Endurance exercise training increases adipose tissue glucocorticoid exposure: adaptations that facilitate lipolysis.

Glucocorticoids (GCs) have long been thought to be lipolytic in nature. Recently, however, increased exposure to GCs in insulin-sensitive tissues has been associated with lipid accumulation and metabolic complications, regardless of plasma concentrations. Intracellular GC action is determined by both 11-beta hydroxysteroid dehydrogenase type 1 (11betaHSD1) and the GC receptor (GR). We hypothesized that exercise training would increase 11betaHSD1 and GR protein in adipose tissue, resulting in increased lipolysis. To test the effects of exercise on adipose tissue GR and 11betaHSD1 protein, 2 sets of hamsters were trained for 6 weeks: young, diet-induced obese animals and older, overweight animals. Young (6 week old) hamsters, fructose-fed to induce an obese phenotype, and older (6 month old) hamsters were randomly divided into exercising and sedentary groups. Exercise training decreased adipose tissue mass in both fructose-fed and older hamsters. In addition, exercise training increased 11betaHSD1 (31.5% +/- 15% and 20.0% +/- 7%, fructose-fed and older, respectively) and GR (45.6% +/- 14% and 61.1% +/- 27%, fructose-fed and older, respectively) protein expression in the perirenal adipose depot and increased 11betaHSD1 (16.7% +/- 7%, P = .09) and GR (47.4% +/- 19%, P < .05) in the subcutaneous adipose depot of the older hamsters. To determine the metabolic effect of increased GC exposure in adipocytes, 3T3-L1 adipocytes were treated with corticosterone for 24 hours; and measures of lipolytic rates were conducted. Low concentrations of GCs (0.01-0.1 micromol/L) increased GR (44.1% +/- 18%, P < .05) and 11betaHSD1 (95.3% +/- 24%) protein expression, as well as lipolytic rates (34.6% +/- 6%) as measured by glycerol release. The increased lipolysis was blocked by RU486, a GR antagonist, suggesting that the elevated lipolysis was a direct result of GC action. These results suggest that exercise training amplifies the activity of GCs in adipose tissue of overweight animals through alterations in 11betaHSD1 and GR despite differences in age and amounts of adiposity. In vitro, GCs are capable of increasing lipolysis, but depend upon the presence of GR. We propose that GCs play a significant role in changing the phenotype of adipose tissue during exercise training, resulting in decreased fat mass.

Prolonged AICAR-induced AMP-kinase activation promotes energy dissipation in white adipocytes: novel mechanisms integrating HSL and ATGL.

This study was designed to investigate the effects of prolonged activation of AMP-activated protein kinase (AMPK) on lipid partitioning and the potential molecular mechanisms involved in these processes in white adipose tissue (WAT). Rat epididymal adipocytes were incubated with 5'-aminoimidasole-4-carboxamide-1-beta-d-ribofuranoside (AICAR;0.5 mM) for 15 h. Also, epididymal adipocytes were isolated 15 h after AICAR was injected (i.p. 0.7 g/kg body weight) in rats. Adipocytes were utilized for various metabolic assays and for determination of gene expression and protein content. Time-dependent in vivo plasma NEFA concentrations were determined. AICAR treatment significantly increased AMPK activation, inhibited lipogenesis, and increased FA oxidation. This was accompanied by upregulation of peroxisome proliferator-activated receptor (PPAR)alpha, PPARdelta, and PPARgamma-coactivator-1alpha (PGC-1alpha) mRNA levels. Lipolysis was first suppressed, but then increased, both in vitro and in vivo, with prolonged AICAR treatment. Exposure to AICAR increased adipose triglyceride lipase (ATGL) content and FA release, despite inhibition of basal and epinephrine-stimulated hormone-sensitive lipase (HSL) activity. Here, we provide evidence that prolonged AICAR-induced AMPK activation can remodel adipocyte metabolism by upregulating pathways that favor energy dissipation versus lipid storage in WAT. Additionally, we show novel time-dependent effects of AICAR-induced AMPK activation on lipolysis, which involves antagonistic modulation of HSL and ATGL.

Voluntary wheel running initially increases adrenal sensitivity to adrenocorticotrophic hormone, which is attenuated with long-term training.

Although exercise is a common and potent activator of the hypothalamic-pituitary adrenal (HPA) axis, the effects of exercise on the acute stress response are not well understood. Here, we investigated the effects of short- (2 wk) and long-term (8 wk) voluntary wheel running on adrenal sensitivity to ACTH stimulation and the acute stress response to restraint in male rats. Diurnal glucocorticoid patterns were measured on days 7 (all groups) and 35 (8-wk groups). Rats were subjected to 20 min of restraint stress on either week 1 or on week 7 of treatment to assess HPA activation. One week later, exogenous ACTH (75 ng/kg) was administered to assess adrenal sensitivity to ACTH. Following this, adrenals were collected and analyzed for key proteins involved in corticosterone (CORT) synthesis. By the end of week 1, exercising (E) animals had twofold higher peak diurnal CORT levels compared with sedentary (S) animals (P < 0.01). CORT values were not different between groups at week 8. In response to restraint stress at week 2, CORT values in E were approximately threefold greater than in S (P < 0.05). No difference was found between E and S rats in the response to, or recovery from, restraint at week 8. During the ACTH challenge at week 2, E demonstrated a approximately 2.5-fold increase in adrenal sensitivity compared with S, while no difference was found between E and S at week 8. The expression of steroidogenic acute regulatory protein was found to be approximately 50% higher in the adrenals in E compared with S at week 2 (P < 0.05), but no difference existed between groups at week 8. These results show that volitional wheel running initially causes hyperactivation of the HPA axis, due to enhanced adrenal sensitivity to ACTH, but that these alterations in HPA activity are completely restored by 8 wk of training.

Attenuation of type 2 diabetes mellitus in the male Zucker diabetic fatty rat: the effects of stress and non-volitional exercise.

To date, a limited number of studies have investigated the effects of exercise on the maintenance of endocrine pancreatic adaptations to worsening insulin resistance. In particular, the roles of stress hormones that are associated with commonly used forced-exercise paradigms are not fully explained. To examine the effects of exercise per se in ameliorating pancreatic decompensation over time, we investigated the role of forced swimming and sham exercise stress on the development of type 2 diabetes mellitus in the Zucker diabetic fatty (ZDF) rat. Thirty-two male ZDF rats were obtained at 5 weeks of age and all went through a 1-week acclimatization period. They were then divided into 4 groups: basal (euthanized at 6 weeks of age), exercise (1 h/d; 5 d/wk), sham exercise (sham), and non-treated controls (n = 8 per group). After 6 weeks of treatment, an intraperitoneal glucose tolerance test was performed and animals were euthanized for tissue analysis. By 5 weeks of treatment, controls had elevated fed and fasted glycemia (>11.1 and 7.1 mmol/L, respectively; both P < .05), whereas exercise and sham rats remained euglycemic. At euthanasia, there were elevations in fed insulin levels in exercise and sham rats compared with basal animals (both P < .05). Despite improvements in fed and fasting glucose levels in sham rats, glucose tolerance in sham-treated rats (intraperitoneal glucose tolerance test) was similar to controls, whereas glucose levels were similar in exercised trained and basal rats. After 6 weeks, gastrocnemius glycogen content was higher in exercised rats and sham rats when compared with age-matched controls, whereas muscle glucose transporter 4 levels were similar between groups. Compared with controls, the exercise group had increased beta cell proliferation, beta cell mass, and partial maintenance of normal islet morphology. Sham rats also displayed beta cell compensation, as evidenced by increased fasting insulin levels and partial preservation of normal islet morphology. Finally, at the time of euthanasia, plasma corticosterone was increased in sham and control rats but was at basal levels in the exercise group. In summary, both exercise and sham treatment delay the progression of type 2 diabetes mellitus in the male ZDF rat by distinct mechanisms related to pancreatic function and improvements in peripheral glucose disposal.

5-Aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside-induced AMP-activated protein kinase phosphorylation inhibits basal and insulin-stimulated glucose uptake, lipid synthesis, and fatty acid oxidation in isolated rat adipocytes.

The objective of this study was to investigate the effects of 5-aminoimidazole-4-carboxamide-1-beta-D-ribofuranoside (AICAR)-induced AMP-activated protein kinase (AMPK) activation on basal and insulin-stimulated glucose and fatty acid metabolism in isolated rat adipocytes. AICAR-induced AMPK activation profoundly inhibited basal and insulin-stimulated glucose uptake, lipogenesis, glucose oxidation, and lactate production in fat cells. We also describe the novel findings that AICAR-induced AMPK phosphorylation significantly reduced palmitate (32%) and oleate uptake (41%), which was followed by a 50% reduction in palmitate oxidation despite a marked increase in AMPK and acetyl-CoA carboxylase phosphorylation. Compound C, a selective inhibitor of AMPK, not only completely prevented the inhibitory effect of AICAR on palmitate oxidation but actually caused a 2.2-fold increase in this variable. Compound C also significantly increased palmitate oxidation in the presence of inhibitory concentrations of malonyl-CoA and etomoxir indicating an increase in CPT1 activity. In contrast to skeletal muscle in which AMPK stimulates fatty acid oxidation to provide ATP as a fuel, we propose that AMPK activation inhibits lipogenesis and fatty acid oxidation in adipocytes. Inhibition of lipogenesis would conserve ATP under conditions of cellular stress, although suppression of intra-adipocyte oxidation would spare fatty acids for exportation to other tissues where their utilization is crucial for energy production. Additionally, the stimulatory effect of compound C on long chain fatty acid oxidation provides a novel pharmacological approach to promote energy dissipation in adipocytes, which may be of therapeutic importance for obesity and type II diabetes.

Metabolic effects of voluntary wheel running in young and old Syrian golden hamsters.

To explore the metabolic effects of high volume wheel running in the Syrian golden hamster, 6-week old (YOUNG) and 6-month old (OLD) male animals were randomly divided into sedentary (i.e., YOUNG-S or OLD-S) or running wheel (i.e., YOUNG-RW or OLD-RW) groups (n = 8/group). RW groups had 24-h access to activity wheels while S were housed in standard rodent cages. At the start of wheel exposure, the number of revolutions were similar in both groups, but by day 15 were nearly two-fold higher in the YOUNG vs. OLD. OLD ate more than YOUNG and wheel running increased food intake by approximately 50%. YOUNG-RW maintained the same total body mass as YOUNG-S, while OLD-RW had a transient weight loss of approximately 10 g. Perirenal fat mass was smaller in YOUNG- and OLD-RW groups compared with S groups (45% and 66%, respectively. Plantaris muscle cytochrome c oxidase activity was also approximately 2-fold higher in YOUNG-RW than in YOUNG-S hamsters but was similar between OLD-RW and OLD-S groups. Plasma leptin levels were approximately 60% lower in YOUNG-RW compared with YOUNG-S and correlated significantly with visceral fat pad mass (r2 = 0.58, p = 0.001). Corticosterone levels were lower in YOUNG-RW (13.0 +/- 0.36 ng/ml) than in YOUNG-S (16.4 +/- 0.83 ng/ml) hamsters and higher in OLD-RW (22.62 +/- 0.47 ng/ml) than in OLD-S (15.54 +/- 0.13 ng/ml) hamsters. These observations reveal that the hamster is a suitable model for accelerating the effects of exercise on body composition and metabolic alterations associated with training and that the training adaptations are more pronounced in younger compared with older hamsters, possibly as a result of the higher voluntary wheel activity in the former group.

Effect of voluntary wheel running on circadian corticosterone release and on HPA axis responsiveness to restraint stress in Sprague-Dawley rats.

Adaptations of the hypothalamic-pituitary-adrenal (HPA) axis to voluntary exercise in rodents are not clear, because most investigations use forced-exercise protocols, which are associated with psychological stress. In the present study, we examined the effects of voluntary wheel running on the circadian corticosterone (Cort) rhythm as well as HPA axis responsiveness to, and recovery from, restraint stress. Male Sprague-Dawley rats were divided into exercise (E) and sedentary (S) groups, with E rats having 24-h access to running wheels for 5 wk. Circadian plasma Cort levels were measured at the end of each week, except for week 5 when rats were exposed to 20 min of restraint stress, followed by 95 min of recovery. Measurements of glucocorticoid receptor content in the hippocampus and anterior pituitary were performed using Western blotting at the termination of the restraint protocol. In week 1, circadian Cort levels were twofold higher in E compared with S animals, but the levels progressively decreased in the E group throughout the training protocol to reach similar values observed in S by week 4. During restraint stress and recovery, Cort values were similar between E and S, as was glucocorticoid receptor content in the hippocampus and pituitary gland after death. Compared with E, S animals had higher plasma ACTH levels during restraint. Taken together, these data indicate that 5 wk of wheel running are associated with normal circadian Cort activity and normal negative-feedback inhibition of the HPA axis, as well as with increased adrenal sensitivity to ACTH after restraint stress.

Effect of voluntary exercise on peripheral tissue glucocorticoid receptor content and the expression and activity of 11beta-HSD1 in the Syrian hamster.

Recent findings indicate that elevated levels of glucocorticoids (GC), governed by the expression of 11beta-hydroxysteroid dehydrogenase type 1 (11beta-HSD1) and GC receptors (GR), in visceral adipose tissue and skeletal muscle lead to increased insulin resistance and the metabolic syndrome. Paradoxically, evidence indicates that aerobic exercise attenuates the development of the metabolic syndrome even though it stimulates acute increases in circulating GC levels. To investigate the hypothesis that training alters peripheral GC action to maintain insulin sensitivity, young male hamsters were randomly divided into sedentary (S) and trained (T) groups (n = 8 in each). The T group had 24-h access to running wheels over 4 wk of study. In muscle, T hamsters had lower 11beta-HSD1 protein expression (19.2 +/- 1.40 vs. 22.2 +/- 0.96 optical density, P < 0.05), similar 11beta-HSD1 enzyme activity (0.9 +/- 0.27% vs. 1.1 +/- 0.26), and lower GR protein expression (9.7 +/- 1.86 vs. 15.1 +/- 1.78 optical density, P < 0.01) than S hamsters. In liver, 11beta-HSD1 protein expression tended to be lower in T compared with S (19.2 +/- 0.56 vs. 21.4 +/- 1.05, P = 0.07), whereas both enzyme activity and GR protein expression were similar. In contrast, visceral adipose tissue contained approximately 2.7-fold higher 11beta-HSD1 enzyme activity in T compared with S (12.9 +/- 3.3 vs. 4.8 +/- 1.5% conversion, P < 0.05) but was considerably smaller in mass (0.24 +/- 0.02 vs. 0.71 +/- 0.06 g). Thus the intracellular adaptation of GC regulators to exercise is tissue specific, resulting in decreases in GC action in skeletal muscle and increases in GC action in visceral fat. These adaptations may have important implications in explaining the protective effects of aerobic exercise on insulin resistance and other symptoms of the metabolic syndrome.