Effect of insulin on fat and protein deposition in diabetic lean and obese rats

Five-week-old male obese and lean Zucker rats were made comparably diabetic by intracardiac injections of alloxan (65-72 mg/kg body wt). Lean rats were then given daily injections of protamine zinc insulin at 3 doses: 0.25, 1.25, and 4.0 U.100 g body wt-1.day-1 for 3 wk. Obese rats received identical amounts as corresponding lean controls independent of body weights. The drop of blood glucose concentration after injections of regular insulin and the percentage fall in radioactive plasma insulin after injections of 125I-insulin were comparable in lean and obese rats. Weight gain, fat gain, and protein gain over 21 days increased with increasing amounts of insulin administered. However, at the same dose of insulin, although weight gain was comparable, fat gain was higher and protein gain was lower in obese rats when compared to lean controls. These results suggest that the enhanced lipid deposition of the obese Zucker rat is not totally dependent on insulin levels, but is exaggerated by hyperinsulinemia.