Hamish N. Munro

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

Abstract 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 Nτ-methylhistidine. The children were receiving a flesh-free diet, free of exogenous Nτ-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 Nτ-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.

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

Abstract Adrenalectomized young male rats were given a large dose of corticosterone ( 10 mg 100 gm body weight ) either by subcutaneous or intraperitoneal injection. Subcutaneous injection caused immediate cessation of growth and an elevation in urea and in Nτ-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 ( 10 mg 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 ( 0.8 mg 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.

Metabolic, endocrine, and reproductive changes of a woman channel swimmer

We report the coordinated metabolic, hormonal, and reproductive data of a female channel swimmer during the pre-swim training period, immediately post-swim, and in the post-swim untrained state. Urine and blood samples collected at these times were assayed for diurnal urinary catecholamines, urinary C-peptide and 3-methylhistidine, total blood ketone bodies, glycerol, the reproductive hormones, adrenal androgens, and thyroid hormones. Subcutaneous fat was measured by ultrasonography. All of the metabolic and hormonal data post-swim except cortisol reflected the severe physiological stress. Urinary catecholamines returned to near-normal levels by 12 hours post-swim. The metabolic changes were associated with reproductive changes, including a shortened luteal phase, absence of ovulation, and increased LH secretion relative to FSH. The swimmer maintained high levels of body fat; she did not become amenorrheic. Metabolic and reproductive hormone levels returned to normal by 2 months post-swim.

Glycoconjugates as noninvasive probes of intrahepatic metabolism: I. Kinetics of label incorporation with evidence of a common precursor UDP-glucose pool for secreted glycoconjugates

Conditions are described under which hepatic metabolism of administered labeled sugars can be monitored in the intact rat by measuring output of label from the liver in two secreted glycoconjugates derived from intracellular UDP-glucose, namely glucuronic acid linked to an administered drug (acetaminophen), and galactose secreted in glycoproteins. Rats with indwelling intrajugular catheters were given constant infusions of acetaminophen at nontoxic doses, and acetaminophen-glucuronide was isolated from rat urine by HPLC. When an unprimed continuous infusion of [U-14C]glucose or [1-3H]galactose is begun during acetaminophen infusion, labeling of urinary acetaminophen-glucuronide attains a plateau with an apparent half-life of 2.1 h from labeled glucose as the 14C-donor, and 1.2 h from labeled galactose. The times required to attain steady-state are attributable either to formation or to hepatic secretion of the glucuronyl conjugate rather than to mixing in extracellular pools or to delays in renal excretion since the half-time of urinary excretion of intravenously injected acetaminophen-glucuronide is only 21 minutes. Consequently, the excreted glucuronide provides noninvasive evidence of the recent specific activity in the precursor pool of hepatic UDP-glucose feeding glucuronic acid. The second glycoconjugate used for comparison was galactose, isolated from the acute-phase plasma protein alpha 1-acid glycoprotein. This represents hepatic UDP-galactose labeling. Comparison of the labeling patterns of secreted galactose and glucuronic acid demonstrates that they are derived from the same intracellular pool of UDP-glucose. This is in contrast to the patterns of labeling of hepatic glycogen from (1-3H) and [U-14C]glucose which have been shown to diverge from those of secreted glucuronic acid.

Nutritional Requirements in the Elderly

Do the nutritional needs of the elderly differ significantly from those of younger adults, and if so, how? This and other fundamental questions are addressed in the light of recent experimental and clinical findings.

Glycoconjugates as Noninvasive Probes of Intrahepatic Metabolism: III. Application to Galactose Assimilation by the Intact Rat

A tracer methodology has been developed for noninvasive assessment of intrahepatic metabolism of administered labeled sugars. In this procedure, we measure the output of the label from the liver in two glycoconjugates derived from hepatic UDP-glucose, namely, glucuronic acid formed through UDP-glucuronic acid and excreted in the urine following acetaminophen administration, and galactose formed through UDP-galactose and then secreted in the carbohydrate portion of glycoproteins in the plasma. Comparison of the distribution of label from various sugar precursors in these end-products can indicate exchanges between hepatic UDP-glucose, UDP-galactose, and UDP-glucuronic acid. In this study we apply the technique to explore whether the enzyme UDP-galactose-4-epimerase catalyzing the step UDP-galactose to UDP-glucose is nonequilibrium and therefore potentially has a regulatory role for utilization of free galactose. The specific activity in the two glycoconjugates was compared when either [1-3H]galactose or [U-14C]glucose was the infused precursor sugar. In rats under a variety of conditions (fasting, oral refeeding, intravenous administration of galactose), label from [1-3H]galactose accumulated in glycoprotein-bound galactose much more than in acetaminophen-bound glucuronic acid, in comparison to label from [U-14C]glucose, demonstrating limitation of the rate of transfer from UDP-galactose to UDP-glucose at the epimerase step. Accordingly, epimerase is suggested to have a regulatory role in the galactose assimilation pathway.(ABSTRACT TRUNCATED AT 250 WORDS)

Glycoconjugates as noninvasive probes of intrahepatic metabolism: II. Application to measurement of plasma α 1-acid glycoprotein turnover during inflammation

We describe a new method for measuring plasma glycoprotein turnover in vivo using carbohydrate-moiety labeling compared with precursor UDP-sugar monitoring, which allows estimation of the specific activity of the precursor at the biosynthetic site of the glycoprotein. This method has been applied to plasma alpha 1-acid glycoprotein (AGP) kinetics, and has allowed direct quantitation of absolute synthesis rates of AGP in a non-steady state. Following turpentine-induced acute inflammation, AGP was found to undergo a maximum increase in plasma level of eight-fold with a 20- to 25-fold induction in absolute synthesis rate peaking at 25 to 50 h, and a concurrent 2.0 to 2.5-fold increase in fractional degradation rate. The changes in absolute synthesis rates were quite comparable both temporally and quantitatively to changes in hepatic AGP mRNA levels and gene transcription rates reported by others following turpentine inflammation, thus suggesting that AGP synthesis in vivo is predominantly regulated by the level of its mRNA. The carbohydrate moiety labeling method can be applied to other plasma glycoproteins to measure their kinetic parameters in the intact animal or human subject.

Protein Nutriture and Requirements of the Elderly

Protein requirements of populations have been estimated for more than a century (see Munro, 1964a, 1985, for the historical references in the paragraph). In 1853 and again in 1865, Playfair reported the protein intakes of different classes of the population of Britain and concluded that the requirements of adults ranged from 57 g protein per day in the case of a subsistence diet to 184 g for laborers doing heavy work. Later in the same century, Voit and Atwater both supported generous allowances of protein for the average working man, but several subsequent investigators publishing just before World War I concluded that 30 to 50 g protein daily was not only adequate to maintain nitrogen balance but even improved the general health and vigor of young adults. Following the first global war, the League of Nations was formed, and in 1936 a health committee reported that the safe protein intake for adults was 1 g per kilogram body weight and that some of the protein should come from animal sources.

Adaptation of Body Protein Metabolism in Adult and Aging Man

Aging causes a selective loss in lean body mass, muscle being most extensively eroded. This presentation is an analysis of the consequences of this age-related loss for protein and amino acid metabolism. The use of stable isotopes allows us to measure the dynamics of protein metabolism in the intact human subject, while the output of 3-methylhistidine permits estimates of muscle turnover to be made which show a smaller reduction in body protein turnover per kg body weight in elderly subjects. This is due to the reduced contribution by muscle protein turnover, which decreases from 30 per cent of total body protein turnover in the young adult to 20 per cent in the elderly, and can be entirely accounted for by the reduced mass of muscle, the rate of turnover per unit muscle weight being unchanged. The consequences of the changing proportions of major tissues with age has been examined for its impact on amino acid metabolism. The interaction of various organs in order to ensure metabolism of different amino acids is described. After a meal containing abundant protein, the liver selectively degraded the excess of most essential amino acids, while allowing the three branched-chain essential amino acids to pass through, so that in consequence they account for more than 50 per cent of the amino acid outflow from the splanchnic area. During this period of absorption muscle takes up amino acids, over 70 per cent being the branched-chain amino acids where they are catabolised, the nitrogen becoming available for formation of non-essential amino acids for muscle protein synthesis. The release of large amounts of glutamine from muscle is dependent on branched-chain amino acid degradation, whereas the release of large amounts of alanine is regulated by availability of pyruvate from glucose. The study of amino acid metabolism in elderly compared with young adults and its relationship to the declining mass of muscle with age is still in its initial stages. Metabolism of glycine at different levels of protein intake fails to demonstrate any significant effects of aging on metabolism of this amino acid. On the other hand, the flux of leucine shows a significant reduction in fasting elderly female subjects. This observation should be followed up by studying leucine kinetics following a meal of meat, in order to determine whether the reduced mass of muscle in the elderly will restrict the capacity to metabolise branched-chain amino acids. It has been concluded that the extensive age-related change in muscle mass should be explored for its impact on the nutritional requirements for branched-chain amino acids, their metabolism in health and disease, and their therapeutic uses at different ages.

The Challenges of Research into Nutrition and Aging

The role of nutrition in the process of aging has been receiving progressively more attention in recent years, reflecting the increasing numbers of old people in the population and the disproportionate demands they make on health care (Rowe, 1985). The proportion of people over 65 years of age varies from less than 5% in some underdeveloped areas of the world to over 16% in many parts of Western Europe. In the United States the proportion of elderly people has increased from 4% in 1900 to 11% in 1978 and is projected to be close to 14% by the end of the century (Brody and Brock, 1985). The impact of this on health care services can be appreciated from estimates made in Massachusetts (Katz et al., 1983) of the number of years after 65 that remain for independent living followed by the number of years of assisted living, the latter category meaning the need for help in rising from bed, bathing, dressing, or eating (Table I). Thus, for men aged 65–69 years, these periods average 9.3 years of independent living followed by 3.8 years of dependence; for women of 65–69 years, the projected periods are 10.6 and 8.9 years, respectively. The end of the period of independent living is presumably determined by degenerative changes in nervous system function, and the much longer survival of women in an assisted state probably represents the slower development of cardiovascular disease in the female because of the years of estrogen protection.