beta-3 adrenergic agonist restores skeletal muscle insulin responsiveness in Sprague-Dawley rats

Between 7 and 14 weeks of age, male Sprague-Dawley rats develop a greater than 50% loss in insulin-stimulated glucose transport in skeletal muscle. We treated rats aged 14 weeks with the beta-3 adrenergic agonist CL316,243 (1 mg/kg/day by minipump for 14 days). Treatment resulted in a 56% reduction in visceral fat (P < 0.05). Muscle mass and body weight were unchanged. In strips of soleus muscle isolated from rats treated with CL316,243, basal transport of [(3)H]-2-deoxyglucose (2-DOG) was unchanged (105.8 +/- 7.5 nmol/g/min for vehicle vs 122.0 +/- 8.7 for CL316,243). However, in rats treated with CL316,243, the increase in 2-DOG transport in response to a maximal concentration of insulin was substantially increased (55.5 +/- 13.1 nmol/g/min for vehicle vs 102.4 +/- 13.5 for CL316,243, P < 0.03). CL 316,243 caused no significant changes in fasting glucose, insulin, or free fatty acids. Treatment of soleus muscle strips in vitro with CL316,243 (either 0.1 nM or 1.0 nM for 120 min at 37 degrees C) had no effect either on basal 2-DOG transport or on insulin-stimulated transport. We conclude that the CL316,243 causes a reduction in visceral fat and a reversal of muscle insulin resistance. The effect CL 316,243 on muscle insulin responses appears to be indirect, as it did not occur in vitro.