Effect of age and high sucrose diet on 2-deoxyglucose uptake in perfused hindlimb

Consumption of carbohydrate solutions enhances energy intake without increased body weight and impaired insulin action in rat skeletal muscles

OBJECTIVES

In the present study, we investigated whether replacement of tap water by fructose or sucrose solutions affect rat body weight and insulin action in skeletal muscles.

METHODS

Rats were fed standard rodent chow ad libitum with water, or water containing fructose (10.5% or 35%) or sucrose (10.5% or 35%) for 11 weeks. Body weight and energy intake from chow and drinking solutions were measured. Urinary catecholamines secretion was determined after 50-60 days. At the end of the feeding period, soleus and epitrochlearis were removed for in vitro measurements of glucose uptake (with tracer amount of 2-[3H]-deoxy-D-glucose) and PKB Ser473 phosphorylation (assessed by Western Blot) with or without insulin.

RESULTS

Fructose and sucrose solutions enhanced daily energy intake by about 15% without increasing rat body weight. Secretion of urinary noradrenaline was higher in rats drinking a 35% sucrose solution than in rats drinking water. In the other groups, urinary noradrenaline secretion was similar to rats consuming water. Urinary adrenaline secretion was similar in all groups. Insulin-stimulated glucose uptake and insulin-stimulated PKB phosphorylation were not reduced by intake of fructose or sucrose solution.

CONCLUSIONS

Fructose and sucrose solutions enhanced energy intake but did not increase body weight. Although noradrenaline may regulate body weight in rats drinking 35% sucrose solution, body weight seems to be regulated by other mechanisms. Intake of fructose or sucrose solution did not impair insulin-stimulated glucose uptake or signaling in skeletal muscles.

Interaction between aging and syndrome X: new insights on the pathophysiology of fat distribution

Increased fat mass (FM), and in particular a specific increase in visceral fat (VF), may account for the age-associated decrease in insulin action and the development of Syndrome X. Utilizing chronic caloric restriction (CR) with aging in a rodent model, we dissociated the effects of VF and FM, and demonstrated that the decrease in VF accumulation was sufficient to prevent the marked decrease in hepatic insulin action. This suggests that the typical increase in VF with aging, rather than aging per se, determines hepatic insulin resistance. To directly assess the role of VF, we studied rats after surgical removal of VF or sham operation. Surgical extraction of VF (which accounts for approximately 10% of total fat) improved hepatic insulin action by more than twofold. We studied the role of fat-derived peptides in the regulation of body composition and insulin action. While VF extraction resulted in decreased gene expression for leptin and TNF-alpha in the subcutaneous adipose, administration of leptin selectively decreased visceral fat (approximately 60%), and enhanced the action of insulin on inhibiting hepatic glucose production (approximately 80%). Thus, the cause-effect relationship between the age-related increase in VF and the decrease in hepatic insulin action may involve the failure of leptin to "cross talk" with other fat depots to regulate fat distribution.

Cellular proliferation and UCP content in brown adipose tissue of cold-exposed aging Fischer 344 rats

Previous investigations have demonstrated that older vs. younger rats respond to cold exposure with blunted cold-induced nonshivering thermogenesis of brown adipose tissue (BAT). This reduction in nonshivering thermogenesis is associated with reduced mass and blunted nonshivering thermogenic capacity of BAT. The purpose of this study was to test the hypothesis that brown fat in 26-mo-old Fischer 344 (F344) male rats has an impaired capacity to respond to the trophic stimulus of chronic cold exposure with increases in cell number, mass, and uncoupling protein (UCP) content. To test this hypothesis, the response of BAT to chronic cold exposure was evaluated in young and old rats. We exposed 6-, 12-, and 26-mo-old F344 male rats to 10 degrees C for 5 days and measured interscapular BAT (IBAT) mass, cell size and proliferation, and mitochondrial UCP1 content. Plasma concentrations of insulin-like growth factor I (IGF-I) and norepinephrine IBAT mass, cell proliferation, or UCP1 content in response to chronic cold, whereas the 6-mo-old rats had a nearly 2-fold cold-induced increase in IBAT mass, a 26-fold increase in cell proliferation, and a 4-fold increase in UCP1 content. Cold exposure also produced an increase of 29, 19, and 20% in mature brown adipocyte cell size of the 6-, 12-, and 26-mo-old animals, respectively. Plasma levels of IGF-I were unaffected by cold at all ages, whereas NE levels were increased by the cold exposure and by increasing age. These data support the hypothesis that brown fat in old F344 rats does not respond to the trophic stimulus of chronic cold exposure to the same degree as younger animals. Moreover, these data indicate that the observed cold- or age-induced changes in levels of growth factors evaluated in this study were not associated with the lack of cold-induced preadipocyte proliferation or increased UCP1 in brown fat of the 26-mo-old rats.

Some considerations for the development of diets for mature rodents used in long-term investigations

Nutritional requirements for mature rodents used in long-term investigations are virtually unknown. The limited knowledge of the dietary needs of mature rodents is due in part to overreliance on weanling animals fed an experimental diet for relatively short periods. Generalizations made from observations of weanling rodents are not appropriate for all ages. Dietary recommendations for rodents have been established, for the most part, by using the nutritional benchmark of maximal growth rate in animals fed ad libitum. Although this method provides valuable insight into the understanding of nutritional deficiency, it is less effective in determining nutrient requirements for mature animals used for the long term. The implication that maximal growth resulting from ad libitum feeding may not indicate the best dietary regimen in the long term is consistent with the observation that energy-restricted rodents live significantly longer and have lower incidence of disease that do their ad libitum-fed counterparts. These and other findings discussed in the review suggest that nutrient requirements established for young rodents may need re-evaluation to determine their applicability to the dietary recommendations for older animals used in long-term investigations.