Fat accretion and the regulation of insulin-mediated glycogen synthesis after puberty in rats

Peripheral insulin sensitivity decreases after puberty in both humans and rodents and can be explained mostly by a reduction in insulin-mediated glycogen synthesis. We tested the hypothesis that the increase in postpubertal fat mass (FM), reflecting an alternative energy store, regulates a decrease in the capacity to store muscle glycogen. We studied Sprague-Dawley rats (n = 21) before puberty (Pre) or after puberty (at 4 mo of age) in groups that were either ad libitum fed (Post) or moderately caloric restricted (CR). FM (by 3H2O isotope dilution technique) was decreased by >40% in CR compared with Post. Glucose uptake (Rd, by 18 mU x kg(-1) x min(-1) hyperinsulinemic clamp) was 63 +/- 8 mg x kg(-1) x min(-1) in Pre and decreased to 39 +/- 2 mg x kg(-1) x min(-1) in Post (P < 0.001). However, it increased in CR to 53 +/- 2 mg x kg(-1) x min(-1) (P < 0.001 vs. Post). This increase in Rd was mainly accounted for by an increase in glycogen synthesis (Rd glycolysis determined by the rate of conversion of 3H-labeled glucose to 3H2O) from 23 +/- 2 in Post to 33 +/- 2 mg x kg(-1) x min(-1) in CR (P < 0.001; 38 +/- 7 mg x kg(-1) x min(-1) in Pre). Correction of glycogen synthesis in CR to near-prepubertal levels was further supported by directly assayed muscle glycogen content after insulin stimulation that was 45% higher and by a 35% enhanced accumulation of [3H]glucose into glycogen. No changes in the enzyme kinetics of glycogen synthase or phosphorylase were observed. An additional group of 2-mo-old postpubertal ad libitum-fed rats was matched with CR for lean body mass but had more FM. This group demonstrated 25% lower rates of insulin-mediated glycogen synthesis compared with CR, further supporting the notion that a moderate reduction of FM prevents the decline in insulin responsiveness and glycogen synthesis occurring after puberty. These data suggest a cause-effect relationship between the increased deposition of fat and the reduced ability to store glucose in skeletal muscle after puberty.