Insulin responsiveness of diaphragm tissue and adipose cellularity in mice selected for rapid growth.

Four lines of mice, two lines selected for rapid growth (large body weights) (M16 and H6) and their unselected control lines (ICR and C2, respectively), were examined for traits related to obesity. The M16 line is obese while the other three lines are normal in body composition at 10 weeks of age when fed stock diet. In experiment I, mice were fed stock diet and examined at intervals from 4 to 22 weeks of age. Mice were fed either a high-carbohydrate or high-fat diet in experiment II and examined at 4, 6 and 10 weeks of age. The traits measured were serum glucose, serum insulin and in vitro insulin responsiveness of diaphragm muscle. Epididymal adipose cellularity was determined at 10 weeks of age. Insulin responsiveness was determined for the diaphragm muscle by 2-deoxyglucose uptake. Body weights differed significantly (M16 greater than H6 greater than ICR greater than C2, P less than 0.01). The M16 obese line was hyperglycemic and mildly hyperinsulinemic while the H6 line was hypoglycemic and normoinsulinemic. Basal 2-deoxyglucose uptakes by diaphragm muscle were similar among lines while the insulin-stimulated uptake by M16 and H6 lines was less (P less than 0.01) than the stimulated uptake by their control lines. Diet did not affect basal uptakes, but high-fat diets reduced (P less than 0.01) the insulin stimulated uptakes when compared to the high-carbohydrate diet. Selected lines had more and larger epididymal adipocytes than control lines when fed the stock diet (P less than 0.01). Decreased insulin responsiveness of muscle tissue among the lines occurred concomitantly with an increase in adipocyte size.