Effects of fasting on hepatic and peripheral glucose metabolism in conscious rats with near-total fat depletion

Effects of chronic spinal cord injury on body weight and body composition in rats fed a standard chow diet

The inability to maintain body weight within prescribed ranges occurs in a significant portion of the human spinal cord injury (SCI) population. Using a rodent model of long-term high thoracic (spinal level T3) spinal cord transection (TX), we aimed to identify derangements in body weight, body composition, plasma insulin, glucose tolerance, and metabolic function, as measured by uncoupling protein 1 (UCP1) expression in interscapular brown adipose tissue (IBAT). Sixteen weeks after SCI, body weights of injured female rats stabilized and were significantly lower than surgical control animals. At the same time point, SCI rats had a significantly lower whole body fat:lean tissue mass ratio than controls, as measured indirectly by NMR. Despite lower body weight and fat mass, the cumulative consumption of standard laboratory chow (4.0 kcal/g) and mean energy intake (kcal.day(-1).100 g body wt(-1)) of chronic SCI rats was significantly more than controls. Glucose tolerance tests indicated a significant enhancement in glucose handling in 16-wk SCI rats, which were coupled with lower serum insulin levels. The post mortem weight of gonadal and retroperitoneal fat pads was significantly reduced after SCI and IBAT displayed significantly lower real-time PCR expression of UCP1 mRNA. The reduced fat mass and IBAT UCP1 mRNA expression are contraindicative of the cumulative caloric intake by the SCI rats. The prolonged postinjury loss of body weight, including fat mass, is not due to hypophagia but possibly to permanent changes in gastrointestinal transit and absorption, as well as whole body homeostatic mechanisms.

Antisense oligonucleotides targeted against glucocorticoid receptor reduce hepatic glucose production and ameliorate hyperglycemia in diabetic mice

Abstract Specific blockade of glucocorticoid receptor (GCCR) action in the liver without affecting the hypothalamus-pituitary-adrenal axis could be a novel pharmaceutical approach to treat type 2 diabetes. In the present study, we applied an antisense oligonucleotide (ASO) against GCCR (ASO-GCCR) to reduce the expression of liver GCCR and examined its impact on the diabetic syndrome in ob / ob and db / db mice. A 3-week treatment regimen of ASO-GCCR (25 mg/kg IP, twice per week) markedly reduced liver GCCR messenger RNA and protein expression with no alteration of GCCR messenger RNA expression in the hypothalamus, pituitary, or adrenal gland. The ASO-GCCR treatment lowered blood glucose levels by 45% and 23% in ob / ob and db / db mice, respectively, compared with those observed in the control group. The ASO-GCCR-treated mice also showed significant enhancement of insulin-mediated inhibition of hepatic glucose production during a euglycemic-hyperinsulinemic clamp as well as marked reduction of phosphoenolpyruvate carboxykinase and glucose 6-phosphatase activity compared with control mice. The ASO-GCCR treatment did not change peripheral insulin sensitivity during the clamp. The ob / ob mice treated with ASO-GCCR had no significant difference in the plasma corticosterone and corticotropin levels compared with control mice. Lean mice receiving a similar treatment regimen of ASO-GCCR exhibited no change in blood glucose levels, oral glucose tolerance tests, or insulin tolerance tests. Our results demonstrate that selective inhibition of GCCR expression in the liver by the ASO-GCCR treatment reduced hepatic glucose production and improved blood glucose control under diabetic conditions.

Microdialysis and 2-[18F]fluoro-2-deoxy-D-glucose (FDG): a study on insulin action on FDG transport, uptake and metabolism in rat muscle, liver and adipose tissue

A combination of microdialysis (MD) and 2-[18F ]fluoro-2-deoxy-D-glucose (FDG) was used to assess FDG uptake, phosphorylation and the glucose metabolic index (Rg') in certain tissues of fed and fasting anesthetized Sprague-Dawley rats which received an i.v. bolus injection of insulin or saline during the course of the study. The relative recovery for FDG for the MD probes was also measured as a function of flow rate and temperature. The elimination half-life (T(1/2 FDG)) of FDG from the plasma and the extracellular fluid of muscle and liver was studied with MD. The phosphorylation of FDG in muscle, liver, subcutaneous fat and mesenteric fat from homogenates of these tissues was analyzed by a radioHPLC-method and the Rg' was calculated. The results show that the nutritional status does not affect the T(1/2 FDG), the total uptake of FDG 6-phosphate or the Rg' values in the studied tissues at ambient glucose. Insulin stimulation decreased T(1/2 FDG), and increased the total FDG 6-P accumulation and Rg' in the muscle of fed and fasted rats. In adipose tissues the insulin stimulation enhanced the phosphorylation but in muscle the proportion of FDG 6-P remained unchanged. Rg' in adipose tissue was higher after insulin administration in fed rats than without insulin but with fasted rats there were no differences in Rg' values with or without insulin, although the proportion of FDG 6-P did increase. The Rg' values for the livers were unaffected by any of the manipulations, but fasted rats accumulated proportionately more FDG 6-P after insulin administration than did fed rats. These results indicate that the combination of MD and FDG is a valuable and reliable tool when studying glucose metabolism in physiological and pathological models in vivo.

Mechanisms by which liver-specific PEPCK knockout mice preserve euglycemia during starvation

Liver-specific PEPCK knockout mice, which are viable despite markedly abnormal lipid metabolism, exhibit mild hyperglycemia in response to fasting. We used isotopic tracer methods, biochemical measurements, and nuclear magnetic resonance spectroscopy to show that in mice lacking hepatic PEPCK, 1) whole-body glucose turnover is only slightly decreased; 2) whole-body gluconeogenesis from phosphoenolpyruvate, but not from glycerol, is moderately decreased; 3) tricarboxylic acid cycle activity is globally increased, even though pyruvate cycling and anaplerosis are decreased; 4) the liver is unable to synthesize glucose from lactate/pyruvate and produces only a minimal amount of glucose; and 5) glycogen synthesis in both the liver and muscle is impaired. Thus, although mice without hepatic PEPCK have markedly impaired hepatic gluconeogenesis, they are able to maintain a near-normal blood glucose concentration while fasting by increasing extrahepatic gluconeogenesis coupled with diminishing whole-body glucose utilization.

Differential effects of rexinoids and thiazolidinediones on metabolic gene expression in diabetic rodents

Both retinoid X receptor (RXR)-selective agonists (rexinoids) and thiazolidinediones (TZDs), PPAR (peroxisome proliferator-activated receptor)-gamma-specific ligands, produce insulin sensitization in diabetic rodents. In vitro studies have demonstrated that TZDs mediate their effects via the RXR/PPAR-gamma complex. To determine whether rexinoids lower hyperglycemia by activating the RXR/PPAR-gamma heterodimer in vivo, we compared the effects of a rexinoid (LG100268) and a TZD (rosiglitazone) on gene expression in white adipose tissue, skeletal muscle, and liver of Zucker diabetic fatty rats (ZDFs). In adipose tissue, rosiglitazone decreased tumor necrosis factor-alpha (TNF-alpha) mRNA and induced glucose transporter 4 (GLUT4), muscle carnitine palmitoyl-transferase (MCPT), stearoyl CoA desaturase (SCD1), and fatty acid translocase (CD36). In contrast, LG100268 increased TNF-alpha and had no effect or suppressed the expression of GLUT4, MCPT, SCD1, and CD36. In liver, the rexinoid increased MCPT, SCD1, and CD36 mRNAs, whereas rosiglitazone induced only a small increase in CD36. In skeletal muscle, rosiglitazone and LG100268 have similar effects; both increased SCD1 and CD36 mRNAs. The differences in the pattern of genes induced by the rexinoids and the TZDs in diabetic animals found in these studies suggests that these compounds may have independent and tissue-specific effects on metabolic control in vivo.

Regulation of the glucose-6-phosphatase gene by glucose occurs by transcriptional and post-transcriptional mechanisms. Differential effect of glucose and xylitol

To understand how glucose regulates the expression of the glucose-6-phosphatase gene, the effect of glucose was studied in primary cultures of rat hepatocytes. Glucose-6-phosphatase mRNA levels increased about 10-fold when hepatocytes were incubated with 20 mm glucose. The rate of transcription of the glucose-6-phosphatase gene increased about 3-fold in hepatocytes incubated with glucose. The half-life of glucose-6-phosphatase mRNA was estimated to be 90 min in the absence of glucose and 3 h in its presence. Inhibition of the oxidative and the nonoxidative branches of the pentose phosphate pathway blocked the stimulation of glucose-6-phosphatase expression by glucose but not by xylitol or carbohydrates that enter the glycolytic/gluconeogenic pathways at the level of the triose phosphates. These results indicate that (i) the glucose induction of the mRNA for the catalytic unit of glucose-6-phosphatase occurs by transcriptional and post-transcriptional mechanisms and that (ii) xylitol and glucose increase the expression of this gene through different signaling pathways.

Role of glucose and insulin in thiazolidinedione-induced alterations in hepatic gluconeogenesis

Previous studies from our laboratory as well as from others have suggested that the thiazolidinediones have the capacity to act as insulinomimetic agents, especially in the liver. In order to further characterize this insulinomimetic action, we evaluated the effect of troglitazone, a representative thiazolidinedione, on lactate- and glucagon-stimulated gluconeogenesis, in the presence or absence of insulin (10 nM) in isolated rat hepatocytes. The antigluconeogenic effect of troglitazone under basal (lactate-stimulated) conditions was found to be due to an elevation in the fructose 2,6-bisphosphate content, which was, however, not mediated by an activation of 6-phosphofructo 2-kinase. Troglitazone (125 and 250 microM) in the absence of insulin, produced a dose-dependent reduction in glucagon-stimulated gluconeogenesis, thereby suggesting an insulinomimetic effect. In addition, troglitazone (125 and 250 microM), in combination with insulin, produced an additive inhibition of gluconeogenesis during glucagon-stimulated conditions. However, unlike insulin, the metabolic mechanism responsible for these effects (in the presence or absence of insulin) does not involve fructose 2,6-bisphosphate.

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.

Decreased visceral adiposity accounts for leptin effect on hepatic but not peripheral insulin action

Leptin decreases visceral fat (VF) and increases peripheral and hepatic insulin action. Here, we generated similar decreases in VF using leptin (Lep), beta(3)-adrenoreceptor agonism (beta3), or food restriction (FR) and asked whether insulin action would be equally improved. For 8 days before the in vivo study, Sprague-Dawley rats (n = 24) were either fed ad libitum [control (Con)], treated with Lep or beta3 (CL-316,243) by implanted osmotic mini-pumps, or treated with FR. Total VF was similarly decreased in the latter three groups (Lep, 3.11 +/- 0.96 g; beta3, 2.87 +/- 0.48 g; and FR, 3.54 +/- 0.77 g compared with 6.91 +/- 1.41 g in Con; P < 0.001) independent of total fat mass (by (3)H(2)O) and food intake. Insulin (3 mU. kg(-1). min(-1)) clamp studies were performed to assess hepatic and peripheral insulin sensitivity. Decreased VF resulted in similar and marked improvements in insulin action on glucose production (GP) (Lep, 1.19 +/- 0.51; beta3, 1.46 +/- 0.68; FR, 2.27 +/-0.71 compared with 6.06 +/- 0.70 mg. kg(-1). min(-1) in Con; P < 0.001). By contrast, reduction in VF by beta3 and FR failed to reproduce the stimulation of insulin-mediated glucose uptake ( approximately 60%), glycogen synthesis ( approximately 80%), and glycolysis ( approximately 25%) observed with Lep. We conclude that 1) a moderate decrease in VF uniformly leads to a marked increase in hepatic insulin action, but 2) the effects of leptin on peripheral insulin action are not due to the associated changes in VF or beta3 activation.

Glycemia-lowering effect of cobalt chloride in the diabetic rat: role of decreased gluconeogenesis

Results of previous studies indicated that treatment of diabetic rats (induced by streptozotocin) with cobalt chloride (CoCl2) resulted in a significant decrement in serum glucose concentration. The present study was designed to determine the potential role of enhanced glucose uptake vs. decreased glucose production in the above response. The rate of systemic appearance of glucose, measured under fasting conditions using [3-3H]glucose tracer, was reduced from 35.5 +/- 2.5 to 17.5 +/- 1.8 micromol . kg-1 . min-1 in diabetic rats treated with 2 mM CoCl2 added to the drinking water for 10-14 days (P < 0.01). Tissue accumulation of intravenously administered 2-deoxy-[14C]glucose was significantly reduced in kidney and eye of diabetic rats treated with CoCl2, whereas the uptake remained unchanged in several other tissues including cerebrum, red and white skeletal muscle, heart, and liver. The relative content of phosphoenolpyruvate carboxykinase (PEPCK) mRNA was increased 3.1-fold in livers of diabetic compared with normal rats (P < 0.001), and treatment of diabetic rats with CoCl2 decreased hepatic PEPCK mRNA levels to normal. The content of PEPCK mRNA in the liver was decreased by 33% in CoCl2-treated normal rats (P < 0.05). Treatment with CoCl2 resulted in no change in cAMP levels in the livers of either diabetic or normal rats. These results suggest that the glycemia-lowering effect of CoCl2 is mediated by reductions in the rate of systemic appearance of glucose and hepatic gluconeogenesis.

Carbon flux via the pentose phosphate pathway regulates the hepatic expression of the glucose-6-phosphatase and phosphoenolpyruvate carboxykinase genes in conscious rats

Hepatic gene expression of P-enolpyruvate carboxykinase (PEPCK) and glucose-6-phosphatase (Glc-6-Pase) is regulated in response to changes in the availability of substrates, in particular glucose (Glc; Massillon, D., Barzilai, N., Chen, W., Hu, M., and Rossetti, L. (1996) J. Biol. Chem. 271, 9871-9874). We investigated the mechanism(s) in conscious rats. Hyperglycemia per se caused a rapid and marked increase in Glc-6-Pase mRNA abundance and protein levels. By contrast, hyperglycemia decreased the abundance of PEPCK mRNA. Importantly, inhibition of glucokinase activity by glucosamine infusion blunted both the stimulation of Glc-6-Pase and the inhibition of PEPCK gene expression by Glc, suggesting that an intrahepatic signal (metabolite) generated by the metabolism of glucose at or beyond Glc-6-P was responsible for the regulatory effect of Glc. The effect of Glc on the L-type pyruvate kinase gene is mediated by xylulose-5-P (Doiron, B., Cuif, M., Chen, R., and Kahn, A. (1996) J. Biol. Chem. 271, 5321-5324). Thus, we next investigated whether an isolated increase in the hepatic concentration of this metabolite can also reproduce the effects of Glc on Glc-6-Pase and PEPCK gene expression in vivo. Xylitol, which is directly converted to xylulose-5-P in the liver, was infused to raise the hepatic concentration of xylulose-5-P by approximately 3-fold. Xylitol infusion did not alter the levels of Glc-6-P and of fructose-2,6-biphosphate. However, it replicated the effects of hyperglycemia on Glc-6-Pase and PEPCK gene expression and resulted in a 75% increase in the in vivo flux through Glc-6-Pase (total glucose output).

Leptin selectively decreases visceral adiposity and enhances insulin action

Intraabdominal adiposity and insulin resistance are risk factors for diabetes mellitus, dyslipidemia, arteriosclerosis, and mortality. Leptin, a fat-derived protein encoded by the ob gene, has been postulated to be a sensor of energy storage in adipose tissue capable of mediating a feedback signal to sites involved in the regulation of energy homeostasis. Here, we provide evidence for specific effects of leptin on fat distribution and in vivo insulin action. Leptin (LEP) or vehicle (CON) was administered by osmotic minipumps for 8 d to pair-fed adult rats. During the 8 d of the study, body weight and total fat mass decreased similarly in LEP and in CON. However, while moderate calorie restriction (CON) resulted in similar decreases in whole body (by 20%) and visceral (by 21%) fat, leptin administration led to a specific and marked decrease (by 62%) in visceral adiposity. During physiologic hyperinsulinemia (insulin clamp), leptin markedly enhanced insulin action on both inhibition of hepatic glucose production and stimulation of glucose uptake. Finally, leptin exerted complex effects on the hepatic gene expression of key metabolic enzymes and on the intrahepatic partitioning of metabolic fluxes, which are likely to represent a defense against excessive storage of energy in adipose depots. These studies demonstrate novel actions of circulating leptin in the regulation of fat distribution, insulin action, and hepatic gene expression and suggest that it may play a role in the pathophysiology of abdominal obesity and insulin resistance.

Short term effects of leptin on hepatic gluconeogenesis and in vivo insulin action

Long term administration of leptin decreases caloric intake and fat mass and improves glucose tolerance. Here we examine whether leptin acutely regulates peripheral and hepatic insulin action. Recombinant mouse leptin (0.3 mg/kg.h, Leptin +) or vehicle (Leptin -) were administered for 6 h to 4-month-old rats (n = 20), and insulin (3 milliunits/kg.min) clamp studies were performed. During physiologic hyperinsulinemia (plasma insulin approximately 65 microunits/ml), the rates of whole body glucose uptake, glycolysis, and glycogen synthesis and the rates of 2-deoxyglucose uptake in individual tissues were similar in Leptin - and Leptin +. Post-absorptive hepatic glucose production (HGP) was similar in the two groups. However, leptin enhanced insulin's inhibition of HGP (4.1 +/- 0.7 and 6.2 +/- 0.7 mg/kg.min; p < 0.05). The decreased HGP in the Leptin + group was due to a marked suppression of hepatic glycogenolysis (0.7 +/- 0.1 versus 4.1 +/- 0.6 mg/kg.min, in Leptin + versus Leptin -, respectively; p < 0.001), whereas the % contribution of gluconeogenesis to HGP was markedly increased (82 +/- 3% versus 36 +/- 4% in Leptin + and Leptin -, respectively; p < 0.001). At the end of the 6-h leptin infusion, the hepatic abundance of glucokinase mRNA was decreased, whereas that of phosphoenolpyruvate carboxykinase mRNA was increased compared with Leptin -. We conclude that an acute increase in plasma leptin 1) enhances insulin's ability to inhibit HGP, 2) does not affect peripheral insulin action, and 3) induces a redistribution of intrahepatic glucose fluxes and changes in the gene expression of hepatic enzymes that closely resemble those of fasting.

Glucoregulation during progressive starvation in late pregnancy in the rat

The response of glucose utilization (transport and phosphorylation) by individual skeletal muscles to progressive starvation in late pregnancy in the rat was investigated in relation to changes in whole body glucose turnover. Compared with insulin-stimulated values, the decline in muscle glucose utilization evoked by short-term (6-h) starvation was about twofold greater in pregnancy. Suppression of glucose utilization by slow-twitch muscles was observed as the starvation period was extended from 6 to 24 h only in unmated rats. Extending starvation to 24 h did not further reduce glucose utilization by fast-twitch skeletal muscles in either group. Suppression of whole body glucose disposal was observed between 6 and 24 h of starvation in unmated, but not pregnant, rats. The results demonstrate that metabolic adaptation of almost complete suppression of glucose utilization by slow-twitch muscle, normally elicited only by prolonged (24-h) starvation, is already established after acute (6-h) starvation in late pregnancy. The present study supports the concept of "accelerated starvation" in late pregnancy with respect to muscle glucose utilization after short-term food withdrawal but demonstrates that further glucose conservation cannot be achieved after more prolonged starvation.

Glucose-6-phosphatase flux and the hepatic glucose balance model

Mice were studied with the euglycemic hyperinsulinemic and the hyperglycemic clamp techniques after a 6-h fast: 1) euglycemic (6.7 +/- 0.2 mM) hyperinsulinemia (approximately 800 microU/ml); 2) hyperglycemic (15.3 +/- 0.4 mM) hyperinsulinemia (approximately 800 microU/ml). All mice received an infusion of [3-3H]glucose and [U-14C]lactate. Basal hepatic glucose production (HGP) averaged approximately 170 mumol.kg-1.min-1 in both groups. During euglycemic and hyperglycemic hyperinsulinemia, HGP decreased by 53% (to 76.7 +/- 11.1 mumol.kg-1.min-1; P < 0.01) and 74% (to 43.3 +/- 7.2 mumol.kg-1.min-1; P < 0.01), respectively. Hyperglycemia increased glucose cycling (by 2.1-fold; P < 0.01) and the contribution of gluconeogenesis to HGP (88 vs. 43%; P < 0.01) while decreasing that of glycogenolysis (12 vs. 57%; P < 0.01). The percentage of neosynthetized hepatic glycogen formed via the direct pathway was markedly increased during hyperglycemia (53 +/- 2% vs. 23 +/- 3%; P < 0.01). These data indicate that the assessment of hepatic glucose fluxes can be accomplished in conscious unrestrained mice and that, in the presence of hyperinsulinemia, hyperglycemia causes 1) a further inhibition of HGP mainly via inhibition of glycogenolysis and increase in hepatic glucose cycling; and 2) about a fivefold stimulation in the direct pathway of hepatic glycogen formation.

Age-related changes in body composition are associated with hepatic insulin resistance in conscious rats

Age-dependent changes in body composition and hepatic ([3H]glucose) glucose metabolism were examined in 2-, 4-, and 14-mo-old (n = 26) conscious Sprague-Dawley rats. Hepatic glucose production (HGP) and hepatic glucose-6-phosphatase maximum velocity were decreased 18 and 30%, respectively, between 2 and 4 mo but were unchanged with further aging. However, between 4 and 14 mo, twofold higher plasma insulin levels were required to maintain similar HGP, suggesting that hepatic insulin resistance develops with age. Utilizing hepatic-pancreatic clamp technique, we showed that a much higher rate of insulin infusion (1.6 +/- 0.1 vs. 0.8 +/- 0.1 mU.kg-1.min-1) was needed to achieve similar plasma glucose levels and HGP. Furthermore, when 4-mo-old rats were infused with insulin at similar rates as the 14-mo-old rats, HGP was decreased by approximately 30%. Because hepatic insulin sensitivity was inversely related to the increase in body weight (r2 = 0.876) and free fatty acid levels (r2 = 0.843), we suggest that age-related changes in body composition may lead to the impairment of hepatic glucose metabolism.