Noninvasive measurements of [1-¹³C]glycogen concentrations and metabolism in rat brain in vivo

Using a specific ¹³C NMR localization method, ¹³C label incorporation into the glycogen C1 resonance was measured while infusing [1- ¹³C]glucose in intact rats. The maximal concentration of [1-¹³C]glycogen was 5.1 ± 0.6 μmol g^-1 (mean ± SE, n = 8). During the first 60 rain of acute hyperglycemia, the rate of ¹³C label incorporation (synthase flux) was 2.3 ± 0.7 μmol g^-1 h^-1 (mean ± SE, n = 9 rats), which was higher (p < 0.01) than the rate of 0.49 ± 0.14 μmol g^-1 h^-1 measured ≥2 h later. To assess whether the incorporation of ¹³C label was due to turnover or net synthesis, the infusion was continued in seven rats with unlabeled glucose. The rate of ¹³C label decline (phosphorylase flux) was lower (0.33 = 0.10 μmol g^-1 h^-1) than the initial rate of label incorporation (p < 0.01) and appeared to be independent of the duration of the preceding infusion of [1-¹³C]glucose (p > 0.05 for correlation). The results implied that net glycogen synthesis of ~3 μmol g^-1 had occurred, similar to previous reports. When infusing unlabeled glucose before [1-¹³C]glucose in three studies, the rate of glycogen C1 accumulation was 0.46 ± 0.08 μmol g^-1 h^-1. The results suggest that steady-state glycogen turnover rates during hyperglycemia are ~1% of glucose consumption.