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Obesity and dysfunctional energy partitioning can lead to the development of insulin resistance and type 2 diabetes. The antidiabetic thiazolidinediones shift the energy balance toward storage, leading to an increase in whole-body adiposity. These studies examine the effects of pioglitazone (Pio) on adipose tissue physiology, accumulation, and distribution in female Zucker (fa/fa) rats. Pio treatment (up to 28 days) decreased the insulin-resistant and hyperlipidemic states and increased food consumption and whole-body adiposity. Magnetic resonance imaging (MRI) analysis and weights of fat pads demonstrated that the increase in adiposity was not only limited to the major fat depots but also to fat deposition throughout the body. Adipocyte sizing profiles, fat pad histology, and DNA content show that Pio treatment increased the number of small adipocytes because of both the appearance of new adipocytes and the shrinkage and/or disappearance of existing mature adipocytes. The remodeling was time dependent, with new small adipocytes appearing in clusters throughout the fat pad, and accompanied by a three- to fourfold increase in citrate synthase and fatty acid synthase activity. The appearance of new fat cells and the increase in fat mass were depot specific, with a rank order of responsiveness of ovarian > retroperitoneal > subcutaneous. This differential depot effect resulted in a redistribution of the fat mass in the abdominal region such that there was an increase in the visceral:subcutaneous ratio, as confirmed by MRI analysis. Although the increased adiposity is paradoxical to an improvement in insulin sensitivity, the quantitative increase of adipose mass should be viewed in context of the qualitative changes in adipose tissue, including the remodeling of adipocytes to a smaller size with higher lipid storage potential. This shift in energy balance is likely to result in lower circulating free fatty acid levels, ultimately improving insulin sensitivity and the metabolic state.

Administration of the murine-selective beta3 adrenoceptor agonist CL-316,243 corrects obesity and elevated blood glucose in diabetic rodents. This antiobesity effect is attributed to an increase in the thermogenic activity of brown adipose tissue (BAT). The antidiabetic effect is unknown, but has been attributed to the decline in body weight and plasma free fatty acids (FFAs). This study using the euglycemic-hyperinsulinemic clamp method was performed in nonobese, nondiabetic Sprague-Dawley rats fed normal rodent chow to determine if the beta3 agonist could improve insulin sensitivity and/or responsiveness in the absence of weight loss or lowering of circulating FFAs. Subcutaneous miniosmotic pumps delivered either saline to control or 1 mg x kg(-1) x day(-1) of CL-316,243 for 10-12 days. Fed plasma glucose, insulin, and FFA levels were similar between the groups. Significant increases in food consumption, resting metabolic rates, and body core temperatures occurred, but only after 7 days of treatment. A 14% decrease in the respiratory quotient was also observed. Plasma glucose and insulin excursions in response to an oral glucose load (2 g/kg) on day 11 were unaltered. Cl-316,243 treatment resulted in a decrease in abdominal and epididymal white fat pad weights, while interscapular brown adipose tissue (IBAT) weight doubled. Basal and insulin-stimulated whole-body glucose disposal rates were increased, while hepatic glucose output was suppressed to a greater extent in the CL-316,243 animals after 10 days of uninterrupted treatment. Chronic treatment with CL-316,243 resulted in an increase in basal and insulin-stimulated [3H]2-deoxyglucose (2-DG) uptake by the retroperitoneal and epididymal white tissue and IBAT, but skeletal muscle 2-DG uptake under the same conditions was unaltered. These studies demonstrate that treatment with CL-316,243 improves basal and insulin-stimulated glucose disposal, and these effects occurred in the absence of a decrease in body weights and FFA concentrations. A particularly interesting observation was that the tissues responsible for this effect were white and brown adipose tissue, while skeletal muscle remained unaffected.

The role of insulin resistance in the impaired glucose-stimulated insulin release of Zucker fatty rats was investigated using the insulin-sensitizing thiazolidinedione drug pioglitazone. Fatty rats had fasting hyperinsulinemia yet a blunted secretory response to intravenous glucose compared with lean age-matched controls. Islets from fatty rats secreted less insulin (based on islet DNA) in response to high glucose than islets from lean rats but secreted normal amounts of insulin when tolbutamide or alpha-ketoisocaproic acid (alpha-KIC) was the stimulus. Administering pioglitazone for 9 days diminished basal hyperinsulinemia and increased the insulin response to high glucose by fatty rats but not by lean controls. Pioglitazone pretreatment augmented the secretory response by isolated islets to high glucose, alpha-KIC, and tolbutamide. Augmentation of islet insulin release was not associated with reduced plasma glucose concentration, suggesting that altered glycemia was not involved. Pancreas and islet insulin content was greater in fatty rats than in lean controls and was decreased by pioglitazone; hence, insulin stores and glucose-stimulated insulin release did not correlate. Pioglitazone treatment did not affect the rate of islet glucose usage or ATP/ADP in the presence of 2.75 or 16 mmol/l glucose. These data indicate that ameliorating insulin resistance reverses defective glucose-stimulated insulin release by Zucker fa/fa rats. After pioglitazone administration, insulin secretion may be augmented by increased generation of a metabolic coupling factor from glucose or at a later step in the secretory process that is common to both glucose and nonglucose secretagogues.