Activation of ß3-adrenoceptors increases in vivo free fatty acid uptake and utilization in brown but not white fat depots in high-fat-fed rats.

Activation of brown adipose tissue (BAT) and browning of white adipose tissue (WAT) present potential new therapies for obesity and type 2 diabetes. Here, we examined the effects of ß3-adrenergic stimulation on tissue-specific uptake and storage of free fatty acids (FFA) and its implications for whole body FFA metabolism in diet-induced obese rats using a multi-radiotracer technique. Male Wistar rats were high fat-fed for 12 wk and administered ß3-agonist CL316,243 (CL, 1 mg·kg-1·day-1) or saline via osmotic minipumps during the last 3 wk. The rats were then fasted and acutely infused with a tracer mixture ([14C]palmitate and the partially metabolized R-[3H]bromopalmitate) under anesthesia. CL infusion decreased body weight gain and fasting plasma glucose levels. While core body temperature was unaffected, infrared thermography showed an increase in tail heat dissipation following CL infusion. Interestingly, CL markedly increased both FFA storage and utilization in interscapular and perirenal BAT, whereas the flux of FFA to skeletal muscle was decreased. In this rat model of obesity, only sporadic populations of beige adipocytes were detected in the epididymal WAT depot of CL-infused rats, and there was no change in FFA uptake or utilization in WAT following CL infusion. In summary, ß3-agonism robustly increased FFA flux to BAT coupled with enhanced utilization. Increased BAT activation most likely drove the increased tail heat dissipation to maintain thermostasis. Our results emphasize the quantitative role of brown fat as the functional target of ß3-agonism in obesity.

Dosing profile profoundly influences nicotinic acid's ability to improve metabolic control in rats.

Acute nicotinic acid (NiAc) administration results in rapid reduction of plasma FFA concentrations. However, sustained NiAc exposure is associated with tolerance development resulting in return of FFA to pretreatment levels. The aim of this study was to determine whether a 12 h rectangular exposure profile (intermittent dose group) could avoid tolerance development and thereby reverse insulin resistance induced by lipid overload. FFA lowering was assessed in male Sprague Dawley (lean) and obese Zucker rats (obese) in response to a 5 h NiAc infusion, in either NiAc-naïve animals or after 5 days of continuous (24 h/day) or intermittent (12 h/day) NiAc dosing (via implantable, programmable minipump). We found that intermittent dosing over 5 days preserved NiAc-induced FFA lowering, comparable to dosing in NiAc-naïve animals. By contrast, following 5 days continuous administration, NiAc-induced FFA lowering was lost. The effect of intermittent NiAc infusion on insulin sensitivity was assessed in obese Zucker rats using hyperinsulinemic-isoglycemic clamps. The acute effect of NiAc to elevate glucose infusion rate (vs. saline control) was indeed preserved with intermittent dosing, while being lost upon continuous infusion. In conclusion, an intermittent but not continuous NiAc dosing strategy succeeded in retaining NiAc's ability to lower FFA and improve insulin sensitivity in obese Zucker rats.-Kroon, T., A. Kjellstedt, P. Thalén, J. Gabrielsson, and N. D. Oakes.

Uptake and tissue accretion of orally administered free carboxylic acid as compared to ethyl ester form of docosahexaenoic acid (DHA) in the rat.

AIM: The aim of this study was to compare the plasma exposure and tissue accretion of docosahexaenoic acid (DHA) in response to oral dosing of free carboxylic acid (OM3CA) and ethyl ester (OM3EE) forms. MATERIALS AND METHODS: Sixteen adult male Wistar rats, fed a low-fat, carbohydrate-rich, standard chow diet, were chronically catheterized and gavaged for 5 consecutive days with either OM3CA (n = 9) or OM3EE (n = 7), the last day fasted overnight and spiked respectively with either 14C-DHA or 14C-DHA-ethyl ester (14C-DHA-EE) tracers. Appearance of 14C-labelled plasma polar and neutral lipids over 4 h and retention of 14C-activity (R) in the tissues at 4 h were measured. RESULTS: Compared to OM3EE, OM3CA resulted in 2- and 3-fold higher areas under the plasma 14C-labelled polar and neutral lipid curves (exposures), respectively, as well as, higher R in all tissues examined. For both OM3CA and OM3EE, R varied in a tissue specific manner; highest in liver, followed by red skeletal muscle, adipose tissue, brain and white skeletal muscle. Multiple linear regression analysis revealed that R in each tissue (except liver) was dependent on polar lipid exposure alone (r2>0.87 and P<0.001), but not neutral lipid exposure, and furthermore this dependence was indistinguishable for OM3CA and OM3EE. In the liver, R was found to be dependent on both polar and neutral lipid exposures (r2 = 0.97, P<0.001), with relative contributions of 85±2% and 15±2%, respectively. As for the other tissues, these dependencies were indistinguishable for OM3CA and OM3EE. CONCLUSION: The present results, in fasted low-fat diet fed rats, are consistent with higher oral bioavailability of OM3CA versus OM3EE forms of DHA. Once DHA has entered the circulation, the tissue distribution is independent of the dosed form and uptake in the skeletal muscle, fat and brain is driven by the polar pools of DHA in plasma, while DHA accretion in liver is supplied by both polar and neutral plasma lipids.

The PPAR α / γ Agonist, Tesaglitazar, Improves Insulin Mediated Switching of Tissue Glucose and Free Fatty Acid Utilization In Vivo in the Obese Zucker Rat.

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 µ mol/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,(3)H]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-(3)H]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-(13)C]glucose, 2-deoxy-D-[U-(14)C]glucose, [U-(14)C]palmitate, and [9,10-(3)H]-(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.

Roles of Fatty Acid oversupply and impaired oxidation in lipid accumulation in tissues of obese 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-(3)H]-(R)-2-bromopalmitate ((3)H-R-BrP) and [U-(14)C]-palmitate ((14)C-P) FA tracers, respectively. Whole-body FA appearance (R a ) was estimated from plasma (14)C-P kinetics. Whole-body FA oxidation rate (R ox) was assessed using (3)H2O production from (3)H-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.

AZ 242, a novel PPARalpha/gamma agonist with beneficial effects on insulin resistance and carbohydrate and lipid metabolism in ob/ob mice and obese Zucker rats.

Abnormalities in fatty acid (FA) metabolism underlie the development of insulin resistance and alterations in glucose metabolism, features characteristic of the metabolic syndrome and type 2 diabetes that can result in an increased risk of cardiovascular disease. We present pharmacodynamic effects of AZ 242, a novel peroxisome proliferator activated receptor (PPAR)alpha/gamma agonist. AZ 242 dose-dependently reduced the hypertriglyceridemia, hyperinsulinemia, and hyperglycemia of ob/ob diabetic mice. Euglycemic hyperinsulinemic clamp studies showed that treatment with AZ 242 (1 micromol/kg/d) restored insulin sensitivity of obese Zucker rats and decreased insulin secretion. In vitro, in reporter gene assays, AZ 242 activated human PPARalpha and PPARgamma with EC(50) in the micro molar range. It also induced differentiation in 3T3-L1 cells, an established PPARgamma effect, and caused up-regulation of liver fatty acid binding protein in HepG-2 cells, a PPARalpha-mediated effect. PPARalpha-mediated effects of AZ 242 in vivo were documented by induction of hepatic cytochrome P 450-4A in mice. The results indicate that the dual PPARalpha/gamma agonism of AZ 242 reduces insulin resistance and has beneficial effects on FA and glucose metabolism. This effect profile could provide a suitable therapeutic approach to the treatment of type 2 diabetes, metabolic syndrome, and associated vascular risk factors.