Use of the RatLoft decreases pup mortality in lactating mice.

Lactating mice in laboratory conditions are thought to be under considerable stress. Dams may manifest this stress through a decrease in milk yield and/or increase in infanticide. The purpose of this study was to examine the effect of access to an environmental enrichment device called the RatLoft on milk yield, circulating serotonin, and pup mortality using both wild-type mice and mice genetically deficient in tryptophan hydroxylase 1 (TPH1(-/-); the rate-limiting enzyme in the non-neuronal serotonin synthesis pathway). Presence or absence of the RatLoft did not affect milk yield or circulating serotonin concentrations overall, but serotonin concentrations decreased throughout the 21-day lactation period. Serotonin concentrations were increased in TPH1-deficient mice with access to the RatLoft compared with those without the RatLoft on day 21. Pup mortality tended to decrease for dams with access to the RatLoft as compared to no RatLoft. Within the TPH1(-/-) groups, dams with access to the RatLoft tended to kill less pups per dam than dams without the RatLoft. These results demonstrate that access to the RatLoft during lactation decreases pup infanticide by dams. This environmental enrichment may be particularly beneficial to TPH1(-/-) dams that, in addition to decreased infanticide, had increased serotonin concentrations, possibly indicating enhanced well-being. Use of the RatLoft could prove beneficial to researchers working with lactating mouse models to increase the number of pups weaned and positively impact the welfare of the dam.

Urea production is increased in neonatal piglets infused with alanine at 25, 50, and 75% of resting energy needs .

To study the ability of neonatal piglets to metabolize a nitrogen load and excrete it as urea, 12 newborn piglets, 6 small (0.99 +/- 0. 16 kg; expt. 1) and 6 large (1.86 +/- 0.16 kg; expt. 2), were infused intravenously with alanine (n = 8; 4 large, 4 small; treatment) or glucose (n = 4; 2 large, 2 small; control) at equal ATP equivalents, supplying 25-75% of the resting energy requirements of the piglet over 18 h. To adjust for differences in the baseline urinary urea nitrogen excretion, blood urea nitrogen (BUN) and estimated urea production between groups, the absolute changes from baseline to maximum value for piglets infused with alanine, and from baseline to the 24-h value for piglets infused with glucose were evaluated statistically. There were no differences (0.1 < P < 0.3) in the absolute changes from baseline to maximum values of urinary urea nitrogen, BUN or estimated urea production between small [18.6 +/- 3.8 mg N/(h. kg(0.75)); 19.1 +/- 2.2 mmol N/L; 2.7 +/- 1.2 mmol N/(h. kg(0.75)), respectively] and large [23.6 +/- 7.6 mg N/(h. kg(0. 75)); 21.6 +/- 3.3 mmol N/L; 3.7 +/- 1.5 mmol N/(h. kg(0.75)), respectively] piglets infused with alanine. Differences in the changes from baseline were detected between alanine and glucose (P = 0.001) infusions. Small piglets required more time (P < 0.005) for BUN to maximize after initiation of the alanine infusion, suggesting that small piglets require more time to process a nitrogen load. Infusion of alanine resulted in at least a threefold increase from baseline in the rate of calculated urea production, suggesting that neonatal piglets, small or large, have reserve capacity to metabolize nitrogen and excrete it as urea.

Comparison of total body urea production potential with total body carbamoyl phosphate synthetase (CPS-1) activity in newborn piglets infused with alanine at 50% of resting energy expenditure for 36 hours.

The calculated rate of urea production [U(p); mmol urea/(h. kg(0. 75))], based on urinary urea-N (UUN) excretion and changes in total body urea-N, was compared with the calculated total body V(max) of carbamoyl phosphate synthetase (CPS-1) of 24 neonatal piglets from four treatments as follows: 6 h baseline control (n = 8), 18 h of alanine intravenously (IV) at 50% of resting energy expenditure (REE; n = 4), 36 h of alanine IV at 50% of REE (n = 6), or 36 h of glucose IV at 50% of REE (n = 6). The following significant increases from baseline were seen in piglets infused with alanine for 36 h: 1) UUN excretion [10.6 +/- 5.9 mg N/(h. kg(0.75)) to 53.2 +/- 11.1]; 2) BUN concentrations (9.1 +/- 3.0 mmol urea N/L to 51.2 +/- 7.0); 3) calculated urea production [0.34 +/- 0.21 mmol urea/(h. kg(0.75)) to 2.39 +/- 0.53]; and 4) CPS-1 V(max) [2.0 +/- 0.81 mmol citrulline/(h. kg (0.75)) to 4.4 +/- 1.5], (P < 0.05). With the exception of CPS-1 activity, significant decreases from baseline were seen in these values in piglets infused with glucose for 36 h (P < 0.05). Comparison of calculated urea production with calculated total body CPS-1 V(max) at baseline, 18 or 36 h after the start of infusion of alanine or glucose revealed a positive relationship (slope = 0.263; P < 0.002). At all enzyme activities, infusion of alanine resulted in a significant increase in the rate of urea production compared with controls (P < 0.001). Total body CPS-1 activity varied from 1.8 to 5.8 times that of urea production, suggesting that CPS-1 did not limit urea production.

Effect of sensory stimuli on huddling behavior of pigs.

Two experiments involving pigs at 1, 3, and 8 d of age were conducted to 1) compare huddling between littermates and nonlittermates, 2) study the ability of pigs to distinguish an anesthetized piglet from a piglet-shaped object, and 3) explore the importance of physical contact between pigs on huddling behavior. Experiments were conducted in an enclosed rectangular aluminum test chamber having pressure sensors beneath floor panels to detect test pig location. Test objects were placed on a platform at one end of the chamber and test pig location was monitored during a 45 min trial. Experiment 1 involved a total of 45 pigs (5 pigs/treatment on d 1, 2, and 3). The results indicate that, regardless of age (P > .05), when either a littermate or a nonlittermate occupied the platform, average location of test pigs that "settled" (ceasing to move for 7 min or more) was closer to the platform (P < .01), time spent near the platform was greater (P < .01), and movement about the chamber was less (P < .01) than when the platform was empty. No differences (P > .05) were observed between littermate and nonlittermate stimuli for these variables. During Exp. 2, the platform was covered with wire mesh. A total of 98 pigs were used in the study. Treatments were a cage containing 1) no object (n = 24), 2) a wooden block (n = 25), 3) a pig-shaped latex casting (n = 24), or 4) an anesthetized 8- to 10-d-old pig (n = 25). Pig age and treatment did not affect the percentage of time in each trial that pigs spent within 23.5 cm of the cage or the percentage of pigs settling within 23.5 cm of the cage. These studies show that pigs huddle similarly with littermates and nonlittermates and that physical contact with another piglet but not visual recognition of another piglet affects piglet huddling.

The effect of thermal environment and age on neonatal pig behavior.

Three experiments were conducted to evaluate the ability of a radiant environment and the presence of a littermate to attract pigs during the first 3 d of age. The effect of stimuli on pig movement was studied in an enclosed rectangular aluminum test chamber containing four similar sections that were heated independently. In Exp. 1, all sections were at 34.8 degrees C to evaluate the chamber for biases of where pigs located themselves at 1 (n = 24) and 2 d (n = 26) of age. More (P < .025) pigs settled (e.g., no movement for 7 min) in end sections than in middle sections. Age did not affect time to settle or settling location. The effect on pig location of heating one chamber end section to either 23, 40, 48, 56, or 64 degrees C and leaving the remaining sections unheated (24 degrees C) was determined in Exp. 2. Settling of pigs at 1 (n = 50) and 2 d (n = 50) of age was affected by temperature (P < .001) but not by age. The minimum distance between average pig location and the heated section occurred at 48 degrees C. Experiment 3 involved 15 pigs each at 1 and 3 d during a 1-h trial to compare the relative pig attraction to 1) a heated chamber end section at 44.4 degrees C when remaining sections were at 23.5 degrees C, 2) an anesthetized littermate in an end section when all sections were at 24.1 degrees C, or 3) a choice test involving a 45.5 degrees C end section and an anesthetized littermate in the opposite end section with three unheated sections at 23.7 degrees C. Average distance between the test animal and the heated section was greater (P < .01) than that between the test animal and an anesthetized pig. Pigs that were allowed a choice preferred to lie near an anesthetized littermate in a cold section rather than alone in a 45 degrees C section (P < .01), and they were less (P < .005) active when an anesthetized littermate was present in the chamber. Although radiant heat effectively attracted pigs, heat was less attractive than an anesthetized littermate. The greater attraction of pigs to a littermate than to radiant heat may explain why pigs remain near the sow and littermates during d 1 and 2 after birth.

Mitochondrial lysine uptake limits hepatic lysine oxidation in rats fed diets containing 5, 20 or 60% casein.

Sixty male Sprague-Dawley rats were randomly allotted to receive diets containing 5, 20 or 60% casein. Rats had access to the diet only during the initial 8 h of the daily 12-h dark period. Hepatic mitochondrial lysine uptake, lysine alpha-ketoglutarate reductase (LKR) and saccharopine dehydrogenase (SacD) activities, and in vitro lysine oxidation (LOX) were measured 0, 6, 12, 18 and 24 h after the start of the dark period. Diurnal variation of mitochondrial lysine uptake was not detected (P > 0.10) although uptake varied 3-fold over 24 h. Mitochondrial lysine uptake was greater (P < 0.05) for rats fed diets containing 60% casein than for rats fed diets containing 5% casein. Diurnal variation of LKR was detected (P < 0. 05) in rats fed diets containing 20 and 60% casein. Diurnal variation of SacD was detected (P < 0.05) in rats fed diets containing 60% casein. Increased casein consumption resulted in increased LKR and SacD activities (4- to 5-fold; P < 0.05). Diurnal variation of LOX was detected in rats fed diets containing 20 and 60% casein (P < 0.05). Increasing the casein concentration in the diet from 5 to 60% resulted in a 7-fold increase in LOX (P < 0.05). To make rate comparisons, LKR and SacD activities and LOX were predicted from a range of substrate concentrations (0.1 to 5.0 mmol/L). Overall, LKR and SacD were 6-107 times that of LOX, suggesting that, in liver, mitochondrial lysine uptake limits LOX.

Rates of lysine catabolism are inversely related to rates of protein synthesis when measured concurrently in adult female rats induced to grow at different rates.

To test the effect of changes in the rate of protein synthesis on amino acid oxidation, both were studied concurrently in individual 200-g female Sprague-Dawley rats. In a growth trial (Experiment 1), recombinant bovine somatotropin (rbST) was injected subcutaneously (0, 2 or 12 mg/d) over 6 d (n = 4 rats per rbST level). Weight gain increased with rbST level (P < 0.01); 1.96 +/- 0.8, 4.24 +/- 0.8 and 8.67 +/- 0.8 g/d, respectively. After treatment with rbST (0 or 12 mg/d) for 4 d (Experiment 2), rats were injected via a tail vein catheter with valine (400 mmol, 4.07 mBq L-[3,4(n)-3H]valine) at 0, 4, 10, 13 or 16 h after the daily rbST injection and killed 20 min later. This flooding dose was 5 to 6 times, not 10 times, the free pool as hoped. Protein synthesis in rbST-treated rats increased 46% in muscle (P < 0.001) and 36% in liver (P < 0.01). The ks was unaltered with time after rbST injection (0-16 h, P > 0.05). When 600 mmol valine (4.4 mBq L-[3,4(n)-3H]valine) was used in Experiment 3, specific activity (SA) of free valine was constant over 20 min and was 94 +/- 4% of that injected. Finally, in Experiment 4, protein synthesis and amino acid oxidation rates measured in the same rat revealed a 35% increase (P < 0.01) in protein synthesis in hind leg muscle and a 29% increase in liver (P < 0.05) from rbST-injected (12 mg/d) rats (n = 6). Lysine oxidation was estimated by continuous (12 h) infusion of L-[1-14C]lysine via the opposite tail vein catheter. Expired CO2 was collected over 20-min intervals and SA at plateau was estimated by fitting an exponential model. Lysine oxidation was reduced (P < 0.05) by 44% in rbST-treated rats. The idea that an increase in protein synthesis results in decreased amino acid oxidation remains tenable.

Feeding diets containing high levels of milk products or cellulose decrease urease activity and ammonia production in rat intestine.

Three studies were done to determine the effect of feeding diets containing high levels of a readily fermentable carbohydrate (lactose in milk or yogurt, or pure lactose) or an undigestible, unfermentable diluent (alpha-cellulose) on urease (EC 3.5.1.5) activity and net ammonia production in the rat gastrointestinal (GI) contents. Rats (170-200 g) were fed a control diet or diets containing 55% dried milk or 55% dried yogurt, 25% lactose or 10% alpha-cellulose. Feeding diets containing milk or yogurt decreased urease activity to approximately 11% of the control value in the small intestine (on the basis of grams of collected contents or total contents), and to 50% in the large intestine (only on the basis of grams of collected contents). Feeding the diet containing 25% lactose also decreased urease activity (on the basis of grams of collected contents or total contents) to about 20% of the control value in the small intestine, but not (P > 0.05) in the large intestine. Net ammonia production rate was correlated (r2 = 0.98) with urease activity in the large intestinal contents, and the rate of ammonia production from ureolysis represented about two thirds of the total. Feeding the cellulose diet decreased (P < 0.05) both urease activity and net ammonia production in the large intestine to approximately 30% of the control value. Weights of tissue and contents of the large intestine were much higher (P < 0.01) in rats fed diets containing milk products or lactose than in the control rats, but were not affected by consumption of the cellulose diet. Results of our studies indicate that feeding diets containing high levels of milk products (lactose) or cellulose reduces urease activity and net ammonia production in the rat intestine, and thus may be beneficial for improving animal and human health.

The effect of farrowing crate heat lamp location on sow and pig patterns of lying and pig survival.

Three experiments were conducted to study sow and pig behavior during the 1st 3 d after birth and pig survival during the 1st 2 wk after farrowing. In Exp. 1, 23 sows were housed in conventional farrowing crates that were divided into five sections: a .5- x 1.5-m front creep section and the remaining area divided into four sections, .75 x 1.05 m each. Air temperature was maintained at 19 degrees C, and a 250-W heat lamp was placed at the right side of the front creep in Treatment 1 (T1), or in the creep at the right side of the sow for Treatment 2 (T2). The percentage of pigs within 8 cm of the sow's trunk was not affected by treatment, but it decreased (P < .001) from 61.8 +/- 3.4% on d 1 to 28.1 +/- 3.5% on d 3. As the percentage of pigs near the sow decreased, the percentage of pigs within the section containing the heat lamp increased (T1, P < .05; T2, P < .10). Experiment 2 involved 15 sows and litters housed as in Exp. 1, except that heat lamps were not provided, and average air temperature was 27.3 +/- .2 degrees C during behavioral observations. Even though the portion of the litter near the sow decreased (P < .001) from d 1 to d 3 (d 1, 57.0 +/- 3.4%; d 2, 42.9 +/- 3.3%; d 3, 31.7 +/- 3.3%), pigs did not concentrate in any specific section as they moved away from the sow. The average number of pigs within the front creep section (Section 1) for the 3-d period was less than (P < .01) the number in any other crate section. Experiment 3 involved 147 sows and tested the effect of solid creep floor covering on pig survival for each of the heat lamp locations used in Exp. 1. Neither heat lamp location nor floor covering affected pig survival. During the 1st 3 d of life, pigs tend to lie near the sow regardless of heat lamp location or air temperature. Heat lamp position and floor covering under the lamp do not affect pig survival.

Synthesis of [9-14C]nonanoic acid via 2-thienyl(14CH3)(cyano)cuprate and its oxidation by newborn piglet muscle strips.

To determine if newborn piglet muscle could oxidize propionyl-CoA formed by catabolism of odd-chain fatty acids, an odd-chain fatty acid labeled in the terminal three carbons was needed. The synthetic scheme described is based upon the displacement of a primary alkyl iodide, ethyl 8-iodooctanoate, by a [14C]methyl group via an activated 2-thienyl(14CHa)(cyano)cuprate intermediate, forming ethyl [9-14C]nonanoate. Ethyl [9-14C]nonanoate was hydrolyzed in 6 N KOH and [9-14C]nonanoic acid recovered by ion-exchange chromatography. The yield of [9-14C]nonanoic acid was 40%, based on the initial amount of [14C]methyl iodide. The cuprate and other precursors were commercially available or readily synthesized from available precursors. Mass spectroscopy of commercial and synthesized nonradioactive nonanoate determined an m/z of 159 for the product molecular ion, as expected. The 14C-labeled product phenacyl ester was found to cochromatograph in a C-18 reverse-phase HPLC system with similarly derivatized commercially obtained nonanoic acid. The synthesis should be generally applicable to labeling of compounds by displacement of primary alkyl iodides, where other reactive groups (e.g., carboxylic acid), if present, can be protected (e.g., converted to an ester). Muscle strips isolated from the triceps muscle of newborn piglets oxidized [9-14C]nonanoic acid to 14CO2. Newborn piglet muscle can oxidize propionyl-CoA produced during odd-chain fatty acid oxidation.

Efficiency of lysine or threonine retention in growing rats fed diets limiting in either lysine or threonine.

Over a 21-d experiment, the efficiency of lysine and threonine retention was determined in 80 male Sprague-Dawley rats (65.9 +/- 0.3 g, means +/- SE) fed purified diets containing an amino acid mix limiting in either lysine or threonine. With additional increments of the first limiting amino acid, lysine concentration in total body protein (g/16 g N) increased (P < 0.01) in rats fed lysine-limiting diets but, when fed threonine-limiting diets, lysine concentration in body protein first increased and then decreased (P < 0.01). As increments of the first limiting amino acid were added, the threonine concentration in total body protein increased then decreased when both lysine- (P < 0.01) and threonine- (P < 0.06) limiting diets were fed. Lysine and threonine retention were calculated based on comparative slaughter. Sixteen rats were killed on d 0 to estimate the grams of amino acid in the body. Retention responses were analyzed using a logistic equation in which lysine or threonine intake was used to predict retention. The maximum marginal efficiency (dr/dI, retention/intake) was observed at <40% of maximum retention. For lysine retention, it was 81% when lysine was limiting and 70% when threonine was limiting. For threonine retention, it was 58% when threonine was limiting and 49% when lysine was limiting. The maximum cumulative efficiency (retention adjusted for maintenance relative to cumulative intake) for lysine retention was 62% when lysine was limiting or 58% when threonine was limiting. For threonine retention, it was 51% when threonine was limiting and 35% when lysine was limiting. Thus, amino acid concentration in body protein is not constant, and amino acids are used with higher efficiency when first limiting.

Recombinant bovine somatotropin decreases hepatic amino acid catabolism in female rats.

The effect of recombinant bovine somatotropin (rbST) on hepatic amino acid catabolism in female rats was investigated. Daily injections of rbST for 5 d decreased liver homogenate lysine alpha-ketoglutarate reductase (EC 1.5.1.8) activity (P < 0.05) and liver homogenate lysine oxidation (P < 0.05) approximately 35%. Liver homogenate methionine and valine oxidation were depressed approximately 20 (P = 0.13) and 35% (P < 0.05), respectively. These data show a decrease in hepatic capacity to oxidize amino acids in rats administered rbST. Whether depressed liver amino acid degrading enzyme activity plays a role in amino acid oxidation in vivo remains to be evaluated.

Contribution of D-(-)-3-hydroxybutyrate to the energy expenditure of neonatal pigs.

In vivo oxidation rate of arterially infused D-(-)-3-hydroxybutyrate (3HB) was measured in 1-2-d-old-piglets. Twelve piglets (1.4 kg) were randomly assigned to a 12 h continuous infusion of 3HB at 19.5, 37.8, 55.8 or 74.5 mumol/min along with -31 kBq/h of [3-14C]3HB. Piglets were housed in respiration chambers allowing collection of total expired CO2 over 20-min intervals for the 12 h infusion and 6 h washout. Oxidation of 3HB was calculated from the quantity and specific radioactivity of expired CO2 for 20-min collection periods at 6, 9 and 12 h for each piglet and collectively plotted against plasma 3HB concentration measured in blood drawn during those 20-min periods. A Lineweaver-Burk plot of these data yielded a Km of 0.62 +/- 0.07 mmol/L and Vmax of 0.74 +/- 0.02 mmol ATP equivalents/(min.kg 0.75) (parameter estimate +/- SD), which could account for 32% of the piglet mean total ATP turnover of 2.3 mmol/(min.kg 0.75). These data show that 3HB oxidation is a linear function of plasma concentration in the physiologic range measured in piglets (0.006 mmol/L to 0.1 mmol/L) and within this range would account for 0.3% to 4.5% of piglet energy requirement. Oxidation of 3HB can meet a maximum of 30 to 40% of piglet energy requirement at unphysiologically high 3HB concentrations (> 3 mmol/L).

Diminishing returns in weight, nitrogen, and lysine gain of pigs fed six levels of lysine from three supplemental sources.

Diminishing returns responses to increments of lysine intake were evaluated in 82 crossbred barrows (10.2 +/- SE = .31 kg) fed one of six concentrations of lysine expressed as 55, 80, 90, 100, 115, or 140% of a concentration (10.9 g/kg of diet) that supported maximum weight gain. Supplemental lysine was provided from either soybean meal (SBM), SBM+synthetic lysine (SBM + L), or SBM+corn gluten meal (SBM + CGM) additions to a basal diet consisting of corn, minerals, and vitamins. A logistic equation was used to describe weight (kilograms/day), nitrogen (grams/day) and lysine (grams/day) gain as a function of lysine intake from each diet. The parameter Rmax, asymptotic maximum response at infinite intake, was shared for SBM and SBM+L (.70 +/- .02, 17.6 +/- .4 and 7.6 +/- .5, respectively) but was different (P < .05) for SBM + CGM (.62 +/- .02, 15.7 +/- .4 and 6.4 +/- .4, respectively) diets. A plateau in weight gain response was observed at approximately 100% of our estimate of the lysine requirement (9.8 g/kg, based on analyzed values), but nitrogen and lysine gain responses did not approach a plateau until 120 or 145% of the requirement. No differences were detected among pigs fed the SBM and SBM+L sources; however, the efficiency of lysine use by pigs fed the SBM+CGM diet was lower. Maximum marginal efficiency (dr/dI) occurred at 45, 40, and 35% of Rmax for weight, nitrogen, and lysine gain, respectively. The maximum marginal efficiency of lysine gain was 81% for pigs fed the SBM and SBM+L diets but only 68% for pigs fed the SBM+CGM diet. Consequently, diminishing returns were apparent for at least the upper 55 to 65% of the response curve. Whole-body lysine content increased (P < .03) from 5.5 to 6.0 g/16 g of N and glycine (9.6 to 7.9) and proline (6.4 to 5.4) content decreased (P < .001) as lysine intake increased. No changes were detected in body threonine content (pooled average = 2.8).

Lysine-alpha-ketoglutarate reductase and saccharopine dehydrogenase are located only in the mitochondrial matrix in rat liver.

In rat liver, comparisons of marker enzyme activities (beta-hexosaminidase, lysosomes; catalase, peroxisomes; cytochrome oxidase, mitochondrial-inner membrane; monoamine oxidase, mitochondrial outer membrane; ornithine aminotransferase, mitochondrial matrix) show that lysine-alpha-ketoglutarate reductase and saccharopine dehydrogenase, the initial enzymes of saccharopine-dependent lysine degradation, are found only in the mitochondrial matrix. These results are consistent with obligatory uptake of lysine into the matrix for lysine catabolism and raise the possibility that lysine transport into the mitochondrion may control lysine degradation.

Role of protein synthesis in amino acid catabolism.

Except for branched chain amino acids, the site of indispensable amino acid degradation is the liver. Location of amino acid degradation capacity in a single organ may play an important role in the reutilization of amino acids derived from protein turnover. The importance of preferential utilization of amino acids for protein synthesis on catabolism of amino acids is demonstrated in two ways. First, by minimal oxidation of an amino acid at dietary concentrations below that required for maximum gain followed by a near proportionate oxidation with increased dietary level, and second, by increased oxidation of an indispensable amino acid when another amino acid limits protein synthesis. A direct effect of protein synthesis on amino acid catabolism can be shown by a marked increase in amino acid catabolism when protein synthesis is inhibited.

Comparison of measured carbon dioxide production with that obtained by the isotope dilution technique in neonatal pigs: observations on site of infusion.

Fifteen newborn pigs (1.5 kg) were used to estimate CO2 "production" over a 5-h experiment. In each piglet, the average expired air CO2 irreversible loss rate measured over 15-min intervals was compared with an estimate of CO2 "produced" by a continuous infusion of NaH14CO3 into either the aortic artery (seven pigs) or the portal vein (eight pigs). The specific radioactivity (Bq/mumol) of the expired CO2 taken over consecutive 15-min intervals during the 5-h period was fitted to an exponential model to predict the specific radioactivity at steady state. This specific radioactivity was used to calculate the total CO2 irreversible loss rate and to correct for the difference in tracer infused in relation to tracer excreted at 5 h (76 to 78%). The estimated CO2 "produced" tended to be higher (P < 0.12) when the aortal rather than the portal infusion site was used [571 vs. 498 mumol/(min.kg0.75)]. The isotope dilution technique significantly (P < 0.05) overestimated the expired air CO2 regardless of whether the aortal [571 vs. 469 mumol/(min.kg0.75)] or portal [498 vs. 447 mumol/(min.kg0.75)] routes of infusion were used.

Evaluation of [1-14C]-medium-chain fatty acid oxidation by neonatal piglets using continuous-infusion radiotracer kinetic methodology.

In vivo oxidation rates of systemically infused medium-chain fatty acids were evaluated using neonatal pigs. Unanesthetized piglets (1.6 kg, n = 20) were housed in respiration chambers for total collection of expired CO2 and were continuously infused with [1-14C]-7:0, 8:0, 9:0 or 10:0 fatty acids via a central catheter. The fatty acids were administered at rates of 25, 50 or 100 mumol/min for 5 h and provided 74 kBq of 14C per h. Total expired CO2 was collected over consecutive 15-min intervals for determination of the amount and specific radioactivity of expired CO2. Portal blood samples were drawn from umbilical vein catheters for determination of 3-hydroxy-butyrate and plasma fatty acid concentrations. Infusion and oxidation rates (mmol/min) were multiplied by the molar ATP yield for each fatty acid (i.e., 52, 61, 69 and 78 mmol ATP/mmol fatty acid for 7:0 through 10:0, respectively) to adjust for differences in molar energy content of the various fatty acids. Expressed in this way, fatty acid oxidation rate was proportional to the rate of infusion and accounted for 63% of that infused. The various fatty acids were oxidized equally well, regardless of chain length, and satisfied up to 60 to 70% of the animals' energy requirements, depending on the rate of infusion. These data are discussed in relation to previous work from our laboratory that has shown significant effects of fatty acid chain length on utilization of orally administered medium-chain triglyceride supplements.