Amino Acid Flux Research Articles

Immunohistochemical localization of the amino acid transporter SNAT2 in the rat brain

SNAT2 is a neutral amino acid carrier that belongs to the system A family. Since its function in the nervous system remains unclear, we have analyzed its distribution in the rat CNS using specific antisera. Although SNAT2 is expressed widely in the CNS, it is enriched in the spinal cord and the brainstem nuclei, especially those of the auditory system. At the cellular level, SNAT2 was preferentially located in neuronal cell bodies and processes, although it was also strongly expressed in the meninges and ependyma. In astrocytes, the localization of SNAT2 was more restricted since it was intensely expressed in the perivascular end-feet, glia limitans, cerebellar astrocytes and Bergmann glia, but it was less intense in astrocytes of the cerebral parenchyma. Among neurons, the primary sensory neurons of the mesencephalic trigeminal nucleus appeared to be those that most strongly express SNAT2, but many other neurons, including cortical pyramidal cells and their dendrites were also intensely stained. In several regions the transporter was detected in axons, especially in the brainstem, and its presence in both dendrites and axons was confirmed by confocal microscopy and ultrastructural studies. However, while SNAT2 was observed in the large principal dendrites and the small distal dendrites, it was only found in axonal shafts and was excluded from terminals. Some glutamatergic neurons were among the more intensely labeled cells whereas SNAT2 was not detected on GABAergic neurons. The expression of SNAT2 partially coincides with that reported for SNAT1, especially in glutamatergic neurons. Hence, both proteins could fulfill complementary roles in replenishing glutamate pools and be differentially regulated under different physiological conditions. They also seem to co-localize in non-neuronal cells probably contributing to amino acid fluxes through the blood–brain barrier.

Effect of Level of Metabolizable Protein on Splanchnic Flux of Amino Acids in Lactating Dairy Cows

The response of splanchnic tissue metabolism to different levels of metabolizable protein (MP) was measured in 6 catheterized multiparous lactating Holstein cows. Three diets, balanced to provide similar energy intakes and increasing amounts of MP (g/d)—1922 (low), 2264 (medium), and 2517 (high)—were fed during 21-d experimental periods according to a replicated Latin square. On d 18, 19, or 20, six hourly blood samples were collected simultaneously from the portal and hepatic veins plus an artery to determine net fluxes of nutrients across the portal-drained viscera and the liver. Yields of milk and protein increased, as did urinary N excretion with increasing MP. Portal absorption of essential amino acids (EAA) increased linearly with increasing MP supply, as did liver removal of His, Met, and Phe. In contrast, liver removal of the branched-chain AA (BCAA) and lysine was unaffected by diets. With increasing MP, the ratio of milk output to postliver supply of BCAA, Thr, and Lys decreased linearly, indicating oxidation of these AA in the peripheral tissues. Concomitant to a decreased catabolism of EAA in the liver (His, Met, Phe, and Thr) and/or in peripheral tissues (BCAA, Lys, and Thr), the efficiency of transfer of absorbed EAA into milk protein decreases markedly as protein supply increases. The efficiency of transfer of absorbed AA into milk also varies greatly between AA. These 2 important factors should be taken into account when building predictive schemes for milk protein output.

Large-scale analysis of the barley transcriptome based on expressed sequence tags.

To provide resources for barley genomics, 110,981 expressed sequence tags (ESTs) were generated from 22 cDNA libraries representing tissues at various developmental stages. This EST collection corresponds to approximately one-third of the 380,000 publicly available barley ESTs. Clustering and assembly resulted in 14,151 tentative consensi (TCs) and 11 073 singletons, altogether representing 25 224 putatively unique sequences. Of these, 17.5% showed no significant similarity to other barley ESTs present in dbEST. More than 41% of all barley genes are supposed to belong to multigene families and approximately 4% of the barley genes undergo alternative splicing. Based on the functional annotation of the set of unique sequences, the functional category 'Energy' was further analysed to reveal tissue- and stage-specific differences in gene expression. Hierarchical clustering of 362 differentially expressed TCs resulted in the identification of seven major clusters. The clusters reflect biochemical pathways predominantly activated in specific tissues and at various developmental stages. During seed germination glycolysis could be identified as the most predominant biochemical pathway. Germination-specific glycolysis is characterized by the coordinated expression of phosphoenolpyruvate carboxylase and phosphoenolpyruvate carboxykinase, whose antagonistic actions possibly regulate the flux of amino acids into protein biosynthesis and gluconeogenesis respectively. The expression of defence-related and antioxidant genes during germination might be controlled by the ethylene-signalling pathway as concluded from the coordinated expression of those genes and the transcription factors (TF) EIN3 and EREBPG. Moreover, because of their predominant expression in germinating seeds, TF of the AP2 and MYB type are presumably major regulators of germination.

Comparison of predictions of digestible supply and measurements of net portal fluxes of essential amino acids in lactating dairy cows

Comparison of predictions of digestible supply and measurements of net portal fluxes of essential amino acids in lactating dairy cows

Polyethylene glycol reduces the net flux of branched-chain amino acids across the mammary gland in ewes fed sulla

Polyethylene glycol reduces the net flux of branched-chain amino acids across the mammary gland in ewes fed sulla

Effect of urea supplementation on urea kinetics and splanchnic flux of amino acids in dairy cows

1. Effect of method of conservation of timothy on endogenous nitrogen flows in lactating dairy cows D.R. Ouellet, R. Berthiaume, G. Holtrop, G.E. Lobley, R. Martineau, H. Lapierre Journal of Dairy Science CrossRef

A quantitative analysis of cell volume and resting potential determination and regulation in excitable cells.

This paper quantifies recent experimental results through a general physical description of the mechanisms that might control two fundamental cellular parameters, resting potential (Em) and cell volume (Vc), thereby clarifying the complex relationships between them. Em was determined directly from a charge difference (CD) equation involving total intracellular ionic charge and membrane capacitance (Cm). This avoided the equilibrium condition dEm/dt = 0 required in determinations of Em by previous work based on the Goldman-Hodgkin-Katz equation and its derivatives and thus permitted precise calculation of Em even under non-equilibrium conditions. It could accurately model the influence upon Em of changes in Cm or Vc and of membrane transport processes such as the Na+-K+-ATPase and ion cotransport. Given a stable and adequate membrane Na+-K+-ATPase density (N), Vc and Em both converged to unique steady-state values even from sharply divergent initial intracellular ionic concentrations. For any constant set of transmembrane ion permeabilities, this set point of Vc was then determined by the intracellular membrane-impermeant solute content (X-i) and its mean charge valency (zX), while in contrast, the set point of Em was determined solely by zX. Independent changes in membrane Na+ (PNa) or K+ permeabilities (PK) or activation of cation-chloride cotransporters could perturb Vc and Em but subsequent reversal of such changes permitted full recovery of both Vc and Em to the original set points. Proportionate changes in PNa, PK and N, or changes in Cl- permeability (PCl) instead conserved steady-state Vc and Em but altered their rates of relaxation following any discrete perturbation. PCl additionally determined the relative effect of cotransporter activity on Vc and Em, in agreement with recent experimental results. In contrast, changes in Xi- produced by introduction of a finite permeability term to X- (PX) that did not alter zX caused sustained changes in Vc that were independent of Em and that persisted when PX returned to zero. Where such fluxes also altered the effective zX they additionally altered the steady state Em. This offers a basis for the suggested roles of amino acid fluxes in long-term volume regulatory processes in a variety of excitable tissues.

Paradoxical changes of muscle glutamine release during hyperinsulinemia euglycemia and hypoglycemia in humans: Further evidence for the glucose-glutamine cycle

Insulin suppresses and counterregulatory hormones increase proteolysis. Therefore, if proteolysis were a major factor determining amino acid fluxes in plasma, one would expect release of glutamine into plasma to be suppressed by insulin under euglycemic conditions and to be stimulated under hypoglycemic conditions. However, release of glutamine into plasma remains unaltered or increases during euglycemic hyperinsulinemia and decreases during insulin-induced hypoglycemia. To investigate the mechanisms for these paradoxical observations and the role of skeletal muscle, we infused overnight fasted volunteers with [U- 14C] glutamine and measured release of glutamine into plasma, its removal from plasma, and forearm glutamine net balance, fractional extraction, uptake and release during 4-hour euglycemic (∼5.0 mmol/L, n = 7) and hypoglycemic (∼3.1 mmol/L, n = 8) hyperinsulinemic (∼230 pmol/L) clamp experiments. During the euglycemic clamps, plasma glutamine uptake and release (both P < .05) and forearm muscle glutamine fractional extraction ( P < .05), uptake ( P < .02) and release ( P < .01) all increased, whereas forearm glutamine net balance remained unchanged. The increase in muscle glutamine release (from 1.85 ± 0.26 to 2.18 ± 0.30 μmol · kg −1 · min −1) accounted for approximately 60% of the increase in total glutamine release into plasma (from 5.54 ± 0.47 to 6.10 ± 0.64 μmol · kg −1 · min −1) and correlated positively with the increase in muscle glucose uptake ( r = 0.80, P < .03). During the hypoglycemic clamps, plasma glutamine uptake and release and forearm glutamine release remained unaltered, but forearm glutamine fractional extraction and uptake decreased approximately 25% (both P < .01) so that forearm glutamine net release increased from 0.37 ± 0.06 to 0.61 ± 0.09 μmol · kg −1 · min −1 ( P < .03). We conclude that skeletal muscle is largely responsible for the increased release of glutamine into plasma during euglycemic hyperinsulinemia in humans, and that this may be due to increased conversion of glucose to glutamine as part of the glucose-glutamine cycle; during hypoglycemic hyperinsulinemia decreased glutamine uptake by skeletal muscle may be important for providing substrate for increased glutamine gluconeogenesis.

Isolated perfused liver model: the rat and guinea pig compared

Objective Although the rat is the most commonly used species for the study of hepatic metabolism, the physiology of the guinea pig is closer to human physiology. We compared the model of isolated perfused guinea pig liver with the classic model of isolated perfused rat liver, especially with respect to amino acid metabolism. Methods After validation of an anesthetic mixture of ketamine, diazepam, and xylazine for the guinea pig, isolated perfused livers were harvested for both species. Three groups of animals were compared for the study of liver metabolic fluxes: 6-wk-old male Sprague-Dawley rats (R; 230 ± 10 g, n = 5), young male Hartley guinea pigs (YG; 223 ± 8 g, n = 6) matched to rats by liver weight, and adult male Hartley guinea pigs (AG; 389 ± 5 g, n = 6) matched to rats by age. Results (mean ± standard error of the mean) were compared by analysis of variance and Newman-Keuls tests. Results Both models displayed a satisfactory hepatic viability, but differences were noted, with higher portal flows (R: 3.1 ± 0.3 versus YG: 4.5 ± 0.3 and AG: 4.2 ± 0.3 mL · min −1 · g −1; P < 0.05, YG and AG versus R) and bile flows (R: 0.34 ± 0.01 versus YG: 2.38 ± 0.22 versus AG: 3.17 ± 0.28 μL · min −1 · g −1; P < 0.05, YG and AG versus R, and YG versus AG) and higher amino acid fluxes ( P < 0.05) leading to greater nitrogen uptake ( P < 0.05) in guinea pigs. We performed a second set of experiments to evaluate the influence of anesthesia and portal flow on this last parameter. In these experiments, rats were anesthetized with ketamine, diazepam, and xylazine and guinea pig livers were perfused at rat blood flow. Apart from a 50% anesthesia-related mortality for rats, bile flow and metabolic parameters were only slightly modified. However, some amino acid fluxes were statistically different (aspartate, serine, and histidine; P < 0.05), as confirmed by a higher transfer constant. Conclusion Our results indicate that the isolated perfused guinea pig liver is a suitable model for the study of hepatic metabolism.

Distribution and dynamics of nitrogen and microbial plankton in southern Lake Michigan during spring transition 1999–2000

Ammonium and amino acid fluxes were examined as indicators of N and microbial food web dynamics in southern Lake Michigan during spring. Either 15NH4+ or a mixture of 15N‐labelled amino acids (both at 4 μM N final concentration) was added to Lake Michigan water. Net fluxes were measured over 24 h under natural light and dark conditions using deck‐top incubators and compared to microbial food web characteristics. Isotope dilution experiments showed similar light and dark NH4+ regeneration rates at lake (6 versus 5 nM N h−1) and river‐influenced (20 versus 24 nM N h−1) sites. Ammonium uptake rates were similar to regeneration rates in dark bottles. Dark uptake (attributed mainly to bacteria) accounted for ∼70% of total uptake (bacteria plus phytoplankton) in the light at most lake sites but only ∼30% of total uptake at river‐influenced sites in or near the St. Joseph River mouth (SJRM). Cluster analysis grouped stations having zero, average, or higher than average N‐cycling rates. Discriminant analysis indicated that chlorophyll concentration, oligotrich ciliate biomass, and total P concentration could explain 66% of N‐cycling rate variation on average. Heterotrophic bacterial N demand was about one third of the NH4+ regeneration rate. Results suggest that, with the exception of SJRM stations, bacterial uptake and protist grazing mediated much of the N dynamics during spring transition. Since NH4+ is more available to bacteria than NO3−, regenerated NH4+ may have a strong influence on spring, lake biochemical energetics by enhancing N‐poor organic matter degradation in this NO3− ‐replete ecosystem.

Infusion of glucose directs circulating amino acids to the mammary gland in well-fed dairy cows.

The effect of intestinal glucose supply on mammary utilization of amino acids (AA) was studied in four lactating dairy cows. Glucose (0, 443, 963, and 2398 g/d) was continuously infused in the duodenum over 14-d periods using a Latin square design. A grass silage-based diet was formulated so that treatments (diet + infusions) were isoenergetic and isonitrogenous and met 100 and 110% of energy and protein requirements, respectively. Mammary AA uptake was determined by arteriovenous difference and continuous blood flow measurement. The milk protein yield tended to be quadratically increased (to +88 g/d for 963 g of glucose) by glucose infusion, but milk protein content was not significantly affected. Treatments did not change significantly arterial concentrations of urea and glucogenic AA. Mammary arterial fluxes of essential AA increased linearly with glucose infusion, whereas fluxes of nonessential and glucogenic AA were not significantly affected. Mammary arteriovenous differences and extraction rates were roughly unchanged by treatments. Mammary uptake of all essential AA, excluding Arg and Val, increased linearly with increasing supply of glucose. Ratio of blood AA uptake to milk protein output increased significantly for His, Met, and Leu. For the highest infused dose of glucose, all AA except for His were taken up in excess relative to their secretion in milk. Based on evolution of extraction rate and ratio of uptake to output, His and Leu could have limited the milk protein yield response to glucose infusions.

Effects of host plant drought stress on the performance of the bird cherry‐oat aphid, Rhopalosiphum padi (L.): a mechanistic analysis

Abstract. 1. The growth (increase in height and leaf number) of four grass species was reduced by a −0.5 MPa drought stress, but the performance of an associated herbivore, Rhopalosiphum padi (L.), was not affected consistently. The intrinsic rate of increase of R. padi was reduced by drought stress on three grass species, including Dactylis glomerata (L.), but was unaffected on Arrhenatherum elatius (L.). Therefore, there is no general relationship in the effect of plant drought on an insect herbivore, even among closely related host plant species.2. Drought stress increased the quality of plant phloem sap, as indicated by increased sieve element osmotic pressure and essential amino acid concentrations. Thus, diet quality could not account for the reduced performance of R. padi under drought stress. The concentration of essential amino acids in the phloem of well‐watered A. elatius was, however, lower than that of well‐watered D. glomerata, correlating with the decreased performance of aphids on well‐watered A. elatius.3. There were no differences in aphid feeding duration between watering treatments or plant species but sap ingestion rates were reduced significantly under drought stress.4. Using the measure of dietary amino acid concentrations and the estimate of sap ingestion, the essential amino acid flux through aphids was calculated. Compared with the flux through aphids feeding on well‐watered D. glomerata, there was a reduction in aphids feeding on drought‐stressed D. glomerata and drought‐stressed A. elatius due to lower sap ingestion rates. The flux through aphids on well‐watered A. elatius was also reduced due to low phloem essential amino acid concentrations. Thus, the performance of an aphid is correlated with the availability and accessibility of essential amino acids.

Ryegrass-based diet and barley supplementation: partition of nitrogenous nutrients among splanchnic tissues and hind limb in finishing lambs.

Splanchnic metabolism of nitrogenous nutrients and their uptake by the hind limb were studied in finishing lambs receiving ryegrass harvested at grazing stage with or without barley supplementation. Six multicatheterized lambs (40.2 +/- 1.5 kg) were fed with frozen ryegrass (RG) at 690 kJ of ME intake (MEI) x d(-1) x BW(-0.75) and 20.8 g of N intake (NI)/d successively without and with barley supplementation (RG + B), according to a crossover design. Barley supplementation represented 21% of DM intake and increased the MEI and the NI by 32 and 24% respectively, (P < 0.01). In the ruminal fluid, barley increased acetate and butyrate concentrations by 21.2 and 49.6%, respectively (P < 0.04), without any effect on the ammonia concentration. Consequently, the net portal appearance (NPA) of ammonia was not modified, but the NPA of total amino acids (TAA; +38%) and nonessential amino acids (NEAA; +45%) was increased (P < 0.05) by barley supplementation. Taken individually, the NPA of the essential amino acids (EAA) was increased for isoleucine (+32%; P < 0.05), threonine (+151%; P < 0.03), and lysine (+26%; P < 0.06), with no effect for the other EAA. In contrast to what was observed at the PDV level, no significant alteration in the net hepatic amino acid flux was observed for TAA, EAA, NEAA, branched-chain amino acids (BCAA), urea, and ammonia after barley supplementation, showing a relatively minor role of the liver in the regulation of the supply of amino acids to the peripheral tissues. However, taken individually, the net hepatic uptake of some NEAA involved in gluconeogenesis and/or ureagenesis was altered with barley supplementation: the alanine uptake was increased by 44% (P < 0.05), aspartate + asparagine (asx) uptake was decreased by 18% (P < 0.01), and glutamate + glutamine (glx) release tended (P < 0.10) to be increased by 208%. With barley supplementation, NI increased by 5 g of N/d, and net splanchnic release increased by 4.63 g of N/d. Consequently, the additional dietary N supply (together with energy supply) was nearly exclusively available to peripheral tissues as AA-N (N as amino acids), but no strong effect of this additional supply of AA to the hind limb could be demonstrated in terms of net AA hind limb fluxes. Consequently, barley supplementation of a ryegrass-based diet increased the net AA release by the splanchnic tissues, with little effect on the AA net uptake by the peripheral tissues.

Evidence for the absence of an intestinal adaptive mechanism to compensate for C. parvum-induced amino acid malabsorption in suckling rats.

In order to assess the impact of Cryptosporidium parvum on host intestinal physiology, we investigated absorption of the two principal amino acids in dam's milk (leucine, glutamate), using Ussing chambers and RT-PCR analyses. Experiments were performed in both heavily (ileum) and mildly (duodenum) infected segments of the small intestine at the peak of infection [day 8 post-infection (PI)] and after spontaneous clearance of the parasite (day 17 PI). At day 8 PI, amino acid fluxes across the mucosa were decreased throughout the small intestine (P<0.01) and EAAT3 mRNA expression was reduced ( from -49% to -28%). At day 17 PI, leucine and glutamate fluxes were normalized but the decrease in EAAT3 mRNA levels persisted (from -31% to -46%). Our results demonstrate that cryptosporidiosis induces major amino acid malabsorption involving the entire small intestine which is not counterbalanced by any up-regulation, even after spontaneous clearance of the parasite.

Whole-body fluxes and partitioning of amino acids to the mammary gland of cows fed fresh pasture at two levels of intake during early lactation.

The utilisation of essential amino acids (EAA) by the mammary gland of lactating dairy cows fed fresh forages was studied to provide basic information useful in designing strategies to increase the production of milk protein from pasture-fed dairy cows. The relationship between the flux of EAA in the whole body and their uptake by the mammary gland was determined in four cows in early lactation (length of time in milk 44 (SD 14.5) d) producing 21 (SD 4.0) kg milk/d. The cows were maintained in metabolism stalls and fed fresh perennial ryegrass (Lolium perenne) and white clover (Trifolium repens) pasture ad libitum or restricted to 75 % ad libitum intake. The whole-body fluxes of amino acids (AA) were measured using an arterio-venous infusion of universally (13)C-labelled AA. Whole-body fluxes of fourteen AA were estimated. Isotope dilution indicated that mammary utilisation accounted for one-third of the whole-body flux of EAA, with individual AA ranging between 17 and 35 %. Isoleucine, leucine, valine and lysine were the EAA with the greatest partitioning towards the mammary gland (up to 36 % of the whole-body flux), which could reflect a potentially limiting effect on milk protein synthesis. In the case of AA with low partitioning to the mammary gland (for example, histidine), it is suggested that non-mammary tissues may have priority over the mammary gland and therefore the supply of this AA may also limit milk protein synthesis.

Pulmonary glutamine production: effects of sepsis and pulmonary infiltrates

To define the role of the lung in the production of glutamine in the critically ill and to determine the effects of the presence of pulmonary infiltrates and the presence and severity of sepsis. Prospective clinical study in a single center; interdisciplinary intensive care unit at a university hospital. Eleven critically ill patients were compared to ten patients prior to cardiac bypass surgery. Fluxes of glutamine and other amino acids were measured. Chest radiography was performed, and APACHE II and multiple-organ failure scores were calculated. Septic patients showed significantly higher glutamine efflux from the lungs than controls. At least one-half of this glutamine is estimated to result from protein breakdown. Severity of illness had no impact on glutamine fluxes. In the presence of pulmonary infiltrates on chest radiographs glutamine efflux did not differ from zero. The lungs produce significant amounts of glutamine in septic patients. Pulmonary infiltrates decrease the glutamine efflux from the lung in septic patients. We suggest that this is caused by uptake of glutamine by white cells in the lung exerting immunological functions.

Interorgan Amino Acid Transport and its Regulation

Interorgan amino acid transport is a highly active and regulated process that provides amino acids to all tissues of the body, both for protein synthesis and to enable amino acids to be used for specific metabolic functions. It is also an important component of plasma amino acid homeostasis. Net movement of amino acids depends on the physiological and nutritional state. For example, in the fed state the dominant flux is from the intestine to the other tissues. In starvation the dominant flux is from muscle to the liver and kidney. A number of general principles underlie many amino acid fluxes: i) The body does not have a store for amino acids. This means that dietary amino acids, in excess of those required for protein synthesis, are rapidly catabolized; ii) Amino acid catabolism must occur in a manner that does not elevate blood ammonia. Thus, extrasplanchnic amino acid metabolism often involves an innocuous means of transporting nitrogen to the liver; iii) Because most amino acids are glucogenic, there will be a considerable flux of amino acids to the gluconeogenic organs when there is a need to produce glucose. In addition to these bulk flows, fluxes of many specific amino acids underlie specific organ function. These include intestinal oxidation of enteral amino acids, the intestinal/renal axis for arginine production, the brain uptake of neurotransmitter precursors and renal glutamine metabolism. There is no single means of regulating amino acid fluxes; rather, such varied mechanisms as substrate supply, enzyme activity, transporter activity and competitive inhibition of transport are all found.

Digestion and nutrient net fluxes across the rumen, and the mesenteric- and portal-drained viscera in sheep fed with fresh forage twice daily: net balance and dynamic aspects.

Digestion and portal net flux of nutrients were studied in sheep fed twice daily with fresh orchard-grass. Digestive flows were measured in six fistulated sheep using the double-marker technique. Three sheep were fitted with catheters and blood-flow probes, allowing nutrient net flux measurements across the portal-drained viscera (PDV), the mesenteric-drained viscera (MDV) and the rumen. Total tract apparent digestion of N was similar to portal net appearance of N, calculated as the sum of free amino acids (FAA), peptide amino acids (PAA), NH3, and urea net fluxes. PAA accounted for 25 % of non-protein amino acid net release across the PDV. With the exception of glycine and glutamate, the small intestine was the main contributor to this PAA net release. The essential amino acid (EAA) apparent disappearance between the duodenum and the ileum was lower than the net appearance of EAA (FAA + PAA) across the MDV. The value of PDV:MDV flux of free EAA was, on average, 78 %. The rumen accounted for 30 % of the net uptake of EAA by the PDV tissues not drained by the mesenteric vein. Rumen net release of acetate, propionate, butyrate, 3-hydroxybutyrate, and lactate accounted for 70, 55, 46, 77 and 52 %, respectively, of their portal net releases. Conversely, the small intestine was a net consumer of arterial acetate and 3-hydroxybutyrate. Dynamic study of nutrient net fluxes across the PDV showed that throughout a feeding cycle, the liver faced a constant flux of amino acids (AA), whereas volatile fatty acid and NH3 net fluxes varied in response to the meal. The present study specified, in forage-fed sheep, the partitioning of nutrient net fluxes across the PDV and the role of peptides in portal net release of AA.

Vertical and latitudinal variations in amino acid fluxes and compositions of settling particles along 175oE in the North Pacific Ocean

Vertical and latitudinal variations in amino acid fluxes and compositions of settling particles along 175oE in the North Pacific Ocean

Minimum protein intake for the preterm neonate determined by protein and amino acid kinetics.

Lower limits of protein needs in prematurely born neonates have not been adequately studied, yet providing protein in amounts maximizing accretion without excess is a goal in these infants' nutritional care. We hypothesized that with the use of amino acid oxidation methodology, it would be possible to define minimum protein requirement. Our objective was to investigate protein kinetics during short-term changes in protein intake by measurement of nitrogen balance and amino acid flux and oxidation using [(15)N]glycine, [(13)C]phenylalanine, and [(13)C]leucine tracers. Protein kinetics were examined in 21 preterm infants (gestational age: 29 +/- 3 wk; birth weight: 1091 +/- 324 g) at five protein intakes (1.0, 1.5, 2.0, 2.5, and 3.0 g x kg(-1) x d(-1)) with 1 d of adaptation to the test intakes. From nitrogen balance data, a protein need of 0.74 g x kg(-1 x -1) was estimated to achieve zero balance. For all three amino acids, flux and oxidation estimates were not different across protein intakes. Whole-body protein synthesis and breakdown estimates from [(15)N]ammonia data were 14.6 +/- 3.4 and 14.4 +/- 4.1 g x kg(-1) x d(-1), respectively. Glycine flux (680 +/- 168 micromol x kg(-1) x h(-1)) was greater than leucine flux (323 +/- 115 micromol x kg(-1) x h(-1)), which was greater than phenylalanine flux (84.3 +/- 35.2 micromol x kg(-1) x h(-1)). Leucine oxidation (36.7 +/- 15.6 micromol x kg(-1) x h(-1)) was also greater than phenylalanine oxidation (6.64 +/- 4.41 micromol x kg(-1) x h(-1)). Infants in our study were able to adapt to short-term changes in protein intake with little consequence to the overall whole-body protein economy, as measured by the three test amino acids.