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Biomedical subjects

H N Munro

Publications and source records attributed to H N Munro.

At least 73 records · Page 4Linked to original sources
PubMed1981

Interaction of thyroid status and diet on muscle protein breakdown in the rat, as measured by N tau-methylhistidine excretion.

The effects of thyroid status and of dietary protein and energy content on muscle protein breakdown have been compared in young rats, using urinary N tau-methylhistidine (3-methylhistidine; 3-Mehis) output as a measure of myofibrillar protein breakdown. Recently, thyroidectomized young rats receiving an adequate diet grew slowly and showed a reduction in 3-Mehis output, relative to the weight of their leg muscles. When the thyroidectomized rats were given 2-5 microgram thyroxine (T4) per 100 g body weight, output of 3-Mehis increased, even when growth was prevented by feeding a diet low in protein. This demonstrates that thyroid-status affects muscle protein breakdown through a mechanism other than via a change in growth rate. In confirmation of previous studies, administration of a diet low in protein to young rats prevented growth and reduced output of 3-Mehis relative to leg muscle weight. The reduction in protein breakdown is not likely to be due to reduced thyroid function, since the diet low in protein increased plasma total triiodothyronine (T3) levels. Furthermore, thyroidectomized rats fed the low-protein diet also underwent a decrease in output of 3-Mehis. In contrast to protein deficiency, a moderate dietary energy deficiency allowed some growth to occur, and output of 3-Mehis, relative to muscle mass, was not depressed. Nevertheless, total T3 concentration in the plasma of these animals was considerably reduced, indicating once more that total circulating thyroid hormone levels are not the only determinants of muscle protein breakdown. Finally, reduced plasma total T3 levels occurred in energy-deficient diets even in thyroidectomized rats receiving exogenous T4. This suggests that diet affects metabolism of the hormone independently of changes in secretory output from the thyroid gland.

Animals↗
PubMed1981

The antiscorbutic action of L-ascorbic acid and D-isoascorbic acid (erythorbic acid) in the guinea pig.

Female guinea pigs were fed a scorbutigenic diet supplemented with either L-ascorbic acid or D-isoascorbic acid or combinations of these. Their responses were judged by changes in body weight, serum alkaline phosphatase levels, wound healing, and tooth structure. Large additions (100 mg daily) of D-isoascorbic acid to the scorbutigenic diet resulted in normal growth over a 7-wk period and normal serum alkaline phosphatase levels, tooth structure development, and collagen formation after wounding. The addition of 0.5 or 5.0 mg of L-ascorbic acid to this high D-isoascorbic diet improved neither growth rate nor collagen deposition during wound healing. On the basis of changes in tooth structure, D-isoascorbic acid has 1/20 the potency of L-ascorbic acid. Its effect is additive to subminimal maintenance levels of L-ascorbic acid implying that there is no competitive inhibition in the utilization of the two compounds. The relatively weak activity of D-isoascorbic acid is probably due to poor transport to the tissues and ineffective binding to functional sites. This explains why the onset of scurvy is much more rapid after withdrawal of D-isoascorbic acid from the diet when it had been the sole antiscorbutic dietary constituent. It is concluded that D-isoascorbic acid is a "weakly" antiscorbutic agent on the basis that it is both poorly absorbed and retained by the tissue; that in fact it may, to the degree that it is taken up by the tissues and retained, be equal in antiscorbutic potency to L-ascorbic acid.

Alkaline Phosphatase↗
PubMed1981

Effect of excess dietary zinc on tissue storage of iron in rats.

Earlier investigators observed that addition of large amounts of zinc to the diet or rats can retard growth, lower their hemoglobin levels and reduce storage of iron. In the present studies, addition of 0.75% zinc to a synthetic diet confirmed the reduced storage of iron in the livers and spleens of growing rats, but failed to show an effect on growth rate or hemoglobin levels. The adverse effects of zinc excess on growth and hemoglobin level could, however be reproduced by replacing the Rogers-Harper salt mixture used in the present studies with the Wesson salt mixture use in the earlier studies. Rats fed excess zinc along with the Wesson salt mixture grew less well, had anemia and also had low level in copper in their livers. It is suggested that addition of zinc to the low copper Wesson salt mixture reduced copper absorption sufficiently it deplete liver copper to a level at which mobilization of liver iron stores by a copper-dependent mechanism became impaired, thus depriving red cell production and tissue enzymes of stored iron. The mechanism by which iron stores are depleted by addition of zinc to the better balanced Rogers-Harper salt mixture remains unexplained It is not due to interference by zinc with iron adsorption from the diet nor with cellular uptake of iron from circulating transferrin, and the capacity of tissues to store iron as ferritin is not impaired.

Absorption↗
PubMed1981

Whole-body leucine and lysine metabolism: response to dietary protein intake in young men.

Whole-body leucine and lysine metabolism was explored in young adult men by a primed constant intravenous infusion of a mixture of L-[1-13C]leucine and L-[alpha-15N]lysine over a 4-h period. Subjects were studied after an overnight fast (postabsorptive state) or while consuming hourly meals (fed state) after adaptation to diets providing either a surfeit level of protein (1.5 g.kg body-1.day-1), a level approximating maintenance requirements (marginal intake) (0.6 g.kg body wt-1.day-1), or a grossly inadequate level (0.1 g.kg-1.day-1). The change in protein intake from a marginal to a surfeit level was associated with an increased leucine flux and incorporation of leucine into body protein. In the fed state, oxidation of leucine increased sharply and release of leucine from tissue protein diminished. When dietary protein intake was reduced from the requirement to inadequate level, leucine flux and body protein synthesis and protein breakdown were reduced, together with a smaller reduction in leucine oxidation. The response of the metabolism of [15N]lysine was responsible for maintenance of leucine and other essential amino acid economy, and they appear to be related to the nitrogen and amino acid requirements of the subject. These findings also demonstrate an effect of meals, modulated by their protein content, on the dynamics of whole-body amino acid metabolism.

Adolescent↗
PubMed1981

Effect of the thyroid status and protein-calorie malnutrition on the rate of myofibrillar protein degradation in mature male rats.

The urinary excretion of N tau-methylhistidine (3-methylhistidine: 3-Mehis), an index of the rate of myofibrillar protein catabolism, was determined in intact and thyroid ectomized mature male rats, receiving intraperitoneally either vehicle (0.9% NaCl) or thyroxine (T4) replacement (2 microgram/100 g body weight/day) during 20 days. Rats were fed either an adequate control of a low-protein low-energy diet. In addition, body weight changes and food intake were recorded throughout the experiment. At the end of the 20-day period, livers and several muscles from hind limbs were excised and weighed. A sample of blood was then taken for serum insulin, triiodothyronine (T3) and thyroid-stimulating hormone (TSH) determination. As compared to the well-nourished animals, a significant (p less than 0.05) reduction in the rate of growth, food intake, 3-Mehis and serum insulin and T3 concentrations was observed in the rats fed the low-protein low-energy diet. In both dietary groups, thyroidectomy increased serum TSH levels and tended to reduce 3-Mehis output and liver and muscle sizes, although there was a different response according to the type of muscle excised. T4 replacement improved growth and restored T3 levels, especially in the well-fed animals, but it failed to restore either serum insulin concentrations or 3-Mehis output in either dietary groups. In conclusion, both thyroidectomy and protein-calorie malnutrition reduced the rate of myofibrillar protein breakdown in the mature rat and T4 replacement had no effect in restoring the normal range of myofibrillar protein degradation.

Animals↗
PubMed1981

Effect of isocaloric fat or glucose on albumin synthesis and nitrogen balance in patients receiving amino acid infusion.

Twelve patients who had undergone colon operations were randomized prospectively in the postoperative recovery room. Seven received a hypocaloric intravenous solution consisting of 3.5% amino acids plus 2.5% glucose and five received 3.5% amino acids plus 10% fat. Nitrogen balance data indicated that isocaloric amounts of glucose and fat seem equally effective when combined with amino acids. Although the albumin synthesis rates (measured by the 14C technique on the fourth postoperative day) were not significantly different between the two groups, when the results were included with pooled data from previous similar studies, there was a statistically significant difference which indicated a higher rate of albumin synthesis in patients who received the combination of amino acids and fat compared with those who received amino acids and glucose P < 0.05). In this clinical setting, fat may favor the uptake and synthesis of amino acids into visceral proteins, while glucose may have a more direct role in the synthesis of skeletal muscle protein.

Aged↗
PubMed1981

Effect of corticosterone on rate of myofibrillar protein breakdown in adult male rats.

The urinary excretion of N tau-methylhistidine (3-methylhistidine: 3-Mehis) was measured in 280-g intact and adrenalectomized (AdX) male rats to assess the effect of corticosterone on the rate of myofibrillar protein breakdown near the end of their growth period. Urinary outputs of urea-N and creatinine were also determined. Rats were given either vehicle or 10 mg of corticosterone subcutaneously/100 g body weight/day during 8 days. At the termination of the experiment, samples of blood were taken for insulin and corticosterone determination, and the livers and selected hind limb muscles were excised and weighed. As compared to either intact or AdX rats receiving vehicle injection, significant (p less than 0.05) and extensive reductions in body weight and muscle weight were found in the AdX rats receiving the steroid, whereas there was a significant increase (p less than 0.05) in liver weight, and in insulin and corticosterone plasma concentrations. Urinary output of 3-Mehis increased considerably during the first 4 days of corticosteroid treatment and then declined. This later adaptive change may account for findings by some authors of a lack of response of muscle protein breakdown some days after commencement of steroid treatment. It is concluded that mature rats, like growing rats, undergo acceleration in rate of myofibrillar protein breakdown when given high doses of corticosteroids, and consequently, that the effects on muscle metabolism are not an indirect result of suppression of growth.

Adrenalectomy↗
PubMed1980

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 x kg-1 for 7 days) or a low protein intake (0.4 g x 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.

Adult↗
PubMed1980

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 micromole/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.

Adult↗