Brian J. Lampi

Metabolic adjustments during daily torpor in the Djungarian hamster

Djungarian hamsters (Phodopus sungorus) acclimated to a short photoperiod (8:16-h light-dark cycle) display spontaneous daily torpor with ad libitum food availability. The time course of body temperature (Tb), metabolic rate, respiratory quotient (RQ), and substrate and enzyme changes was measured during entrance into torpor and in deep torpor. RQ, blood glucose, and serum lipids are high during the first hours of torpor but then gradually decline, suggesting that glucose is the primary fuel during the first hours of torpor, with a gradual change to lipid utilization. No major changes in enzyme activities were observed during torpor except for inactivation of the pyruvate dehydrogenase (PDH) complex in liver, brown adipose tissue, and heart muscle. PDH inactivation closely correlates with the reduction of total metabolic rate, whereas in brain, kidney, diaphragm, and skeletal muscle, PDH activity was maintained at the initial level. These findings suggest inhibition of carbohydrate oxidation in heart, brown adipose tissue, and liver during entrance into daily torpor.Djungarian hamsters ( Phodopus sungorus) acclimated to a short photoperiod (8:16-h light-dark cycle) display spontaneous daily torpor with ad libitum food availability. The time course of body temperature (Tb), metabolic rate, respiratory quotient (RQ), and substrate and enzyme changes was measured during entrance into torpor and in deep torpor. RQ, blood glucose, and serum lipids are high during the first hours of torpor but then gradually decline, suggesting that glucose is the primary fuel during the first hours of torpor, with a gradual change to lipid utilization. No major changes in enzyme activities were observed during torpor except for inactivation of the pyruvate dehydrogenase (PDH) complex in liver, brown adipose tissue, and heart muscle. PDH inactivation closely correlates with the reduction of total metabolic rate, whereas in brain, kidney, diaphragm, and skeletal muscle, PDH activity was maintained at the initial level. These findings suggest inhibition of carbohydrate oxidation in heart, brown adipose tissue, and liver during entrance into daily torpor.

Enzymes of carbohydrate metabolism in young and adult rats fed diets differing in fat and carbohydrate

Glycogen content as well as glycolytic, gluconeogenic and fatty acid synthesis enzyme activities were monitored in young and adult male rats fed diets differing in fat content: 11 % (low), 22% (medium) and 42% (high) of total energy from fat. The results showed significant differences in the responses of young and adult rats to changes in dietary fat and carbohydrate. In young animals, increasing dietary fat decreased total liver glycogen phosphorylase (GP), pyruvate kinase (PK), glycerol 3-phosphate dehydrogenase, glucose 6-phosphate dehydrogenase, malic enzyme (ME), ATP-citrate lyase (ATP-CL) and fatty acid synthase (FAS). Increasing dietary fat also affected enzyme levels in other tissues: hexokinase (HK) and pyruvate dehydrogenase (PDH) activities decreased whereas skeletal muscle PK activity increased. The pattern of enzyme changes was similar in livers of fed adults with the exception that liver GP was not affected by dietary manipulations. Overnight food deprivation decreased liver glucokinase (GK), ME, ATP-CL, and FAS activities and increased liver phosphoenolpyruvate carboxykinase (PEPCK) and phosphofructokinase in both young and adult animals. In young animals, food deprivation also: (i) reduced liver GK and PK, (ii) increased kidney PEPCK, (iii) decreased muscle PEPCK and (iv) decreased kidney PDH. Food-deprived adults had increased skeletal muscle PEPCK and kidney glycogen synthetase as well as decreased kidney PEPCK muscle GP activity. These differences suggest that young animals are somewhat more responsive to changes in dietary manipulations. They also show that overnight food restriction causes a more profound metabolic re-organization in younger than in older animals.

The effect of changing dietary fat and carbohydrate on the enzymes of amino acid metabolism

Changes in the activities of alanine aminotransferase (Ala-AT), aspartate aminotransferase (Asp-AT), glutamate dehydrogenase (GDH), serine dehydratase (SDH), and branched chain amino acid dehydrogenase were monitored in weanling and adult male rats fed diets differing in fat content: 11% (low), 22% (medium), and 42% (high) of total energy from fat. The carbohydrate and fiber contents were adjusted to maintain the caloric density of the diets. The results showed that changing the total dietary fat and carbohydrate altered the activities of enzymes predominantly in liver and white muscle. This effect was dependent on the age of the animal. In adult animals, high fat diets decreased liver Ala-AT and SDH activity and increased GDH activity. Muscle Ala-AT activity was lowered in adults fed a high fat diet. In weanlings, high fat diets lowered liver Ala-AT and Asp-AT activities as well as muscle Ala-AT activity. Animals were fasted overnight to determine the effect of a high fat diet on fasting enzyme levels. In adults, fasting eliminated the decrease in muscle Ala-AT activity seen in high fatfed animals and decreased liver Ala-AT and SDH activity. In weanlings, fasting increased liver GDH activity and white muscle Ala-AT and Asp-AT activities. These results suggest a complex dependence of amino acid utilization that may depend on dietary regime and the age of the animal. (J. Nutr. Biochem. 6:414-421, 1995.)

Effects of dietary protein and fat on cholesterol and fat metabolism in rats

Abstract The influence of two fat sources (soybean oil and a 4:1 mixture of coconut oil and soybean oil) fed at three different levels (5, 10 and 20% by weight) and two protein sources (casein and gelatin supplemented with limiting amino acids) on cholesterol and fat metabolism in rats was determined. Fat or protein type had a significant ( P P de novo fat synthesis and cholesterol kinetics. De novo fat synthesis was highest in animals fed gelatin-soybean oil diets. De novo cholesterol synthesis followed the same patterns observed for fat synthesis. Since overall serum cholesterol levels were lower in rats fed gelatin-soybean oil diets, cholesterol clearance rates must have been higher to compensate for the increased synthesis observed in rats fed these diets.

Time Course of Enzyme Changes After a Switch From a High-Fat to a Low-Fat Diet

This study was conducted to determine the time course of metabolic changes associated with a switch from a high-fat to a low-fat diet in rats. Adult rats, maintained on a high-fat diet (42% of energy from fat) for 4-5 weeks were switched to a low-fat diet (11% of energy from fat), and the activities of several liver enzymes were followed. Three different phases could be distinguished. The early phase, complete by 2 days after the switch in diets, included an increase in the activity of glucose 6-phosphate dehydrogenase (pentose phosphate pathway), an increase in pyruvate kinase and pyruvate dehydrogenase activities (terminal end of the glycolytic pathway) and an increase in ATP-citrate lyase and fatty acid synthetase (fatty acid synthesis pathway). The early phase also included a decrease in the activity of phosphoenolpyruvate carboxykinase (PEPCK, gluconeogenesis) and a lower branched-chain amino acid dehydrogenase activity (BCAADH, branched-chain amino acid degradation). The concentration of the allosteric phosphofructokinase regulator, fructose 2,6-bisphosphate (Fru-2,6-P2, glycolysis), decreased during the early phase. An intermediate phase could also be discerned between 3 and 10 days after the switch in diets. In this phase, the decreased Fru-2,6-P2 concentration and the decreased PEPCK and BCAADH activities observed in the early phase were reversed. The late phase occurred 10 days after the dietary switch and was characterized by an increase in the activities of glucokinase (glycolytic pathway) and glycogen phosphorylase (associated with glycogenolysis) and by a decrease in glutamate dehydrogenase, PEPCK and BCAADH activities. These measurements indicate that at least 20 days are required before metabolic changes associated with a switch in diet are complete.

Hard Wheat Bran and Hard Wheat Bran Fiber Energy Values Measured in Rats after 6 and 16 Weeks

The difficulties in determining the energy values of dietary fiber (DF) sources in human studies include the following: 1) The control over food intake, 2) the long-term maintenance of dietary changes (Stevens et al., 1987), 3) the cost and the large number of dietary combinations to be assessed. Models for predicting the energy value of DFs in humans have improved over the last decade (Livesey, 1995) but energy-balance experiments in animals are important due to the limited number of human studies (Livesey, 1990a). The rat model is particularly useful for long-term experiments and it has been recommended for predicting the digestibility of dietary components and energy (Bach Knudsen et al., 1994; Wisker et al.,1996).

Fatty acid oxidation and fatty acid synthesis in energy restricted rats

The importance of fat oxidation and fatty acid synthesis were examined in rats fed approximately one half their ad libitum food intake for a period of 13 days followed by 7 days of ad libitum feeding (refed rats). This study was undertaken because previous reports demonstrated that refed rats rapidly accumulated body fat. Our results confirmed this observation: refed rats accrued body fat and body weight at rates that were approximately 3 times higher than controls. Evidence for a period of increased metabolic efficiency was demonstrated by measuring the net energy requirement for maintenance over the refeeding period: refed rats had a reduced metabolic rate during the period of energy restriction (approximately 30% lower than control) and this persisted up to 2 days after the reintroduction of ad libitum feeding. The major factor responsible for the rapid fat gain was a depressed rate of fatty acid oxidation. Calculations of protein and carbohydrate intake over the refeeding period showed that the simplest explanation for the decrease in fatty acid oxidation is fat sparing. This is possible because of the large increase in dietary carbohydrate and protein intake during the refeeding period when metabolic rates are still depressed. The increased carbohydrate and protein may adequately compensate for the increasing energy requirements of the ER rats over the refeeding period affording rats the luxury of storing the excess dietary fat energy.