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

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