We studied the effects of thiazolidinedione treatment (rosiglitazone 1 or 10 micromol.kg(-1).day(-1) or darglitazone 1.3 micromol.kg(-1).day(-1) for 3 weeks) on lipid metabolism in obese Zucker rats. In the basal 7-h fasted state, rosiglitazone (10 micromol.kg(-1).day(-1)) and darglitazone corrected the hypertriglyceridemia by increasing plasma triglyceride (TG) clearance and decreasing hepatic TG production, as assessed using Triton WR 1339. Free fatty acid (FFA) metabolism was assessed using 3H-palmitate tracer by estimating rates of plasma FFA appearance (Ra), whole-body FFA oxidation (Rox), and tissue-specific nonoxidative FFA disposal (Rfs). Basal Ra, plasma FFA levels, and clearance were increased by both thiazolidinediones. Detailed studies were conducted with darglitazone, which under basal conditions increased Ra (+114%), Rox (+51%), and Rfs in adipose tissues. During euglycemic clamps performed at insulin levels corresponding to those observed postprandially, darglitazone increased the glucose infusion rate from 4.7 to 13.3 mg.min(-1) and, in contrast to the basal state, it decreased Ra (-67%), Rox (-84%), and Rfs in adipose tissue, muscle, and liver. We concluded that thiazolidinediones 1) ameliorate hypertriglyceridemia by lowered hepatic TG production and augmented TG clearance (two separate kinetic effects), 2) enhance insulin-mediated suppression of systemic FFA mobilization while increasing the capacity to mobilize FFA during fasting, 3) increase FFA trafficking into adipose tissue by increasing the ability of adipose tissue to take up and store FFA, and 4) enhance metabolic flexibility by improving glucoregulation under hyperinsulinemic conditions (possibly involving reduced skeletal muscle and liver exposure to fatty acids) and augmenting the capacity to utilize FFAs during fasting.

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Studies of cardiac fuel metabolism in mice have been almost exclusively conducted ex vivo. The major aim of this study was to assess in vivo plasma FFA and glucose utilization by the hearts of healthy control (db/+) and diabetic (db/db) mice, based on cardiac uptake of (R)-2-[9,10-(3)H]bromopalmitate ([3H]R-BrP) and 2-deoxy-D-[U-14C]glucose tracers. To obtain quantitative information about the evaluation of cardiac FFA utilization with [3H]R-BrP, simultaneous comparisons of [3H]R-BrP and [14C]palmitate ([14C]P) uptake were first made in isolated perfused working hearts from db/+ mice. It was found that [3H]R-BrP uptake was closely correlated with [14C]P oxidation (r2 = 0.94, P < 0.001). Then, methods for in vivo application of [3H]R-BrP and [14C]2-DG previously developed for application in the rat were specially adapted for use in the mouse. The method yields indexes of cardiac FFA utilization (R(f)*) and clearance (K(f)*), as well as glucose utilization (R(g)'). Finally, in the main part of the study, the ability of the heart to switch between FFA and glucose fuels (metabolic flexibility) was investigated by studying anesthetized, 8-h-fasted control and db/db mice in either the basal state or during glucose infusion. In control mice, glucose infusion raised plasma levels of glucose and insulin, raised R(g)' (+58%), and lowered plasma FFA level (-48%), K(f)* (-45%), and R(f)* (-70%). This apparent reciprocal regulation of glucose and FFA utilization by control hearts illustrates metabolic flexibility for substrate use. By contrast, in the db/db mice, glucose infusion raised glucose levels with no apparent influence on cardiac FFA or glucose utilization. In conclusion, tracer methodology for assessing in vivo tissue-specific plasma FFA and glucose utilization has been adapted for use in mice and reveals a profound loss of metabolic flexibility in the diabetic db/db heart, suggesting a fixed level of FFA oxidation in fasted and glucose-infused states.

explored and provide evidence supporting this concept. This includes inactivation of hormone sensitive lipase (HSL) ( 5, 6 ) and A 1 -adenosine receptor agonists ( 7 ). In addition, several other G protein-coupled receptors (GPRs) are involved in controlling adipocyte FFA release, including GPR43, GPR81, and GPR109A ( 8-10 ).The GPR109A agonist nicotinic acid (NiAc) has been used clinically ever since its antidyslipidemic effects (HDL elevation and reductions of total cholesterol, LDL-cholesterol, and TG) were discovered more than 50 years ago ( 11-15 ). Although NiAc potently lowers FFA acutely, large-scale clinical studies, with repeated oral NiAc administration, often report increased levels of fasting glycemia ( 16-19 ). NiAc has not been optimized to achieve durable and therapeutically meaningful FFA lowering. By this, we specifi cally mean reducing around-the-clock FFA area under the curve (AUC). In theory, this might be achieved by sustained NiAc exposure; however, the FFA-lowering effect seen initially appears to be lost over time despite maintained NiAc exposure (tolerance development) ( 20 ). Time-dependent loss of both FFA lowering and glucose control improvement also occur in patients with type 2 diabetes, treated with the NiAc analog acipimox ( 21,22 ). To avoid tolerance development, drug holidays are needed. However, at the end of each NiAc exposure period, there is the risk of FFA rebound (here referring to the situation where FFA overshoots pretreatment levels in connection with NiAc decline) due to the short NiAc plasma half-life ( 23 ). FFA rebound is associated with impaired glucose control ( 24, 25 ). The question of whether there might be an optimal balance between periods of continuous exposure (which would minimize rebound) and drug holidays (which would minimize tolerance) in order to achieve maximal FFA lowering has not been addressed. Lipid overload in nonadipose tissues has been linked to the pathogenesis of insulin resistance and atherogenesis ( 1-4 ). A potential means for reversing peripheral lipid overload is to restrict the release of FFAs from adipose tissues. A number of independent mechanisms have been N.D. Oakes, P. Thalén, and A. Kjellstedt Abbreviations: ATGL, adipocyte triglyceride lipase; AUC, area under the curve; ER, extended release; GIR, glucose infusion rate; GPR, G protein-coupled receptor; HOMA-IR, homeostasis model assessment for insulin resistance; HSL, hormone sensitive lipase; NiAc, nicotinic acid; PDE-3B, phosphodiesterase-3B .

Insulin resistance, impaired glucose tolerance, high circulating levels of free fatty acids (FFA), and postprandial hyperlipidemia are associated with the metabolic syndrome, which has been linked to increased risk of cardiovascular disease. We studied the metabolic responses to an oral glucose/triglyceride (TG) (1.7/2.0 g/kg lean body mass) load in three groups of conscious 7-h fasted Zucker rats: lean healthy controls, obese insulin-resistant/dyslipidemic controls, and obese rats treated with the dual peroxisome proliferator-activated receptor alpha/gamma agonist, tesaglitazar, 3 mumol.kg(-1).day(-1) for 4 wk. Untreated obese Zucker rats displayed marked insulin resistance, as well as glucose and lipid intolerance in response to the glucose/TG load. The 2-h postload area under the curve values were greater for glucose (+19%), insulin (+849%), FFA (+53%), and TG (+413%) compared with untreated lean controls. Treatment with tesaglitazar lowered fasting plasma glucose, improved glucose tolerance, substantially reduced fasting and postload insulin levels, and markedly lowered fasting TG and improved lipid tolerance. Fasting FFA were not affected, but postprandial FFA suppression was restored to levels seen in lean controls. Mechanisms of tesaglitazar-induced lowering of plasma TG were studied separately using the Triton WR1339 method. In anesthetized, 5-h fasted, obese Zucker rats, tesaglitazar reduced hepatic TG secretion by 47%, increased plasma TG clearance by 490%, and reduced very low-density lipoprotein (VLDL) apolipoprotein CIII content by 86%, compared with obese controls. In conclusion, the glucose/lipid tolerance test in obese Zucker rats appears to be a useful model of the metabolic syndrome that can be used to evaluate therapeutic effects on impaired postprandial glucose and lipid metabolism. The present work demonstrates that tesaglitazar ameliorates these abnormalities and enhances insulin sensitivity in this animal model.

Metabolic flexibility was assessed in male Zucker rats: lean controls, obese controls, and obese rats treated with the dual peroxisome proliferator activated receptor (PPAR) α/γ agonist, tesaglitazar, 3 μmol/kg/day for 3 weeks. Whole body glucose disposal rate (R d) and hepatic glucose output (HGO) were assessed under basal fasting and hyperinsulinemic isoglycemic clamp conditions using [3,3H]glucose. Indices of tissue specific glucose utilization (R g′) were measured at basal, physiological, and supraphysiological levels of insulinemia using 2-deoxy-D-[2,6-3H]glucose. Finally, whole body and tissue specific FFA and glucose utilization and metabolic fate were evaluated under basal and hyperinsulinemic conditions using a combination of [U-13C]glucose, 2-deoxy-D-[U-14C]glucose, [U-14C]palmitate, and [9,10-3H]-(R)-bromopalmitate. Tesaglitazar improved whole body insulin action by greater suppression of HGO and stimulation of R d compared to obese controls. This involved increased insulin stimulation of R g′ in fat and skeletal muscle as well as increased glycogen synthesis. Tesaglitazar dramatically improved insulin mediated suppression of plasma FFA level, whole body turnover (R fa), and muscle, liver, and fat utilization. At basal insulin levels, tesaglitazar failed to lower HGO or R fa compared to obese controls. In conclusion, the results demonstrate that tesaglitazar has a remarkable ability to improve insulin mediated control of glucose and FFA fluxes in obese Zucker rats.

To test the roles of lipid oversupply versus oxidation in causing tissue lipid accumulation associated with insulin resistance/obesity, we studied in vivo fatty acid (FA) metabolism in obese (Obese) and lean (Lean) Zucker rats. Indices of local FA utilization and storage were calculated using the partially metabolizable [9,10-3H]-(R)-2-bromopalmitate (3H-R-BrP) and [U-14C]-palmitate (14C-P) FA tracers, respectively. Whole-body FA appearance (R a) was estimated from plasma 14C-P kinetics. Whole-body FA oxidation rate (R ox) was assessed using 3H2O production from 3H-palmitate infusion, and tissue FA oxidative capacity was evaluated ex vivo. In the basal fasting state Obese had markedly elevated FA levels and R a, associated with elevated FA utilization and storage in most tissues. Estimated rates of muscle FA oxidation were not lower in obese rats and were similarly enhanced by contraction in both lean and obese groups. At comparable levels of FA availability, achieved by nicotinic acid, R ox was lower in Obese than Lean. In Obese rats, FA oxidative capacity was 35% higher than that in Lean in skeletal muscle, 67% lower in brown fat and comparable in other organs. In conclusion, lipid accumulation in non-adipose tissues of obese Zucker rats appears to result largely from systemic FA oversupply.

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