1,721,026 research outputs found
Effetto dell'esercizio sulla sintesi delle proteinemuscolari e della fibronettina nell'uomo
No full textHigh-protein intake alters the response of fasting in normal humans subjects
No full textTwo groups of normal volunteers were studied for 5 d of dietary control followed by 3 d of fasting. One group (n = 5) was given a control diet of 0.9 g protein.kg-1.d-1 and the other group (n = 7) was given a high-protein (HP) diet (2.5 g protein.kg-1.d-1). Both groups received 175.56 kJ.kg-1.d-1 (42 kcal.kg-1.d-1). The HP diet but not the control diet caused a significant retention of nitrogen. Postabsorptive leucine kinetics as assessed with [1,2-13C]leucine were similar in the two groups. In the control subjects, the rate of nitrogen excretion did not change in response to fasting, but leucine oxidation increased. In contrast, nitrogen excretion progressively decreased with fasting after the HP diet. Leucine rate of appearance was increased after fasting after the HP diet but oxidation was not increased, meaning that the calculated rate of whole-body protein synthesis was higher than in the control group. The response to a short period of food deprivation is dependent on prior protein intake
Urea kinetics in humans at two levels of exercise intensity
No full textA primed constant infusion of [15N2]urea was used to quantify the response of urea production to exercise at 40 and 70% maximal oxygen consumption on a treadmill. Total urea production, urea production from recycled N, urea production from nonrecycled N, and urea N recycled back into body protein were calculated. Most components of urea kinetics were unaffected by exercise at either intensity. The rate of urea reincorporated into protein was significantly increased during exercise and recovery at both levels of exercise. We conclude that exercise does not stimulate urea production but that there may be an accelerated reincorporation of urea N back into body protein
Measurement of 15N enrichment in multiple amino acids and urea in a single analysis by gas chromatography/mass spectrometry
No full textA precise and accurate procedure to measure the 15N isotopic enrichment of 18 common plasma amino acids and singly (15N1) and doubly (15N2) labeled urea in a single analysis by selected ion monitoring electron impact ionization gas chromatography/mass spectrometry analysis is presented. The choice of tert-butyldimethylsilyl derivatives allowed the enrichments in the amide and amino nitrogens of glutamine to be resolved. The ions monitored contained all the nitrogen atoms from the parent compounds except for arginine, which lost one guanidino nitrogen. Isotope ratios were determined with a coefficient of variation (within-assay precision) of 0.35% (range, 0.1-1.0%) on replicate measures averaged over all components; thus, the standard deviation associated with a nominal [m + 1]/[m + 0] isotope ratio of 0.2000 was 0.0007. The average error between measured and theoretical [m + 1]/[m + 0] isotope ratios was +0.0001 +/- 0.0086 for samples at natural abundance isotopic composition. The utility of the procedure is demonstrated by monitoring the incorporation of 15N into 18 plasma amino acids and urea during a 6 h oral administration of 15NH4Cl to a human volunteer. Highest levels of enrichment were achieved in arginine and urea, followed by glutamine. Approximately 80% of the label in glutamine was in the amino nitrogen. Excess 15N enrichment was observed in all plasma amino acids monitored with the exception of the essential amino acids phenylalanine, lysine and histidine. This method will facilitate the measurement of isotopic enrichment of multiple amino acids by a single analysis when it is necessary to monitor multiple stable-isotopically labeled amino acids in studies of amino acid and protein metabolic kinetics
Alanine kinetics in humans during low-intensity exercise
No full textThere is little doubt that pyruvate contributes to the increased alanine flux in exercise, but the role of protein breakdown is less clear. To quantify the relative contributions of pyruvate and protein breakdown to the increase in alanine flux observed in exercise, we used a primed, constant infusion of 15N-alanine and 1-13C-lactate. The rate of appearance of alanine, the de novo synthesis of alanine, and rate of alanine release from protein breakdown were determined in five healthy subjects at rest and during exercise. The exercise was performed for 120 min on a treadmill at 45% of the subject's VO2max. The total rate of appearance of alanine, calculated with the 15N-alanine tracer, increased significantly during exercise from 4.9 +/- 0.5 to 7.9 +/- 0.9 mumol.kg-1. The amount of alanine derived from pyruvate also significantly increased during exercise (3.2 +/- 0.3 vs 4.5 +/- 0.7), but the proportion of the total decreased from 65% at rest to 57% during exercise (statistically significant, P < 0.05). Consequently, the alanine derived from protein breakdown significantly increased (1.7 +/- 0.5 vs 3.4 +/- 0.8) and was also increased as percent of total alanine flux. Thus, we conclude that during low-intensity exercise, whole body protein catabolism is accelerated
Quantification of incorporation of [15N]ammonia into plasma amino acids and urea
No full textThe incorporation of 15N into individual plasma amino acids and urea was quantified in five human subjects who received 15NH4Cl either orally or intravenously for 6 h. After oral tracer administration, the highest enrichment was achieved by arginine, followed by urea and glutamine; distribution of 15N within glutamine was 55% amide and 45% amino N. Glutamine achieved the highest enrichment after the intravenous administration of tracer, with a distribution of 92% amide and 8% amino N. The relative distribution pattern of 15N incorporation was quantified from the rate at which 15N initially appeared in each plasma component. Amino acids (especially arginine, glutamine, and glutamate) accounted for greater than one-half (54%) of the orally administered tracer that was initially recovered in plasma components, compared with 46% initial appearance for urea; for the intravenous tracer, amino acids accounted for 78% of initial appearance of tracer compared with 22% for urea. Our results highlight the involvement of the splanchnic bed in the utilization of orally administered ammonia (preferential incorporation of oral tracer into arginine, urea, glutamate, and the amino N of glutamine) in contrast to the preferential incorporation of systemically administered ammonia into the amide N of glutamine and alanine
Isotopic determination of fibronectin synthesis in humans
No full textFibronectin is an opsonic protein that, among other functions, activates the reticuloendothelial system. Accurate measurement of its rate of synthesis is necessary to more fully understand its physiological role in normal and pathological conditions. We have determined the rate of fibronectin synthesis in three normal volunteers using a primed-constant infusion of 15N-glycine and 1,2-13C-leucine, and measuring the incorporation of the isotopes into the protein over 5 days of infusion. In nine additional subjects, the fractional synthetic rate (FSR) of fibronectin was calculated during a 24-hour infusion using urinary hippurate and plasma alpha-ketoisocaproic acid enrichment to represent the precursors for incorporation of labeled glycine and leucine, respectively, into fibronectin. The FSR using glycine and leucine was 1.56 +/- 0.14 and 1.29 +/- 0.04 (%/h), respectively, in the 5-day infusion study, and 1.56 +/- 0.10 versus 1.83 +/- 0.09 (%/h), respectively, in the 24-hour study. The results of the 5-day infusion of 15N-glycine justify the use of urinary hippurate to reflect the precursor enrichment for the determination of the FSR of fibronectin during a shorter (less than 24 hour) infusion period
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