Effect of corticosterone and its route of administration on muscle protein breakdown,measured in vivo by urinary excretion of Nτ-methylhistidine in rats: Response to different levels of dietary protein and energy

Adrenalectomized young male rats were given a large dose of corticosterone ($\text{10 mg}\text{100 gm body weight}$) either by subcutaneous or intraperitoneal injection. Subcutaneous injection caused immediate cessation of growth and an elevation in urea and in N^τ-methylhistidine (3-Mehis) output in the urine, the latter indicative of accelerated muscle protein breakdown. On the other hand, intraperitoneal administration resulted in only slight retardation of growth and no change in 3-Mehis output. Since the plasma level of corticosterone was more elevated by subcutaneous than by intraperitoneal administration of the hormone, it was concluded that direct passage of the intraperitoneal dose through the liver inactivated more of the administered hormone and thus prevented the level in the peripheral blood from rising to the critical concentration necessary to cause acceleration of muscle protein breakdown. This explains some reports in the literature of lack of action of corticosteroids on muscle protein breakdown. When corticosterone ($\text{10 mg}\text{100 gm body weight}$) was administered subcutaneously to young adrenalectomized rats receiving diets deficient in protein and/or energy, they lost weight at the same rate as did rats on an adequate diet. In addition, the gastrocnemius, tibialis, and extensor digitorum longus muscles lost weight and output of 3-Mehis was elevated irrespective of the diet. When a lower dose of corticosterone ($\text{0.8 mg}\text{100 g body weight}$) was given subcutaneously, the pattern of growth or weight loss on the different diets was not affected and the weights of the leg muscles relative to body weight remained unchanged. Output of 3-Mehis was slightly and transiently elevated for rats on the energy-deficient and protein-energy deficient diets receiving this low dose of corticosterone, but not for rats on the protein-deficient or adequate diets. Since the livers of the animals on the low energy intakes were smaller, we attribute the slight action of the lower corticosterone dose on 3-Mehis output to less efficient hepatic removal of the hormones. Thus, diet appears to modify the action of corticosteroids mainly through changes in the rate of hormone inactivation. It is concluded that dietary intake of protein and energy and corticosterone affect muscle protein turnover independently of one another.     

Quantitative contribution by skeletal muscle to elevated rates of whole-body protein breakdown in burned children as measured by Nτ-methylhistidine output

To investigate the distribution of whole-body protein breakdown in children suffering from severe burn injury, the rates of skeletal muscle and whole-body protein breakdown were estimated in a total of 13 studies on 7 children, ages 4–13 yr, with body burns ranging from 36% to 83% of body surface area. Results were compared with values reported for healthy children. Whole-body protein breakdown was determined using ¹⁵N-glycine, and muslcle protein breakdown was estimated from measurements of urinary excretion of N^τ-methylhistidine. The children were receiving a flesh-free diet, free of exogenous N^τ-methylhistidine. The mean rate of whole-body protein breakdown was 5.7 ± 1.3 g protein/kg/day, which was higher than that found previously in healthy children of the same age group. The mean rate of N^τ-methylhistidine excretion was 4.3 μmoles/kg/day, or 205 μmoles/g creatinine; this was higher than the rate for healthy children as determined by interpolation of data reported in the literature. Estimating the rate of muscle protein breakdown from these data revealed that skeletal muscle accounts for 19.1% ± 7.6% of whole-body protein breakdown in burned children. When compared with published data for healthy subjects of varying ages, the present findings suggest that, although the rate of muscle protein breakdown is elevated in children recovering from burn injury, its percentage contribution to the rate of whole-body protein breakdown, also elevated in response to burn injury, remains within approximately normal limits.     

Musle protein breakdown rates in humans based on Ntau-methylhistidine (3-methylhistidine) content of mixed proteins in skeletal muscle and urinary output of Ntau-methylhistidine

Samples of psoas muscle from nine infants (aged 1 day to 14 mo) and of several skeletal muscles from seven adult males (age 19-74 yr) were analyzed for content of protein-bound Ntau-methylhistidine (3-methylhistidine; 3-Mehis). The mean content of 3-Mehis (expressed as mumoles/g mixed protein) was 3.2 (range 2.4-3.7) in infants and 4.2 (range 3.7-4.6) in adults. The daily urinary excretion of 3-Mehis was measured in four young adult males receiving an egg-protein, flesh-free diet. Mean excretion of 3-Mehis was 211 (range 167-252) mumoles/day. From these two sets of data the mean rate of muscle protein breakdown in adult males was estimated to be 50 g/day, or 0.7 +/- 0.1 g/kg body weight/day. These results are compared with reported values for the 3-Mehis content of mixed proteins in muscle of various species, and with published estimates, computed by other techniques, of the rate of muscle protein breakdown in human subjects.     

Potential use of 3-methylhistidine excretion as an index of progressive reduction in muscle protein catabolism during starvation

Based on animal studies, the amino acid 3-methylhistidine, a component of actin and myosin, is not reutilized for protein synthesis following the breakdown of muscle protein but is quantitatively excreted in the urine and should thus provide an index of the rate of muscle protein breakdown. The urinary outputs of total N, urea N, 3-methylhistidine, 1-methylhistidine, and creatinine are reported for three obese subjects during a 20-day starvation period. The excretion of 3-methylhistidine decreased progressively and on day 20 was 34% less than on day 3. This reduction was proportionally similar to the reduction in urinary total N output but was much greater than the 15% decrease in creatinine excretion and in calculated loss of muscle mass. Assuming that tissues other than muscle are not significant sources of urinary 3-methylhistidine, these findings suggest that the reduced output of urinary N is due to an adaptive decrease in the rate of catabolism of muscle proteins as starvation progressed.     

Glucose and insulin effects on the novo amino acid synthesis in young men: studies with stable isotope labeled alanine, glycine, leucine, and lysine

We have explored interrelationships between te dynamic aspects of whole body glucose and alanine and glycine metabolism in adult humans. Using a primed, continuous intravenous infusion of [1-13C] leucine or lysine given simultaneously with [2H3] or [15N]alanine or [15N]glycine, respectively, whole body alanine and glycine fluxes and their rates of de novo synthesis were determined in three experiments with healthy young men. Subjects were studied in the post-absorptive state and during a 150 min period of an intravenous infusion with unlabeled glucose, at a rate of 4 mg.kg-1 min-1. In one experiment, insulin was given together with the glucose infusion to maintain normoglycemia. In the other two studies, subjects received glucose alone. For the post-absorptive state, alanine flux (mean +/- SEM) was 381 +/- 26 and 317 +/- 18 mumole.kg-1 hr-1 in two separate experiments and glycine flux was 240 +/- 22 mumole.kg-1 hr-1. De novo synthesis of alanine and glycine accounted for 75%-81% and 81% of flux, respectively. Infusion with glucose alone raised plasma glucose to a mean level of 152 mg/dl and increased alanine flux, due to a rise in alanine synthesis of 98 mumole.kg-1 hr-1 (p less than 0.01). Glycine flux and synthesis rate were unaffected by the glucose infusion. When insulin was given with glucose to maintain normoglycemia, the rate of alanine synthesis was unchanged. Because glucose uptake rate, measured with [6,6-2H2] glucose was the same whether glucose was infused along or with exogenous insulin, these results support the view that the circulating plasma glucose level itself may affect alanine synthesis and that the hyperglycemic state is an important factor in regulating interorgan nitrogen transfer, via alanine, in various pathophysiologic states.     

Dynamic aspects of whole body glycine metabolism: Influence of protein intake in young adult and elderly males

The influence of adult age and adequacy of dietary protein intake on whole body glycine metabolism was studied in human subjects. Five healthy young adult males (19–25 yr) and six elderly males (64–78 yr) were given an adequate-protein diet (1.5 g protein/kg/day) for 7 days and a low-protein diet (0.4 g protein/kg/day) for 14 days. At the end of each dietary period, whole body glycine flux and rates of glycine synthesis were estimated with the use of a continuous 60 hr oral administration of ¹⁵N-glycine and determination of ¹⁵N enrichment of plasma glycine by gas chromatography-mass spectrometry with selected ion monitoring. Mean whole body glycine flux and the rate of endogenous glycine synthesis were 458 and 351 μmole/kg body weight/hr, respectively, for young adults receiving the diet adequate in protein; similar values were obtained in the elderly group. Feeding the diet low in protein resulted in an extensive and significant reduction in both parameters in young adults and also in elderly subjects to a similar extent. Measurement of ¹⁵N enrichment in plasma serine gave a constant ratio of ¹⁵n enrichment in plasma free serine relative to glycine for both age groups and at the two protein intake levels. It is concluded that aging of adults has little impact on the quantitative aspects of whole body glycine metabolism, but that it responds extensively to changes in protein intake. Thus, it appears that glycine synthesis and flux are integrated with the body's total nitrogen metabolism and requirement for dietary nitrogen.     

The relation between urinary 5-hydroxyindoleacetic acid levels and the ratio of tryptophan to other large neutral amino acids placed in the stomach.

We assayed 5-hydroxyindoleacetic acid (5-HIAA) in urine samples (3- or 6 1/2-hr collection) after individual rats received 6-8 ml of water, of amino acids in solution, or of glucose by stomach tube. Tryptophan (Trp) solutions caused dose-related increases in urinary 5-HIAA; these were blocked when animals received carbidopa, an inhibitor of peripheral aromatic amino acid decarboxylase. The concurrent administration of a large neutral amino acid (LNAA; valine or isoleucine) with oral Trp, in high doses probably sufficient to compete with Trp for transport into gut cells, blocked the Trp-induced rise in urinary 5-HIAA. Concurrent administration of glycine (not a LNAA) in equivalent doses did not. Pretreatment with pyridoxine blocked the Trp-induced rise in urinary 5-HIAA but not that in brain serotonin (5-hydroxytryptamine, 5HT). These observations confirm the previous suggestion that while brain serotonin synthesis depends on the plasma Trp/LNAA ratio (which varies inversely with the proportion of protein to total calories in the diet), gut serotonin synthesis depends largely on the Trp/LNAA ratio in the dietary protein itself (and, probably, within the gut lumen postprandially). The range of molar Trp/LNAA ratios at which free LNAAs significantly diminish the effects of ingested Trp on gut serotonin synthesis (as reflected by urinary 5-HIAA) is similar to the range found in dietary proteins.     

Whole body leucine and lysine metabolism studied with [1-13C]leucine and [α-15N]lysine: Response in healthy young men given excess energy intake

An excess intake of dietary energy in adult subjects enhances body N balance but the mechanism(s) responsible remains unknown. Thus, dynamic aspects of metabolism of whole body leucine and lysine were explored in healthy young adult men, receiving adequate or excess energy intakes, using a primed, continuous intravenous infusion of a mixture of L-[¹³C]leucine and L-[α-¹⁵N]lysine to provide a constant enrichment of plasma free leucine and lysine over a period of 2 hr. Twelve subjects were studied with the labeled amino acids following an overnight fast (post-absorptive state) and 12 additional subjects while consuming small isocaloric, isonitrogenous horly meals (fed state). Preceding each infusion, subjects were adapted for 7 days to experimental diets, providing a constant and barely adequate protein intake of 0.6 g/kg body weight/day, at either a maintenance energy intake, determining from estimates of usual food intake that maintain body weight, or an energy intake 25% greater than the maintenance level. The excess non-protein energy intake was given as an isocaloric mixture of carbohydrate and fat (eight subjects) or entirely as either carbohydrate or fat (eight subjects each). Whole body leucine and lysine flux remained unchanged with excess energy intakes, regardless of the source of energy substrate. Based on the combined data with all energy intake sources, the rate of leucine oxidation was significantly reduced and the rate of leucine incorporation into body protein showed a small increase with excess energy intakes. Thus, when expressed as net protein gain (leucine incorporation into body protein minus leucine release from protein breakdown) a significantly greater rate of body protein retention occurred with excess energy intake and this was more marked for the high carbohydrate diets. Mean change in body leucine retention determined by ¹³C-leucine was in good agreement with that calculated from alterations in overall N balance. In addition, the rate of inflow of leucine and of lysine into the metabolic pool via tissue protein breakdown was reduced with ingestion of meals. These results indicate that excess energy intake improves overall body N balance by reducing amino acid oxidation and enhancing protein synthesis. Furthermore, these effects are particularly evident at a time when passage of amino acids to tissues is stimulated by ingestion and absorption of meals.     

Effect of various oral glucose doses on plasma neutral amino acid levels

Six healthy, nonobese, fasting subjects each received, on different days 0, 6, 12.5, 25, or 50 g of glucose (Glucola) in a total volume of 100 ml. Blood was taken at intervals and assayed for plasma levels of the branched-chain amino acids (valine, isoleucine and leucine); the other major large neutral amino acids (LNAA) (methionine, phenylalanine, tyrosine and tryptophan); and, in some cases, insulin and glucose. Insulin levels were significantly elevated 30 min after consumption of 12.5, 25, or 50 g of glucose, and were higher after the 50 g dose than after 12.5 g. Changes in plasma glucose concentrations were small and did not correlate with glucose dose. Mean percent reductions of LNAA tended to exhibit dose-dependence, most clearly observed after 120 min. In some subjects as little as 6 g of glucose transiently decreased LNAA concentrations. Branched-chain amino acids were most sensitive, decreasing by 35%-41% after 50 g of glucose. Plasma tryptophan concentrations fell only by 23%, hence the ratio of plasma tryptophan to other plasma LNAA (which affects brain serotonin synthesis) increased significantly.     

Albumin synthesis in young and elderly subjects using a new stable isotope methodology: Response to level of protein intake

Albumin synthesis was evaluated in 5 young adult males (19–25 yr) and 6 elderly males (64–78 yr) by a procedure involving oral administration of ¹⁵N-glycine every 3 hr over a 60-hr period. From about 40 hr onwards, urinary urea achieved a plateau of ¹⁵N-enrichment, which was estimated from the average of the last five (low protein) or seven (adequate protein) consecutive three-hourly urinary samples of the 60-hr period. This enrichment plateau was used as an index of the ¹⁵N-enrichment of the guanidine N of hepatic free arginine. The ¹⁵N-enrichment of the guanidine N of arginine in serum albumin was determined and albumin synthesis was estimated by comparing this value with the estimated enrichment of precursor hepatic arginine. Using this methodology, serum albumin concentration, synthesis, rate and plasma volume were measured when the young and elderly subjects had received an adequate protein intake (1.5 g · kg^?1 for 7 days) or a low protein intake (0.4 g · kg^?1 for 14 days). Serum albumin concentration was lower in the elderly at both levels of protein intake; protein intake did not affect this parameter in either age-group. Plasma volume (per kg body weight) did not differ between young and old, but increased in both groups when they were given the low-protein diet, so that the total intravascular albumin mass increased in both age groups significantly in the case of the young, and was probably due to net transfer of albumin from the extravascular pool. The fractional synthesis rate of the whole body albumin pool with adequate intake of protein was 4.0%/day in the young and 3.4%/day in the elderly. This fractional rate was reduced significantly by giving the low-protein diet to the young subjects, but was not reduced in the elderly. Absolute synthesis rates, calculated per kg body weight and per kg body cell mass, led to a similar conclusion. Whole body protein synthesis was also estimated from urinary ¹⁵N-urea enrichment using the Picou and Taylor-Roberts model. Albumin synthesis as a percentage of whole body protein synthesis (5%–6%) was reduced in the young adults by giving the low-protein diet, but was unchanged in the elderly. In conclusion, the rate of albumin synthesis in the young, but not in the elderly, is sensitive to changes in protein intake. It is suggested that albumin synthesis in the elderly is controlled at a lower set point, which prevents its response to higher protein intakes.     

Effect of pyridoxine on the depletion of tissue pyridoxal phosphate by carbidopa

One or two hours after rats received a single dose (100 or 800 mg/kg, intraperitoneally) of carbidopa (MK-486; α-methyl-L-dopa hydrazine), pyridoxal-5′-phosphate (PLP) concentrations were significantly depressed in peripheral tissues (serum, liver, and muscle) and in hypothalami, but not in whole brains. Repeated administration of carbidopa (three 100 mg/kg doses per day for 3 days) lowered PLP concentrations in serum, liver, and muscle (by 88%, 51%, and 18%, respectively) among rats killed 2 hr after the final injection. PLP levels in brain and hypothalamus were also reduced significantly (by 22% and 18%, respectively) after this treatment. The activity of aromatic L-amino acid decarboxylase (AAAD) in hypothalamic homogenates (assayed in vitro) exhibited a parallel small decline; its activity in brain homogenates, however, was not significantly changed. Administration of pyridoxine·HCl (200 mg/kg/day for 3 days) in addition to carbidopa blocked the fall in hypothalamic AAAD activity. Pyridoxine administration did not affect the inhibition of peripheral AAAD by carbidopa in vivo; it failed to interfere with carbidopa's blockade of decarboxylation of exogenous L-dopa (100 mg/kg, given intraperitoneally 1 hr before sacrifice). These findings suggest that large doses of carbidopa can, by depleting tissues of PLP, cause potentially undesirable nonspecific changes in pyridoxine-dependent enzymes. However, the coadministration of pyridoxine with carbidopa can maintain tissue PLP levels and protect against such enzyme changes.     

Relations between dietary choline or lecithin intake, serum choline levels, and various metabolic indices

Choline administration increases blood choline, brain choline, and brain acetylcholine levels in rats. It also increases blood choline levels in humans and appears to be a useful treatment for some patients with tardive dyskinesia, a brain disease probably associated with deficient cholinergic tone. In order to characterize other possible metabolic and hormonal effects of choline-containing compounds, we measured changes in serum choline, glucose, insulin, cortisol, prolactin, cholesterol, and triglyceride levels resulting from ingestion of low- or high-choline meals in 16 normal human subjects. After the consumption of a single meal containing 3 g choline chloride, serum choline rose by 86% (p < 0.01), attaining peak values after 30 min. When the same subjects ate a meal containing an equivalent amount of choline in the form of lecithin, serum choline levels rose by 33% after 30 min, and continued to rise for at least 12 hr, to 265% over control values (p < 0.001). Serum choline concentrations were related to the amount of choline in the diet: they did not vary significantly during 24-hr periods when the subjects consumed a low-choline diet for two consecutive days, but rose substantially (p < 0.01) after each high-choline meal. Serum glucose, insulin, cortisol, and prolactin levels were not significantly modified by choline or lecithin ingestion. Lecithin consumption increased serum triglyceride levels and lowered serum cholesterol concentration.     

Dietary protein intake and dynamic aspects of whole body nitrogen metabolism in adult humans.

The constant isotope-infusion method of Picou and Taylor-Roberts was used to study rates of total body protein synthesis and breakdown in adult subjects following acute changes in the level of dietary protein intake. Six healthy adults, four males and two females, were studied after adaptation to dietary protein intakes of 1.5 and 0.38 g of protein/kilogram body weight/day. Dietary periods were from 7 to 15 days duration. 15N-glycine was used as a tracer, and was administered orally for 60 hr at 3-hr intervals, or by continuous intravenous infusion for 48 hr. Results were similar for both routes of isotope administration for the comparison conducted at the higher protein intake. At the 1.5-g protein level the mean N flux was 28.2 mg nitrogen/kg/hr, with total body protein (N x 6.25) synthesis and breakdown rates being 3.0 g/kg/day and 2.7 g/kg/day, respectively. Reducing the protein intake to 0.38 g/kg/day caused an 8% decrease (p less than 0.05) in N flux, a 27% increase (p less than 0.005) in the rate of total body protein breakdown, and a 15% increase (p less than 0.05) in the rate of protein synthesis. Endogenous amino acids were reutilized more efficiently under these conditions. The findings are discussed in relation to the way in which adult subjects adapt to acute changes in dietary protein intake.     

Relevance of free tryptophan in serum to tissue tryptophan concentrations

The consumption of a carbohydrate diet by fasted rats is followed by major decreases in serum nonesterified fatty acids (NEFA) and nonalbumin-bound tryptophan (unbound tryptophan), but by increases in serum total tryptophan and brain tryptophan; the tryptophan concentrations of liver and small intestine are unchanged, while that of skeletal muscle falls slightly. The addition of 15% or 30% fat to a protein-carbohydrate diet results in dose-related increases in serum NEFA and serum unbound tryptophan, but no significant changes in serum total tryptophan or brain tryptophan. The observation that diet-induced changes in serum unbound tryptophan does not correlate with brain tryptophan concentrations is independent of the method used to separate free from albumin-bound serum tryptophan. These studies confirm that, in the rat, a major physiologic regulator of the extent to which serum tryptophan binds to albumin is the concentration of NEFA in serum. These studies also provide additional evidence that the concentration of tryptophan in the brain is not necessarily determined by the size of the unbound pool of tryptophan in blood as measured in serum.     

Effects on the diet on brain neurotransmitters.

The synthesis of neurotransmitters in mammalian brain responds rapidly to changes in precursor availability. Serotonin synthesis depends largely on the brain concentrations of L-tryptophan, its precursor amino aicd. This relationship appears to be physiologic: when brain tryptophan levels vary because of insulin secretion or meal ingestion, corresponding alterations occur in the rate of serotonin formation. The ability of any food to modify brain tryptophan (and serotonin) depends on how its ingestion changes the serum concentration of not only tryptophan, but also several other large neutral amino acids that compete with tryptophan for uptake into the brain. Such precursor-induced changes in brain serotonin appear to be functionally important: animals having a reduced level of brain serotonin (caused by the chronic ingestion of a naturally tryptophan-poor diet, such as corn) demonstrate a heightened sensitivity to painful stimuli; this pain sensitivity can be acutely restored to normal values by a single injection of L-tryptophan, which rapidly elevates brain serotonin. The synthesis of catecholamines (e.g., dopamine, norepinephrine) in the brain also varies with the availability of the precursor amino acid L-tyrosine. Single injections of this amino acid increase brain tyrosine levels and accelerate brain catechol synthesis, while injections of a competing neutral amino acid (e.g., leucine, tryptophan) reduce brain tyrosine and its rate of conversion to dopa. The rate of catecholamine synthesis, however, appears to be influenced less by precursor levels than is serotonin formation: tyrosine hydroxylase, whcih catalyzes the rate-limiting step in catecholamine synthesis, responds strongly to end-product inhibition and to other controls that reflect variations in neuronal activity. The synthesis of acetylcholine in brain responds to substrate (choline) availability much like serotonin synthesis. Short-term alterations in brain choline levels are mirrored by similar changes in brain acetylcholine concentration. Variations in the daily dietary intake of choline also modify brain choline and acetylcholine. The relationship between choline availability and acetylchyoline synthesis has already foudn a cletween choline availability and acetylchyoline synthesis has already found a clinical application: choline has been used successfully in the treatment of tardive dyskinesia, a disorder of the central nervous system thought to reflect a deficiency in cholinergic transmission. These relationships between precursor availability from the periphery and brain neurotransmitter synthesis may ultimately provide the brain with information about peripheral metabolic state.     

Glucagon infusion in normal man: effects on 3-methylhistidine excretion and plasma amino acids.

In order to assess the role of glucagon in human protein metabolism and to examine its action as a "catabolic" hormone, studies were conducted in two normal male subjects over an 8-day period. After minimum and stable urinary nitrogen excretion had been produced by the continuous nasogastric administration of carbohydrate (720 g/day) for 8 consecutive days, a continuous intravenous infusion of glucagon (1.0 mg/24 hr) was superimposed on days 7 and 8. Excretion of total nitrogen (N) and urea-N increased significantly (p less than 0.05). Excretion of 3-methylhistidine was unaltered, suggesting that the source of the N losses produced by glucagon did not derive from increased muscle proteolysis. Although striking hypoaminoacidemia was produced, the reductions of extracellular amino acids alone could not account for all of the extra urea excreted. These data suggest that hyperglucagonemia in normal man induces mild nitrogen losses by stimulation of hepatic ureogenesis from free intracellular amino acid pools and not by increased rates of muscle protein breakdown.     

Effect of sucrose overfeeding on brown adipose tissue lipogenesis and lipoprotein lipase activity in rats

During cold-induced nonshivering thermogenesis, interscapular brown adipose tissue (BAT) lipoprotein lipase (LPL) activity and lipogenesis are elevated. Because of the many similarities between cold- and diet-induced thermogenesis, we examined the effect of ad libitum access to a 32% sucrose solution on caloric intake, adiposity, and BAT enzyme activities in male rats. Daily caloric intakes of sucrose-fed animals were elevated by 20%-25%, and 8 wk of sucrose feeding doubled carcass fat content. This sucrose-feeding induced obesity was associated with increases in circulating triglyceride and insulin levels as well as increased retroperitoneal white adipose tissue LPL activity. However, the increased carcass lipid content accounted for less than half of the excess calories ingested by the sucrose-fed rats. Sucrose feeding stimulated in vivo lipogenesis in BAT and elevated BAT fatty acid synthetase and acetyl-CoA carboxylase activities but not LPL activity. These findings suggest that overeating enhances endogenous lipogenesis but not uptake of circulating triglyceride in BAT. Thus, both cold- and diet-induced thermogenesis increase BAT lipogenesis, while only cold-induced thermogenesis is associated with elevated LPL activity in BAT.     

Thyroid status and muscle protein breakdown as assessed by urinary 3-methylhistidine excretion: Study in thyrotoxic patients before and after treatment

Urinary 3-methylhistidine (3MH) excretion was studied in nine thyrotoxic patients before and after treatment. Urinary creatinine (Cr) output was also measured and was low in the thyrotoxic subjects before treatment. Thus, although urinary output of 3MH was not greater than among the control population when expressed per subject, it was significantly elevated when expressed as the ratio of 3MH to Cr; this ratio fell significantly, reaching normal control values after a euthyroid state was obtained. In one patient who became hypothyroid, the $\text{3MH}\text{Cr}$ ratio fell under the control value. There was a significant linear correlation between the $\text{3MH}\text{Cr}$ ratio and the hormonal variables (T₃, T₄, FT₄I); moreover, variations in the $\text{3MH}\text{Cr}$ ratio and variations in the T₃ level were closely correlated. 3-Methylhistidine appears to be a reliable index of muscular breakdown in thyrotoxicosis. From our results, it can be concluded, first of all, that hyperthyroidism is accompanied by an increased muscular catabolism, and, second, that the return to a euthyroid state results in an immediate normalization of muscular breakdown.     

Effect of parenteral nutrition on muscle amino acid output and 3-methylhistidine excretion in septic patients

The effects of adequate total parenteral nutrition (TPN) on nitrogen excretion, urea N percentage, 3-methylhistidine excretion, and leg amino acid output, were studied during the ten-day period following abdominal surgery for generalized peritonitis in nine patients. The first two postoperative days were without nutritional intake, TPN was started on the third postoperative day (57 cal/KgBW--40% as Intralipid--0.30 g of N/KgBW). Leg amino acid outputs were done before TPN (DO), then two days (D2) and eight days (D8) after TPN. Total nitrogen and urea N percentage did not significantly differ before and after TPN. Between DO and D2 there was a significant reduction of urinary 3-methylhistidine (467 +/- 37 to 280 +/- 29 mumol/24 h-P less than 0.001) and leg amino acid release (604 +/- 103 to 254 +/- 87 nmol/mn/100 g of calf muscle--P less than 0.01) reflecting reduction in muscle hypercatabolism despite the persistence of the septic state. Between D2 and D8, 3-methylhistidine remained stable while leg amino acid release continued to decrease (254 +/- 87 to 68 +/- 40 nmol/mn/100 g--P less than 0.05). This association suggests an increased muscle protein synthesis. A closer examination of the clinical evolution of these patients, especially concerning their septic evolution, shows that only improved patients with recovery from sepsis increased their muscle protein synthesis. Thus, in septic hypercatabolic patients TPN seems to be able to reduce muscle catabolism while the increase in protein synthesis is mainly the consequence of recovery from the septic state. In such patients TPN should be used as a preventive therapeutic measure.     

The effect of T3 and reverse T3 administration on muscle protein catabolism during fasting as measured by 3-methylhistidine excretion.

Since recent studies have indicated that measurement in urine of the amino acid, 3-methylhistidine, accurately reflects the extent of muscle catabolism, and because it has been suggested that thyroid hormones may influence muscle breakdown, especially during fasting, the effect of T3 and reverse T3 (rT3) administration on the excretion of 3-methylhistidine was examined in obese subjects during fasting. The mean (+/- SE) 3-methylhistidine excretion in patients fed an egg protein diet (devoid of meat protein) was 256 +/- 35 mumoles/day and decreased to 190 +/- 14 mumoles/day during fasting. T3 administration (100 microgram/day x 5 days) increased 3-methylhistidine excretion to 304 +/- 37 mumoles/day during its ingestion and to 485 +/- 46 mumoles/day in the T3 posttreatment interval. T3 doses of 10 microgram every 4 hr (q4h) for the first 6 days of fasting also appeared capable of increasing 3-mehis excretion whereas 5 microgram T3 q4h administered during the first 6 days of fasting did not increase 3-mehis excretion. Reverse T3 administration (80 microgram q6h) during fasting was associated with a mean 3-methylhistidine of 130 +/- 13 mumoles/day, a value no higher than in patients fasted alone. These observations suggest that: (1) skeletal muscle catabolism decreases during fasting: and (2) pathophysiologic doses of T3 (60 microgram/day or more), but not reverse T3, enhance muscle catabolism during fasting.