Intracellular pathways of insulin-mediated glucose uptake before and after puberty in conscious rats

Studies in humans and animals indicate that peripheral insulin sensitivity is decreased after puberty. Although glucose, after its uptake and phosphorylation, will be diverted to either the glycolytic or glycogen synthesis pathway, these pathways have not been characterized after the transition to puberty. Thus, we examined the changes in the pathways of glucose utilization in conscious (n = 22) prepuberty (81 ± 3 g), and postpuberty (258 ± 9 g) Sprague-Dawley rats. Insulin stimulated (by insulin clamp 18 mU/kg/min) glucose uptake [rate of glucose disappearance (R(d))] was decreased by ~30% postpuberty (from 339 ± 22 to 239 ± 28 μmol/kg/min; p < 0.001). Although glycolysis (estimated by the rate of conversion of [³H]glucose to ³H₂O) decreased by ~15% (p < 0.05), glycogen synthesis decreased by ~40% (from 200 ± 17 prepuberty to 122 ± 22 μmol/kg/min postpuberty; p < 0.001), and accounted for ~80% of the decrease in R(d) postpuberty. Decrease in the capacity to store glycogen in response to insulin was also confirmed by ~40% decrease in both glycogen levels, and in ³H accumulation into glycogen (from ³H-glucose) at the end of the clamp study. This occurred in the absence of any changes in either the K(m) or the V(max)of glycogen synthase nor in the activity of glycogen phosphorylase. We conclude that the postpubertal decrease in insulin responsiveness is characterized by decreased ability to store muscle glycogen. We propose that high capacity for muscle glycogen synthesis may be required to sustain the increased metabolic requirements during peripubertal growth.