Glucose utilization by interscapular brown adipose tissue in vivo during nutritional transitions in the rat

Effects of Syzygium aromaticum-derived oleanolic acid on glucose transport and glycogen synthesis in the rat small intestine

BACKGROUND

In the present study, we investigated the effects of oleanolic acid (OA), which has hypoglycemic properties, on glucose transport and glycogen synthesis in the small intestine, an organ that secretes enzymes involved in carbohydrate metabolism.

METHODS

The OA was isolated from Syzygium aromaticum ethyl acetate-soluble fractions followed by recrystallization with ethanol. It was diluted to required concentrations in freshly prepared dimethyl sulfoxide (2 mL) and normal saline (19 mL) before being administered to rats (p.o.). Glycogen concentrations were determined in isolated small intestines from fasted and non-fasted non-diabetic and streptozotocin-diabetic rats after 18 h treatment with 80 mg/kg, p.o., OA or standard hypoglycemic drugs (i.e. 100 μg/kg, s.c., insulin; 500 mg/kg, p.o., metformin). In a separate series of experiments, the effects of 30-min incubation with graded concentrations of OA (0.82-6.56 mmol/L) on d-glucose were evaluated by monitoring changes in glucose concentrations inside and outside of intestinal sacs isolated from fasted, non-diabetic rats and mounted in an organ bath containing Krebs-Henseleit bicarbonate buffer.

RESULTS

All in vivo treatments increased the glycogen concentration in rat small intestine, although the effects of metformin treatment in non-fasted diabetic rats failed to reach statistical significance. In vitro, both OA (1.64-6.56 mmol/L) and phlorizin (10(-5) -10(-3) mol/L) decreased glucose transport from the mucosa to the serosa.

CONCLUSION

The data suggest that OA may be a potential alternative drug treatment for postprandial hyperglycemia because of its inhibition of glucose uptake across the small intestine and its concomitant conversion of glucose to glycogen in the intestinal wall.

Effects of burn injury, cold stress and cutaneous wound injury on the morphology and energy metabolism of murine brown adipose tissue (BAT) in vivo

AIMS

Cold stress has been shown to produce dramatic increases in 2-fluoro-2-deoxy-D-Glucose ((18)FDG) accumulation by brown adipose tissue (BAT) in rodents. However, neither the effects of other types of stress on (18)FDG accumulation nor the effects of stressors on the accumulation of tracers of other aspects of energy metabolism have been evaluated. In this report we studied the effects of cold stress, burn injury and cutaneous wounds on murine BAT at the macroscopic, microscopic and metabolic level.

MAIN METHODS

Glucose metabolism was studied with (18)FDG, fatty acid accumulation was evaluated with trans-9(RS)-(18)F-fluoro-3,4(RS,RS)-methyleneheptadecanoic acid (FCPHA) and tricarboxcylic acid cycle (TCA) activity was evaluated with (3)H acetate.

KEY FINDINGS

All three stressors produced dramatic changes in BAT at the macroscopic and microscopic level. Macroscopically, BAT from the stressed animals appeared to be a much darker brown in color. Microscopically BAT of stressed animals demonstrated significantly fewer lipid droplets and an overall decrease in lipid content. Accumulation of (18)FDG by BAT was significantly (p<0.01) increased by all 3 treatments (Cold: ~16 fold, burn ~7 Fold and cutaneous wound ~14 fold) whereas uptake of FDG by white fat was unchanged. This effect was also demonstrated non invasively by μPET imaging. Although less prominent than with (18)FDG, BAT uptake of FCPHA and acetate were also significantly increased by all three treatments. These findings suggest that in addition to cold stress, burn injury and cutaneous wounds produce BAT activation in mice.

SIGNIFICANCE

This study demonstrates brown fat activated by several stressors leads to increased uptake of various substrates.

Pharmacological targeting of glucagon and glucagon-like peptide 1 receptors has different effects on energy state and glucose homeostasis in diet-induced obese mice

Pharmacologic contributions of directly agonizing glucagon-like peptide 1 (GLP-1) receptor or antagonizing glucagon receptor (GCGR) on energy state and glucose homeostasis were assessed in diet-induced obese (DIO) mice. Metabolic rate and respiratory quotient (RQ), hyperglycemic clamp, stable isotope-based dynamic metabolic profiling (SiDMAP) studies of (13)C-labeled glucose during glucose tolerance test (GTT) and gene expression were assessed in cohorts of DIO mice after a single administration of GLP-1 analog [GLP-1-(23)] or anti-GCGR antibody (Ab). GLP-1-(23) and GCGR Ab similarly improved GTT. GLP-1-(23) decreased food intake and body weight trended lower. GCGR Ab modestly decreased food intake without significant effect on body weight. GLP-1-(23) and GCGR Ab decreased RQ with GLP-1, causing a greater effect. In a hyperglycemic clamp, GLP-1-(23) reduced hepatic glucose production (HGP), increased glucose infusion rate (GIR), increased glucose uptake in brown adipose tissue, and increased whole-body glucose turnover, glycolysis, and rate of glycogen synthesis. GCGR Ab slightly decreased HGP, increased GIR, and increased glucose uptake in the heart. SiDMAP showed that GLP-1-(23) and GCGR Ab increased (13)C lactate labeling from glucose, indicating that liver, muscle, and other organs were involved in the rapid disposal of glucose from plasma. GCGR Ab and GLP-1-(23) caused different changes in mRNA expression levels of glucose- and lipid metabolism-associated genes. The effect of GLP-1-(23) on energy state and glucose homeostasis was greater than GCGR Ab. Although GCGR antagonism is associated with increased circulating levels of GLP-1, most GLP-1-(23)-associated pharmacologic effects are more pronounced than GCGR Ab.

Metabolic response to glucose overload in surgical stress: energy disposal in brown adipose tissue

The metabolic response to total parenteral nutrition (TPN) overload was investigated in clinical studies of surgically stressed humans and in experimental studies of surgically stressed animals. In both studies, all non-protein calories were administered as glucose, and subjects were divided into two groups, classified according to the amount of caloric supplementation after surgical stress, i.e., either overfed or appropriate (groups C1 and C2, respectively, in the clinical study, and groups E1 and E2, respectively, in the experimental study). In the clinical study, the postoperative energy expenditure in group C1 increased from the preoperative value, becoming significantly more elevated than that in group C2. The respiratory quotient (RQ) in group C1 exceeded 1.0, representing lipogenesis from carbohydrate, and the plasma norepinephrine concentration was also higher in group C1, indicating that lipolysis was likely to have been enhanced in this milieu. These findings imply that lipogenesis and lipolysis can occur simultaneously, constituting a futile cycle, and that this activated cycle could be a reason for the increased energy expenditure associated with overfeeding after surgical stress. To gather further evidence, we evaluated the rate of lipogenesis and lipolytic activity in white and brown adipose tissue (WAT; BAT) in an experimental study of surgically stressed rats. In BAT, both lipogenesis and lipolysis were activated in group E1. The results of both studies suggest that glucose overload in surgically stressed individuals increases energy expenditure by activating a futile cycle and that BAT is more involved in this cycle than WAT.

Physiological modulation of the uptake and fate of glucose in brown adipose tissue

Glucose utilization indices (GUI values) and rates of fatty acid synthesis in interscapular brown adipose tissue (IBAT) varied during the diurnal cycle in virgin and late-pregnant rats permitted unrestricted access to food. In virgin rats, peak GUI values and lipogenic rates were observed at the end of the dark (feeding) phase, but were not sustained during the light phase. Whereas peak GUI values were comparable with those observed during re-feeding after 24 h starvation, maximum rates of IBAT fatty acid synthesis in virgin rats during the diurnal cycle were only approx. 25% of those measured during re-feeding after 24 h starvation. Despite hyperphagia, GUI values during the diurnal cycle in late-pregnant rats fed ad libitum were generally lower than those of age-matched virgin controls. The percentage of pyruvate dehydrogenase complex present in the active form (PDHa) was also significantly decreased. Suppression of GUI and PDHa was not parallelled by suppression of fatty acid synthesis. IBAT GUI values in late-pregnant rats during chow re-feeding ad libitum after 24 h starvation were only 25% of those of corresponding virgin controls, and stimulation of fatty acid synthesis was also dramatically attenuated. The suppression of IBAT GUI values after re-feeding in pregnancy was not due to depletion of GLUT 4 protein. The results are discussed in relation to the importance of glucose as a precursor for fatty acid synthesis in IBAT.

Changes in lipoprotein lipase activities in adipose tissue, heart and skeletal muscle during continuous or interrupted feeding

Lipoprotein lipase (LPL) activities in parametrial and interscapular adipose tissue, soleus and adductor longus muscles and hearts of female rats were measured during progressive starvation, chow re-feeding after 24 h starvation and throughout dark and light phases in rats permitted unrestricted access to chow. Adipose-tissue LPL activities declined by 50% after 6 h starvation and continued to fall as the starvation period was extended to 24 h. Skeletal-muscle LPL activities dramatically increased between 9 and 12 h of starvation. Cardiac LPL activities increased 2.5-fold within 6 h of starvation, reaching a maximum after 12 h of starvation. Adipose-tissue LPL activities increased rapidly within 2 h of re-feeding chow ad libitum after 24 h starvation, achieving 'fed ad libitum' values after 6 h. Oxidative-skeletal-muscle LPL activities also increased after 2 h of refeeding and exceeded 'fed ad libitum' values throughout the 6 h re-feeding period. Cardiac LPL activities remained up-regulated for the 6 h of re-feeding. Adipose-tissue LPL activities exceeded those of cardiac or skeletal muscle throughout both light and dark phases. The lowest adipose-tissue LPL activities were observed at 9 h into the light phase. In contrast, cardiac LPL activity declined throughout the dark phase, with a minimum at 9 h into the dark phase. No such variation was observed for skeletal-muscle LPL activities. A diurnal nadir in plasma triacylglycerol (TG) concentrations coincided with the peak in cardiac LPL activities. The results demonstrate that, during unrestricted feeding and re-feeding after prolonged starvation, changes in skeletal-muscle and adipose-tissue LPL activities are neither reciprocal nor co-ordinate. Regulation of cardiac LPL activity during the diurnal cycle may be an important aspect of both of cardiac fuel selection and whole-body TG metabolism.

Glucose transporter expression and glucose utilization in skeletal muscle and brown adipose tissue during starvation and re-feeding

Starvation (48 h) decreased the concentration of mRNA of the insulin-responsive glucose transporter isoform (GLUT 4) in interscapular brown adipose tissue (IBAT) (56%) and tibialis anterior (10%). Despite dramatic [7-fold (tibialis anterior) and 40-fold (IBAT)] increases in glucose utilization after 2 and 4 h of chow re-feeding, no significant changes in GLUT 4 mRNA concentration were observed in these tissues over this re-feeding period. The results exclude changes in GLUT 4 mRNA concentration in mediating the responses of glucose transport in these tissues to acute re-feeding after prolonged starvation.

Glucose disposal by skeletal muscle in response to re-feeding after progressive starvation

We investigated the extent to which increases in glucose utilization indices (GUIs) in individual skeletal muscles during chow re-feeding after 6 h, 24 h or 48 h starvation are related to the antecedent duration of starvation. Chow re-feeding after either acute or prolonged starvation led to an increase in glucose disposal by the muscle mass. Glucose intolerance after prolonged starvation was not associated with lower values of GUI in skeletal muscle. In both working and non-working muscles, the increment in GUI during the first 2 h of re-feeding was less after acute than after prolonged starvation. In non-working muscles the differential responses to re-feeding were due to higher GUI values after re-feeding rather than lower pre-prandial GUI values. Therefore the contribution of non-working muscles to glucose clearance is higher as the antecedent period of starvation is extended. Rates of glycogen deposition in non-working muscles after refeeding were similar to absolute values of GUI, and a strong relationship existed between measured GUI values and rates of glycogen deposition.