Closed-circuit metabolic system with multiple applications

Decreased cerebral glucose utilization in rats during the ebb phase of thermal injury

OBJECTIVE

Thermal injury is associated with the development of encephalopathy. The mechanism(s) for the development of this condition have not been established. In the present study, the effects of thermal injury were determined on rat brain glucose utilization (Rg), using 2-[18F]fluoro-2-deoxy-D-glucose (18FDG).

DESIGN

Four types of studies were performed. In one group of rats, the effect of thermal injury on total rat brain glucose utilization (Rg) was determined at 6 hours, 24 hours, and 3 weeks after injury. The brains of thermally injured rats were also assayed for hexokinase and glucose-6-phosphatase activities, since these enzyme activities are responsible for the phosphorylation and dephosphorylation of the 18FDG. We also measured total body oxygen consumption in the thermally injured rats. We wanted to compare the changes produced by thermal injury on rat brain glucose utilization (Rg) with the effects produced by compounds known to modify energy metabolism and/or rat brain glucose utilization (Rg). For that reason, in a second group of rats, an inflammatory state was produced by lipopolysaccharide injection, and rat brain glucose utilization (Rg) was determined. In the third group of rats, overall metabolism in rats was reduced by pentobarbital injection, followed by hypothermia, and rat brain glucose utilization (Rg) was determined. In the fourth group of rats, overall metabolism in rats was stimulated by 2,4-dinitrophenol injection, and rat brain glucose utilization (Rg) was determined.

MATERIALS AND METHODS

Glucose utilization (Rg) by the brains of these treated rats was determined using 18FDG. Oxygen consumption in vivo, as well as glucose-6-phosphatase and hexokinase activity in vitro, were measured by standard procedures.

MEASUREMENTS AND MAIN RESULTS

Glucose utilization (Rg) by rat brain was significantly reduced (p < 0.01) at 6 and 24 hours after injury, but returned to normal values 3 weeks after injury. These reductions were associated with decreases in rat brain hexokinase activity, increases in rat brain glucose-6-phosphatase activity, and decreased oxygen consumption by rats in vivo. Pentobarbital injection followed by hypothermia reduced rat brain glucose utilization (Rg) (p < 0.01), while 2,4-dinitrophenol treatment elevated rat brain glucose utilization (Rg) (p < 0.01). In contrast, LPS treatment had no effect on rat brain glucose utilization (Rg).

CONCLUSIONS

These data indicate that thermal injury decreases glucose utilization (Rg) in rat brain during the hypometabolic phase. This effect can be explained, at least in part, by alterations in hexokinase and glucose-6-phosphatase activities, as well as reductions in oxygen consumption. Thus, the changes in brain glucose utilization (Rg) appear to be associated with the ebb phase of the thermal injury. The present results observed in burned rats may provide evidence to explain the encephalopathy observed in burned patients.

Practical aspects of indirect calorimetry in laboratory animals

Oxidation of the energetic substrates by the body is associated with oxygen consumption, carbon dioxide production, and heat release specific to the nature of the energetic substrates being oxidized. Therefore, measurement of respiratory exchanges (indirect calorimetry) is a powerful method to investigate heat production of a living organism. In this article, we review the elementary principles of indirect calorimetry and describe the operating principle of the two most typical devices used to perform indirect measurements of energy expenditure in the laboratory animal: the closed-circuit and the open-circuit. We then discuss some practical aspects of the day-to-day use of these devices: respective advantages and limitations of each technique, data processing, calibration, correction for body-size, and computation of the energy expended for activity. In the second part, we review some of the standard formulas of indirect calorimetry that offer the possibility to obtain more precise information such as the rate of oxidation of carbohydrates (CHO), lipids and proteins if some hypotheses are made on the intensity of lipogenic, ketogenic, and gluconeogenic processes. Finally, a practical example of the measurement of energetic cost of activity and thermic effect of food in the rat is given.

Intestinal absorption and excretion of octapeptides composed of D amino acids

Octapeptides synthesized from D amino acids were absorbed from the intestine and excreted in urine of normal rats drinking 5% glucose/1% creatinine containing the 125I-labeled peptides at 0.1-25 mg/dl. The rats ingested fluid at the rate of about 20 ml/hr and produced urine at 15 ml/hr for several hours during the nocturnal feeding period. Sixty-one +/- 4% of the ingested creatinine and 50 +/- 3% of a lipid-insoluble D octapeptide (EASASYSA, 784 Da) were excreted intact in the urine. The steady-state molar rate of absorption-excretion of creatinine equaled or exceeded the maximum rate of carrier-mediated intestinal transport of glucose, suggesting that both the creatinine and the D octapeptide were transported paracellularly by solvent drag through absorptive cell junctions that were dilated by the glucose. More than 70% of the ingested glucose was also absorbed paracellularly. The results demonstrate that intact oligopeptides can be absorbed efficiently from the intestine when they are not hydrolyzed by membrane-bound peptidases of the brush border. The results also provide support for recent theories proposing that coupling of membrane digestion with paracellular solvent drag accounts for a major fraction of normal intestinal absorption of nutrients.

Anti-obesity and anti-diabetic effects of CL 316,243, a highly specific beta 3-adrenoceptor agonist, in yellow KK mice

The anti-obesity and anti-diabetic effects of CL 316,243, a highly specific beta 3-adrenoceptor agonist (beta 1: beta 2: beta 3 = 0:1:100,000), were evaluated in obese diabetic yellow KK mice and C57Bl control mice. The study compound was fed through a gastric tube at a rate of 0.1 mg/kg/day for 2 weeks. The following parameters were compared in the treated and control animals given distilled water: brown adipose tissue thermogenesis, resting metabolic rate, insulin receptors in adipocytes, and blood glucose and serum insulin levels during a glucose overloading test. CL 316,243 significantly increased brown adipose tissue thermogenesis and resting metabolic rate in both yellow KK mice and C57Bl controls. The amount of white adipose tissue decreased, although food intake was not affected. The effects contributed to the mitigation of obesity in yellow KK mice. CL 316,243 also increased the concentration of insulin receptors and decreased the levels of serum insulin and blood glucose during the glucose overloading test in yellow KK mice. These observations suggest that CL 316,243 possesses anti-obesity and anti-diabetic effects and consequently may be useful for treating obesity as well as non-insulin-dependent diabetes mellitus in obese persons, without causing excessive side effects.

Turnover rates of different collagen types measured by isotope ratio mass spectrometry

The rates of collagen turnover in different tissues have been estimated in growing rats previously exposed to gaseous 18O2. The abundance of the stable isotope was measured using isotope ratio mass spectrometry following combustion of isolated collagen-derived hydroxyproline. Using this method, problems of label reutilization associated with radiolabelling methods are avoided. In general the results confirm the slow turnover rates with half-lives of total collagen in skin, muscle and gut of 74, 45 and 244 d, respectively. The use of cyanogen bromide digests of whole tissues followed by isolation of collagen type-specific peptides has allowed the comparison of turnover rates of collagen types I and III, indicating that collagen type III is turned over more rapidly than type I.

Synthesis and degradation of collagens in skin of healthy and protein-malnourished rats in vivo, studied by 18O2 labelling

To explore the effects of growth retardation, caused by restricted protein intake, on collagen turnover in the whole skin, Sprague-Dawley rats (n = 20) were labelled with 18O2 and fed on either an adequate (18%) or a low (3%) lactalbumin diet. Skin biopsies were obtained at intervals during the following 6 months. Independent groups of animals (n = 186) were used to determine the size of the 0.5 M-acetic acid-soluble and -insoluble collagen pools in the entire skin of healthy and malnourished rats. Collagen was estimated by measurement of hydroxyproline. Soluble-collagen synthesis rates were equivalent to 99 +/- 8 mumol of hydroxyproline/day in healthy animals and 11 +/- 2 mumol/day in malnourished rats. Insoluble-collagen synthesis rates were 32 and 5 mumol of hydroxyproline/day in the healthy and protein-depleted rats respectively. The degradation of soluble collagen amounted to 37 +/- 8 and 6 +/- 2 mumol of hydroxyproline/day in the healthy and malnourished groups respectively. Efflux of collagen from the soluble collagen, defined as the sum of the rate of soluble collagen that is degraded plus that which matures into insoluble collagen, was 70 +/- 8 and 11 +/- 2 mumol of hydroxyproline/day in the healthy and malnourished groups respectively. Insoluble collagen was not degraded in either group. The fraction of soluble collagen leaving the pool that was converted into insoluble collagen was 0.46 in both diet groups. It is concluded that the turnover of soluble collagen is markedly decreased with malnutrition, but degradation and conversion into insoluble collagen account for the same proportions of efflux from the soluble-collagen pool as in rapidly growing rats.