 14C tracers ranged from 1·4 to 2·4 g N/day for the three diets without additional urea, while providing urea by intra-ruminal infusions reduced the transfer to 1·1–1·4 g N/day. Transfer of plasma urea to the rumen was not changed by the higher digestible organic matter intake associated with either inclusion of barley grain in the diet or provision of a greater amount of more highly digestible NaOH-treated straw. These studies indicate that plasma urea transfer to the rumen is not always increased by greater organic matter fermentation in the rumen.

The effect of intake on protein metabolism across splanchnic tissues in growing beef steers

The contribution of the total splanchnic tissue (TSP; portal-drained viscera (PDV) plus liver) to whole-body protein metabolism was estimated in relation to intake (0.6, 1.0 and 1.6 x maintenance requirements), in six multicatheterized growing beef steers used in a double 3 x 3 Latin square design. At the end of each 21 d experimental period, [1-13C]leucine was infused into a jugular vein (1.05 mmol/h for 5 h, preceded by a priming dose of 1.05 mmol). Arterial, portal and hepatic blood samples were collected hourly during the infusion. The increment in TSP leucine irreversible loss rate (ILR) observed with increasing intake reached significance (P < 0.10) only for PDV, while whole-body ILR increased markedly (P < 0.001) with intake. The relative contribution of TSP to whole-body leucine ILR averaged 44% (25% from PDV and 19% from the liver). Although these proportions were not affected by intake, on an incremental basis more than 70% of the increase of whole-body leucine ILR between the 0.6 and 1.0 x maintenance originated from the changes in TSP ILR, while the corresponding value was below 13% between 1.0 and 1.6 x maintenance. Total whole-body leucine oxidation and fractional oxidation increased (P < 0.05) with intake. Protein retention increased with intake (P < 0.01), as a result of a greater increase in protein synthesis compared with protein degradation. Protein breakdown had a major impact on protein turnover as 65% of the protein synthesized was degraded when intake varied from 1.0 to 1.6 x maintenance. Net leucine portal absorption increased (P < 0.001) with intake and represented 1, 16 and 23% of whole body leucine ILR, for 0.6, 1.0 and 1.6 x maintenance, respectively. Although leucine oxidation was not a major component of whole body ILR (9.3-19.9%), it represented 69% of the net available leucine (portal absorption) even at 1.6 x maintenance. The lower relative contribution of the TSP to whole-body leucine ILR at higher intake indicates the proportional increase in the metabolic activity of peripheral tissues as the animals moved into positive protein balance.

The influence of extruding corn grain on glucose metabolism in pregnant ewes.

Glucose metabolism was studied in 31 pregnant ewes fed mixtures (50:50) of chopped alfalfa hay and corn grain given in whole (HWC) or extruded form (HEC). Number of fetuses was 1 (n = 10), 2 (n = 11) or 3 (n = 10). Diets were supplied at three dietary levels: maintenance (1.0 M), 1.25 x maintenance (1.25 M) or twice maintenance (2.0 M). Glucose metabolism was estimated by a double isotope dilution procedure at days 97-121 of pregnancy. Glucose entry rate (GER) was higher (P < 0.04), and glucose irreversible loss rate (GILR) tended to be higher (P < 0.07) in ewes given HEC than HWC. Dietary level (P < 0.001) and the number of fetuses (P < 0.005) affected GER and GILR positively. These results provide a probable explanation for higher birth weights previously found in lambs born to HEC-, compared with HWC-fed dams.

Glucose Metabolism and Milk Yield of Cows Infused Abomasally or Ruminally with Starch1,2

The effect of ruminal or abomasal starch infusion on milk yield and glucose metabolism of early lactation cows was measured. Four cows were continuously infused in the rumen or abomasum with partially hydrolyzed starch (1500 g/d) or were not infused (control) for three 14-d periods during wk 4 to 12 postpartum. Milk yield averaged over 40 kg/d throughout the experiment. Milk and milk lactose yields tended to increase when starch was infused and DMI was decreased, regardless of the site of infusion. Starch infusion increased mean insulin concentration and tended to decrease the concentration of serum nonesterified fatty acids. Ruminal starch infusion did not affect glucose irreversible loss rate but tended to increase glucagon concentration and decrease glucose oxidation. The increased milk yield that occurred when starch was infused ruminally relative to the milk yield of control cows could be a result of increased microbial protein supply or increased energy availability. Compared with ruminal starch infusion, abomasal starch infusion tended to increase the irreversible loss rate of glucose and to increase glucose oxidation. Abomasal infusion tended to increase plasma insulin concentration and to decrease the nonesterified fatty acid concentration relative to ruminal infusion. Infusion of starch abomasally resulted in increases of most uses of glucose, including milk lactose production, glucose oxidation, and the possible storage of glucose as body fat, which indicates that the early lactation dairy cow has a greater capacity for glucose metabolism than is provided by voluntary feed intake of average diets, but that not all available glucose is partitioned to the mammary gland. These data should be useful in testing current concepts and equations in nutritional and metabolic models of dairy cattle.

How is whole body protein turnover perturbed in growth hormone-deficient adults?

Adult patients with GH deficiency have reduced lean body mass (LBM), muscle mass, and muscle strength, suggesting an underlying abnormality of protein metabolism. As acute GH administration has previously been reported to decrease protein oxidation and increase protein synthesis in GH-deficient (GHD) adults, we investigated whether the converse might occur in untreated GH deficiency by undertaking studies of whole body protein turnover in 10 GHD and 13 normal subjects using a 3-h primed constant infusion of 1-[13C]leucine. Dual energy x-ray absorptiometry was used to quantify LBM and fat mass (FM). In normal subjects, LBM was the major, independent determinant of leucine appearance (Ra; r=0.80; P=0.0009), leucine oxidation (r=0.81; P=0.0008), and leucine incorporation into protein (r=0.75; P= 0.003). However, in an analysis of covariance, FM was also a significant independent determinant of leucine Ra (P=0.002) and leucine incorporation into protein (P=0.003). After correcting for LBM and FM, GHD patients had significantly reduced rates of leucine Ra (109.9+/-4.4 vs. 125.5+/-3.7 micromol/min, respectively; P=0.02) and leucine incorporation into protein (87.0+/-3.9 vs. 100.3+/-3.3 mmol/ min; P=0.02) compared to normal subjects. There was no significant difference in the corrected rates of leucine oxidation between the two groups (22.9+/-1.3 vs. 25.2+/-1.0, GHD vs. normal; P=0.20). In summary, GHD adults have reduced rates of protein synthesis and protein breakdown, but normal rates of irreversible oxidative loss; these findings are discordant with what was predicted from the acute changes in protein metabolism observed with GH administration. We conclude that normalization of protein oxidation may be a homeostatic mechanism that operates to constrain protein loss in GHD adults.

Whole-body protein synthesis in growing barrows: diurnal and day-to-day variation and the effect of site of tracer infusion and sampling

In the first experiment, two catheters were placed in one of three sites for infusion and sampling (vena cava and vena cava with 7 cm between catheter tips, VC/VC, n = 5; right ventricle and atrium, RV/RA, n = 5; anterior aorta and left ventricle, AA/LV, n = 4) to determine the effect of site on the measurement of CO2 and leucine kinetics. Pigs received a 6-h continuous i.v. infusion of NaH13CO3 (1.45 µmol h−1 kg−1, preceded by a priming dose of 2.05 µmol kg−1) and of L[1-13C]leucine (9.7 µmol h−1 kg−1, preceded by a priming dose of 9.7 µmol kg−1), the following day. No effect (P &gt; 0.10) of site was observed on NaH13CO3 recovery rate, leucine irreversible loss rate (leucine ILR) and leucine used for protein synthesis (LS). A tendency (P &lt; 0.10) for increased leucine oxidation (LO) was observed with the VC/VC compared to the RV/RA sites. In the second experiment, six growing barrows with catheters in the VC/VC site were infused as earlier with NaH13CO3 and L[1-13C]leucine for three different 6h periods (12:00 to 18:00 h, first day; 20:00 to 02:00 h, night; 12:00 to 18:00 h, second day) to determine the diurnal and day-to-day variation of CO2 and leucine kinetics. No effect (P &gt; 0.10) of period was observed on NaH13CO3 recovery rate, leucine ILR and LS. The LO was lower (P &lt; 0.05) on the second day compared with the first day (day-to-day variation) while similar (P &gt; 0.10) LO was observed between the first day and night (diurnal variation). Together, these results indicate that 1) the VC/VC site can be used for L[1-13C]leucine and NaH13CO3 infusions, 2) NaH13CO3 recovery studies can be performed either the day preceding or following the measurement of substrate and 3) protein synthesis estimated from a 6-h infusion period can be extrapolated to the 24 h periods under conditions of frequent feeding. Key words: Pigs, leucine, protein synthesis, tracer techniques, diurnal variation

Gas chromatography/mass spectrometry analyses of [2,3,3-d3] serine, [2,3,3-d3] cysteine and [3-13 C] cysteine in plasma and skin protein: measurement of transsulphuration in young sheep.

A new procedure using stable isotope labelled serine (L-[2,3,3-d3] serine) and cysteine (L[13-3-13 C] cysteine) and analysis by gas chromatography/mass spectrometry (GC/MS) has been developed to measure transsulphuration in sheep. The enrichments of the tracers in plasma and skin biopsy samples were measured by GC/electron impact MS analysis of the t-butyldimethylsilyl derivatives. The measured recoveries of the standards enriched with [3-13 C] cysteine from 0.1% to 8%, or with [2,3,3-d3] serine from 0.14% to 14% were greater than 99% of the theoretical values, and the variation coefficients were less than 3% when the enrichment was higher than 0.5%. The use of dithiothreitold (DTT) as a reducing agent before deproteinization of the sample and during the derivatizations successfully increased the cysteine peak area and significantly improved reproducibility in the analysis. The cysteine residues in protein from the skin biopsy were also during the protein hydrolysis with DTT in 6 n HCI. The method was applied to measure transsulphuration of methionine in young sheep. The amount of cysteine derived from transsulphuration accounted for 17% to 21% of the irreversible loss rate of cysteine, depending on the substrate supplies. The results are consistent with other reports. Compared with conventional methods of measuring transsulphuration using radioactive isotopes, the processes of animal experimentation was sample analysis were simple, and there were no radiation hazards. The method should prove useful in studies on the metabolism of methionine and cysteine in human and animals.

Short‐chain fatty acids in sheep: Portal appearance rates following high intraruminal loads

The portal recovery of SCFA (short‐chain fatty acids) infused intraruminally as a single load was investigated in sheep adapted to a hay (H) or a concentrate/straw (CS) diet. The treatments were control (saline), 1.4 mol and 2.4 mol of SCFA (in % 65:20:15 acetate, propionate and butyrate). Three rumen cannulated and multi‐catheterized sheep were used. Portal and arterial blood as well as rumen liquid was sampled during a period of 480 min following the loads. The PDV (portal‐drained viscera) metabolism of arterial acetate and the ILR (irreversible loss rate) of arterial acetate were estimated by intravenous infusion of sodium‐acetate [1‐13C]. The portal blood flow rate was measured using ultrasonic transit time flow probes. The portal blood flow rate increased by 60–70% following the intraruminal loads of SCFA compared with the preinfusion level. The portal recovery of arterial acetate increased to 0.84 ± 0.02 and 0.95 ± 0.01 in samples obtained 5 min after an intraruminal load of 1.4 or 2.4 mol SCFA, respectively. Arterial concentrations as high as 3.5, 1.0 and 0.7 mmol 1−1 of acetate, propionate and butyrate, respectively, were measured following the loads. The portal appearance of SCFA appeared to be down‐regulated during the period from 20–120 min following the load of 2.4 mol SCFA. The portal recovery of infused acetate and propionate decreased with increasing amounts of infused SCFA. Recoveries were estimated as 0.46 to 0.59 for acetate, 0.52 to 0.61 for propionate and 0.28 to 0.36 for butyrate. Only the recovery of acetate was affected by diet and the recovery increased (0.05 ± 0.02) when the sheep were adapted to the CS diet (P < 0.05). A model of the regulation of SCFA absorption from the rumen implicating a role of peripheral butyrate is discussed in the context of the results from the present study and from literature data.

Stearic acid oxidation in the Alaskan red-backed vole: effects of cold and norepinephrine

Saturated fatty acids constitute a considerable energy reserve that could convey survival value under chronic cold exposure. It was investigated whether acclimation to cold was associated with a change in the ability of a small microtine rodent, the red-backed vole ( Clethrionomys rutilis), to increase the use of saturated fatty acids during thermogenesis. The C-14 labeled stearic acid, a typical saturated fatty acid (FA), was used to determine the rate of utilization through oxidation. In warm acclimated (WA, 20°C) and cold acclimated (CA, 5°C) voles, acute cold exposure (WA at 5°C, −2°; CA at −5°C) and norepinephrine (NE) injection increased metabolic rate ( V ̇ O 2 , V ̇ CO 2 ) and mobilization of lipid reserves. Acute cold exposure increased percent blood stearate oxidized in a linear fashion with both metabolic rate and stearate concentration in both WA and CA voles. The CA voles at 5°C had increased stearate irreversible loss (1.5×) and oxidation rate (2×) compared to WA voles at 20°C. The CA voles at −5°C increased stearate irreversible loss and oxidation linearly with metabolic rate and blood stearate concentration. In CA voles the contribution made by stearate oxidation to V ̇ CO 2 , increased with the level of cold exposure and NE injection. In contrast, for WA voles stearate irreversible loss and the contribution made by stearate oxidation to V ̇ CO 2 were unaffected by acute cold exposure. Thus, cold acclimation involves: (1) a modest increase (1.5×) in the use of stearate as a fuel substrate; and (2) a greater stearate oxidation and contribution to overall metabolism during acute cold exposure (−5°C).

Diurnal Patterns of Ruminal Concentrations and Portal Appearance Rates of Short-chain Fatty Acids in Sheep Fed a Hay or a Concentrate/Straw Diet in Two Meals Daily

Abstract Three rumen fistulated and catheterized sheep were meal-fed and used to study ruminal and arterial concentrations of short-chain fatty acids (SCFA) as well as portal appearance rates of SCFA and irreversible loss rate (ILR) of acetate in 24 h periods on a hay and a concentrate/straw diet, respectively. Ruminal and arterial concentrations as well as portal appearance rates of SCFA and ILR of acetate were significantly affected by the intake of feed. Generally, the highest concentrations and appearance rates were obtained 2 h after feeding. The portal recovery of arterial acetate was not affected by feeding or diet. The 24 h means were 0.68 ± 0.01 and 0.67 ± 0.01 on the hay and the concentrate/straw diet, respectively. Partial correlation coefficients corrected for the effects of time, sheep, and diet were calculated for the relationships evaluated. The portal appearance rate of acetate (r = 0.52, P <0.001) and the portal net appearance rate of propionate (r = 0.68, P <0.001) were linearly related ...

Diurnal pattern of the interrelationships among leucine oxidation, urea production, and hydrolysis in humans.

We investigated in six healthy adult men, who received an adequate intake of protein (1 g.kg-1.day-1), the relationship among urea production, excretion, and hydrolysis. At the end of a 6-day diet-adjustment period, subjects were studied using a 24-h continuous intravenous [1-13C]leucine and [15N,15N]urea tracer protocol (A. E. El-Khoury, N. K. Fukagawa, M. Sánchez, R. H. Tsay, R. E. Gleason, T. E. Chapman, and V. R. Young. Am. J. Clin. Nutr. 59: 1000-1011, 1994) to determine rates of irreversible protein nitrogen loss and urea kinetics. By combining leucine and urea kinetic data, we found a significant degree of urea hydrolysis over the 24-h period but no evidence to support the thesis that there is a net retention or "salvage" of the urea nitrogen liberated. Our measurements revealed little or no urea hydrolysis during the fed 12-h period of the 24-h tracer protocol but substantial hydrolysis during the 12-h fasting phase. Furthermore, a mass balance model and calculations (APPENDIX) indicated that nitrogen salvage, if any, is quantitatively indistinguishable from insensible nitrogen losses and aggregate estimation errors, accounting for no more than 5% of the nitrogen intake. We conclude that urea hydrolysis, via the intestinal microflora, although representing a component of the overall cycles of nitrogen flow within the body, does not contribute via a net retention of amino nitrogen to the maintenance of body nitrogen homeostasis in healthy adults consuming an adequate diet.

Carbon Flow through the Hepatic Folate-Dependent One-Carbon Pool Is Not Altered in Vitamin A-Deficient Rats

Vitamin A status can influence a number of enzymes and coenzymes involved in folate-dependent one-carbon metabolism as well as subsequent methyl group metabolism. Tracer kinetic techniques were used in the present study to assess the physiological importance of vitamin A deficiency on the de novo synthesis of methionine via the hepatic folate-dependent one-carbon pool. Vitamin A-deficient (0 retinol equivalents (RE) retinyl palmitate/g diet) rats were fed their respective diet for 11 wk, whereas control rats (1.2 RE retinyl palmitate/g diet) were food restricted to match the growth rate exhibited by the vitamin A-deficient group. After the dietary treatment period, duodenal cannulated rats were continuously infused with L-[3-14C] serine and L-[methyl-3H] methionine until a plateau specific radioactivity was exhibited with respect to the hepatic serine and methionine pools, indicating a steady state had been achieved. The hepatic concentration of both S-adenosylmethionine and S-adenosylhomocysteine were elevated in vitamin A-deficient rats. However, vitamin A-deficient rats exhibited similar kinetic values compared with control rats fed a vitamin A-sufficient diet. The irreversible loss rate of hepatic serine and methionine, the transfer quotient from serine to methionine and the folate-dependent flow of carbon to methionine from serine were unaffected by vitamin A status. These studies demonstrate that vitamin A deficiency does not affect the reductive carbon flow from serine to methionine because the ability to generate methionine via remethylation of homocysteine with the carbon group originating from serine was not altered in vitamin A-deficient rats. Furthermore, the data illustrate the importance of using tracer kinetic techniques to quantify metabolic flux under steady-state conditions in vivo, thereby evaluating the consequences of an abnormal condition on a physiological and functional basis.

Portal Recovery of Intraruminally Infused Short-chain Fatty Acids in Sheep

Abstract Three rumen fistulated and catheterized sheep were used to study portal recovery of continuously intraruminally infused short-chain fatty acids (SCFA). Each sheep received six treatments: 0, 60, 120, 180, 240 and 300 mmol h−1 SCFA infused for 8 h in a semi-fasted state. The infused SCFA had a molar proportion of 65:20:15 (acetate:propionate:butyrate). [1 - 13C]Na-acetate was infused intravenously during the experiment for measurement of portal drained viscera (PDV) metabolism and irreversible loss rate (ILR) of acetate. Portal blood flow was measured by ultrasonic flowprobes. Portal blood, arterial blood and rumen liquid were sampled during the last 4 h of the experiments. A model of individual metabolite fluxes in the portal drained viscera is presented. The absorption rate of acetate, measured as portal net appearance plus PDV metabolism, and irreversible loss rate of acetate showed a linear response to infusion rate and accounted for 54 ± 8% and 73 ± 9% of infused acetate, respectively. The B-...

The effect of sucrose supplementation on kinetics of nitrogen, ruminal propionate and plasma glucose in sheep

SUMMARYThe effects of dietary sucrose on the metabolic rate of plasma glucose and ruminal propionate as well as the change in nitrogen kinetics were examined in four mature wethers fitted with rumen fistulas in Tsukuba, Japan in 1990. Wethers were fed at 12 equal intervals daily on crushed lucerne hay cubes (1233 g DM/day), with or without 204 g/day of sucrose. Plasma urea and glucose kinetics were determined following a single intravenous injection of [I5N]urea and [U-13C]glucose respectively; and the kinetics of ruminal ammonia and propionate were determined following a single intraruminal injection of [15N]ammonium chloride and [2–13c]sodium propionate respectively. Following supplementation of sucrose to the diet, nitrogen retention was increased (P&lt; 0·05) with a decrease in plasma urea concentration (P&lt; 0·05) and urinary urea excretion (P&lt; 0·05). Sucrose supplementation decreased (P&lt; 005) the concentration and irreversible loss rate of ruminal ammonia. Urinary allantoin excretion did not change with sucrose treatment, but the flow rate of non-ammonia-nitrogen from the rumen was increasedP&lt; 0·05). The transfer rate of ruminal ammonia to plasma urea was also decreased (P&lt; 0·01), whilst the transfer rate of plasma urea to ruminal ammonia was increased (P&lt; 0·05) by dietary sucrose. Sucrose supplementation resulted in a higher concentration of propionate and butyrate (P&lt; 0·05) in the rumen with no significant change in acetate or pH. The concentration of plasma glucose did not change with sucrose treatment, but the concentration of insulin, pool size (P&lt; 0·05) and the irreversible loss rate of glucose (P&lt; 0·01) were increased, reflecting the increase in the production rate of ruminal propionate (P&lt; 0·05). It was concluded that the supplementation of sucrose affected the metabolism of urea and glucose in plasma via a change in ruminal production rate of ammonia and propionate, respectively.

The importance of transmethylation reactions to methionine metabolism in sheep: effects of supplementation with creatine and choline.

The influence of administering the methylated products choline and creatine on methionine irreversible-loss rate (ILR) and recycling from homocysteine has been investigated in sheep fed close to energy and N equilibrium. Two methods to estimate methionine recycling were compared. The first involved [U-13C]methionine infused as part of a labelled amino acid mixture obtained from hydrolysed algal protein. In this approach the isotope dilution of methionine with all five C atoms labelled (m + 5) will represent the ILR which does not recycle through homocysteine, while that which includes molecules with C-1-C-4 labelled will allow for loss of the labelled methyl (5)-C atom and replacement by an unlabelled moiety in the remethylation of homocysteine. The second method involved a combined infusion of [1-13C]- and [S-methyl-2H3]methionine. These two approaches gave similar data for methionine ILR which does not include label recycled to the amino acid from homocysteine but differed for recycled methionine fluxes. Consequently the two procedures differed in the calculated extent of homocysteine methylation under control conditions (6 v. 28%). These extents of remethylation are within the range observed for the fed human subject, despite the fact that fewer dietary methyl groups are available for the ruminant. Using combined data from the infusions, significant depression of methionine recycling occurred in blood (P < 0.05), with a similar trend for plasma (P = 0.077), when choline plus creatine were infused. Wool growth, assessed by intradermal injection of [35S]cysteine, was not altered by supplementation with the methylated products. From changes in the label pattern of free methionine in aortal, hepatic portal and hepatic venous blood during U-13C-labelled algal hydrolysate infusion, the major sites of homocysteine remethylation appear to be the portal-drained viscera and the liver. This was confirmed by analysis of free methionine enrichments in various tissues following dual infusion of [1-13C]- and [S-methyl-2H3]methionine, with the greatest activities occurring in rumen, jejunum and liver. Of the non-splanchnic tissues examined, only kidney exhibited substantial methionine cycling; none was detected in muscle, heart, lung and skin. The implications of methyl group provision under net production conditions are discussed.

Effects of bovine somatotropin and insulin on whole-body and hindlimb glucose metabolism in growing steers

Six Holstein steers (245 kg initial BW) were surgically prepared with chronic catheters to allow measurement of blood flow and nutrient flux across the hindlimb. Steers were used in a single-reversal design with 16-d treatment periods of daily i.m. injection of either excipient (control) or recombinantly derived bovine somatotropin (120 micrograms/kg BW). On d 15 of each period, whole-body and hindlimb glucose metabolism were studied during a primed continuous infusion of [6(-3)H]glucose, under both basal conditions and during a hyperinsulinemic/euglycemic clamp. Somatotropin increased (P < .01) basal blood glucose and serum insulin by 7 and 150% respectively, with no change in glucose irreversible loss rate (ILR). There was no effect of somatotropin on hindlimb blood flow or oxygen consumption. In contrast, hindlimb lactate uptake (P < .02) and the ratio of glucose to oxygen uptake (P < .08) were reduced by somatotropin. Insulin infusion stimulated (P < .01) glucose ILR and inhibited (P < .02) endogenous glucose production to a lesser extent during somatotropin treatment. Basal plasma nonesterified fatty acid concentrations were increased (P < .01) during somatotropin treatment and decreased (P < .02) during insulin infusion. Insulin infusion increased hindlimb blood flow and glucose uptake to similar extents during both treatment periods. These data demonstrate that somatotropin treatment of growing steers decreases response of tissues to insulin (sensitivity in muscle, sensitivity, and responsiveness in liver and adipose tissue), thereby increasing glucose availability to other tissues.