ABSTRACT
We previously reported that long-term treatment of Zucker diabetic fatty (ZDF) rats with the selective beta(3) agonist CL-316243 normalizes glycemia, decreases plasma free fatty acids (FFA) concentration, improves insulin responsiveness, and increases glucose uptake, not only in brown and white adipose tissues, but also in skeletal muscles. Because muscles do not express typical beta(3) adrenoceptors, we postulated that the muscle effect was indirect and that it was possibly mediated by an activation of the glucose-fatty acid cycle. To test this hypothesis, we investigated the effects of Acipimox, a potent inhibitor of lipolysis in adipose tissue. Similar to CL-316243, Acipimox (150 mg/kg orally) markedly decreased plasma FFA, glucose, and insulin concentrations and improved glucose tolerance while reducing the insulin response in obese (350 to 400 g) ZDF rats. Plasma FFA concentrations were significantly correlated with plasma glucose and insulin concentrations (r =.72 and.83, respectively; P <.01), indicating strong metabolic relationships between these parameters. Euglycemic-hyperinsulinemic clamps combined with the 2-[(3)H]deoxyglucose method revealed that Acipimox markedly improved insulin responsiveness and significantly increased glucose uptake (Rg') in the diaphragm, the heart, and various skeletal muscles. Unlike CL-316243, Acipimox did not increase glucose use in brown or white adipose tissues. This selectivity shows that it is possible to improve diabetes in obese ZDF rats without necessarily stimulating thermogenesis in adipose tissues. Thus, decreasing plasma FFA with 2 drugs (Acipimox or CL-316243) that act via different mechanisms (acute inhibition of lipolysis or chronic stimulation of FFA oxidation) is associated with increased glucose uptake in muscles and enhanced insulin responsiveness. These observations support the hypothesis that CL-316243 may indirectly stimulate glucose uptake in muscles of type II diabetic rats by first stimulating brown adipose tissue (increasing uncoupling protein content and fatty acid oxidation) and progressively decreasing the levels of circulating FFA, resulting in activation of the glucose-fatty acid cycle or other mechanisms regulating insulin responsiveness in skeletal muscles.
