Diurnal response in endogenous amino acid oxidation of meal-fed rats

[The effect of the protein level of food on the energy maintenance requirement of growing rats on different feeding regimens]

Male Wistar rats were fed in the live weight range between 70 and 200 g in periods alternately for maintenance and weight gain (semi ad libitum). The diets contained 10, 40 and 70% crude protein, resp., the ambient temperature was 30 degrees C. In agreement with two previous experiments 2 different feed regimes were used. Whereas in the first growth phase (up to about 150 g live weight) the supply of protein was constant for each group, in the second growth phase the protein level was changed between the groups from one period to the other. The energy metabolism on the feeding level of maintenance was measured by indirect calorimetry. At both feeding regimes a dependence of energy maintenance requirement on the protein level of feeding was established. The differences in the maintenance requirement between the 10% and 70% protein level and in the first growth phase also between the 10% and 40% protein level corresponded on average to a growth phase the expected values of which result from the different efficiency of ATP synthesis in the oxidative degradation of nutrients. It follows that--in accordance with the previous findings--the feeding regime does not influence the ratio of energetic utilization of nutrients in the maintenance metabolism of growing rats under the experimental conditions used by us. Further the results of all our studies on the dependence of the energy maintenance requirement of growing rats on the nutrient composition are summarized. We conclude that a dependence on the protein-carbohydrate ratio of the diet according to the different efficiency of ATP synthesis in the protein and carbohydrate catabolism can be reproducibly established by our experimental methods only then, if more than 50% of the energy maintenance requirement are covered from protein energy.

Relationship between plasma and brain large neutral amino acids in rats fed diets with different compositions at different times of the day

Large neutral amino acids (LNAAs) compete with each other for carrier-mediated transport through the blood-brain barrier into the brain. The relative plasma concentration, expressed as the ratio of each LNAA to the sum of LNAAs, is considered the main regulator of brain LNAA concentrations. In order to investigate the consistency of this assumption throughout a 24-h period, we have compared the relationship of plasma LNAAs to brain LNAAs among groups of rats fed diets containing various amounts of protein (in order to obtain a wide range of plasma LNAA levels) at two different phases of the light/dark cycle (0900 and 2100 hours). The relationship between plasma and brain LNAAs was found to be dependent on both diet and the time of day. Similar plasma amino acid concentrations in the morning and in the evening contrasted with different brain concentrations. Furthermore, previous findings that brain LNAA concentrations are influenced by plasma amino acid concentrations were confirmed.

Is rat brain content of large neutral amino acids (LNAAs) a reflection of plasma LNAA concentrations?

Competition between the large neutral amino acids (LNAAs), including the monoamine precursors tyrosine and tryptophan, for the carrier-mediated transport from plasma into the brain is considered the major regulator of brain LNAA content. The plasma ratio of each LNAA to the sum of all LNAAs, previously, in standardized experiments, has proved an excellent predictor of brain LNAA content. To investigate how diurnal variations in plasma LNAAs are reflected in the brain, we have killed rats, in groups of six, every 1 h throughout one 24 h period. The concentration of each LNAA in both plasma and brain varied with time-of-day. Also the total concentration of LNAAs in brain was shown to vary diurnally. Obviously, variations in total brain LNAA concentration cannot be explained by changes in plasma LNAA ratios. Consequently, other factors, contributing to the regulation of the diurnal rhythm in brain LNAA content, must be considered.

Effect of daily exercise and food intake on leucine oxidation

Oxidation of the branched-chain amino acid leucine was studied in 22 male Sprague-Dawley rats (70-90 g) over 3 days following the ingestion on Day 1 of a mixed diet containing a tracer dose (10 muCi) of L-[1-14C]Leu. One group (E) completed 1 hr exercise at 80% VO2 max immediately after a 2-hr feeding period on all 3 days, while a second group served as a control. Rats from group E were sacrificed immediately after the 2 hr feeding on Day 1, following exercise on Days 1 and 3, and at the end of Day 3. The following were determined: (1) continuous 14CO2 production, (2) radioactivity remaining in the gastrointestinal tract, and (3) distribution of free vs protein bound 14C in muscle and liver. The results indicated that (1) 14CO2 production increased during exercise on all 3 days (P less than 0.01), (2) 14CO2 production also increased (P less than 0.05) following food intake (unlabeled diet), (3) 14CO2 production due to exercise was greater than that due to food intake (P less than 0.05), (4) absolute 14CO2 production decreased dramatically by 15 hr of Day 1 (P less than 0.01) with little change thereafter (except with exercise and food intake on Days 2 and 3), (5) greater than 98% of the labeled diet was absorbed from the GIT 51 hr postingestion, and (6) 14C in the free pool of muscle and liver could account for less than 15% of the total 14CO2 production. These results suggest that protein bound 14C in addition to free 14C may be responsible for a significant proportion of the observed increased 14CO2 production during exercise.

Analysis of 1- and 3-methylhistidines, aromatic and basic amino acids in rat and human urine

A procedure based on automated amino acid analysis has been developed to simultaneously quantify 1-methylhistidine (1-MH), 3-methylhistidine (3-MH), tyrosine, phenylalanine, tryptophan, lysine, histidine and arginine levels in human and rat urines. Deproteinized urine samples containing amino acids in the range 1-10 nmol were analyzed using single-column methodology with ninhydrin detection. Standard curves produced correlation coefficients greater than or equal to 0.99 with duplicate analyses agreeing to within +/- 1.9%. Quantitative recovery was ensured by using L-alpha-amino-beta-guanidinopropionic acid as an internal standard. Elution was accomplished in less than 90 min at pH 5.7 with sodium citrate buffers at 45 degrees C and 65 degrees C. Since 3-MH in the rat is acetylated at the alpha-amino group, rat, but not human, urine ultrafiltrates required acid hydrolysis prior to analysis. The utility of the technique of analysis of 1-MH and 3-MH in human urine was demonstrated for an adult male on a meat-free diet for 21 days; urinary excretion rates for 3-MH and 1-MH were determined to be 3.06 +/- 0.10 and 0.72 +/- 0.07 mumol/kg body mass/day, respectively. The technique was also used to measure the effect of disuse atrophy of rat skeletal muscle which induced a 40-60% increase in 3-MH. The procedure is also highly suited for measurement of urinary aromatic and/or basic amino acids.

Leukocyte endogenous mediator alters protein dynamics in rats

Leukocyte endogenous mediator (LEM), a low-molecular-weight peptide synthesized by monocytic cells during phagocytosis, has been implicated as the host's initiator of the protein metabolic response to infection and inflammation. To determine whether administration of LEM would alter protein kinetics, appearance and oxidation of plasma tyrosine as well as the rates of protein synthesis in liver and skeletal muscle were determined in fasted rats that received a 30-hour continuous infusion of either physiologic saline, LEM, or heat-inactivated LEM. The LEM was obtained from rabbit peritoneal exudate and the treatment solutions supplied 2.8 X 10(8) cell equivalents/100 g of body weight (BW) per day. Endogenous tyrosine oxidation increased from 4.0 +/- 0.4 mumol/100 g BW/h to 5.4 +/- 0.7 mumol/100 g BW/h in animals infused with heat-inactivated LEM and to 7.5 +/- 1.5 mumol/100 g BW/h in rats receiving LEM (P less than 0.01). Nonsecretory protein synthesis in the liver was greatest in rats administered LEM (2239 +/- 325 mg/d) when compared with control groups receiving physiologic saline (1122 +/- 195 mg/d) or heat-inactivated LEM (1374 +/- 62 mg/d; P less than 0.01), whereas skeletal protein synthetic rates were unchanged. Rates of muscle and collagen protein breakdown were estimated from the urinary excretion rate of Nt-methylhistidine and hydroxyproline, respectively, and their excretion rose by 30% (P less than 0.05) and 42% (P less than 0.05) with LEM administration. These results suggest that administration of LEM stimulates a mobilization of amino acids from peripheral tissues to support increased visceral protein anabolism while whole body amino acid oxidation is also enhanced. Since similar effects follow fever and infection, these results suggest that LEM may play an underlying role in the protein metabolic response to infection and inflammation.

Rate of endogenous methionine oxidation in rats at different levels of methionine intake

The fractional rates of loss of 14C and body-weight were measured in growing female rats after a single dose of [l-14C]methionine. After the dose, the rats were maintained for 30 d on diets containing graded amounts of methionine and killed at 30 d. By 20 d after the dose the level of labelling of body proteins was fairly uniform. From 20 to 30 d, 14CO2 was collected at intervals. The rats were killed at 30 d and the total radioactivity remaining in the body was measured. The fractional loss of 14CO2 between days 20 and 30 was constant regardless of methionine intake except at the highest intake level. After correcting the fractional loss of 14CO2 for urinary 14C loss, a value of 35 mg/kg body-weight (20 mg/kg body-weight 0.75) was obtained for the rate of endogenous methionine oxidation.