Estimation of glucose turnover in rats in vivo with tritium labeled glucoses

Starved and starved-refed rats were injected intravenously with labelled glucose (a mixture of [2-3H]-, [3-3H]- and [U-14C]glucose with either [5-3H]- or [6-3H]glucose), and the decay of the specific activity of [14C]glucose followed. Glucose was degraded to obtain the 3H/14C ratios for 3 isotope combinations in the same sample. The apparent rates of replacements, apparent carbon recycling, and the body glucose mass were calculated for the different tracers. The 3H/14C ratio from [2-3H, -U-14C]glucose declined much faster than that of the other tracers. Apparent recycling as calculated in fasted rats was 28% for [2-3H, U-14C]- 18% for [5-3H,-U-14C]- 17% for [3-3H, U-14C]- and 14% for [6-3H,U-14C]glucoses. The values in fed rats showed a similar pattern. We estimate that in fasted rats 85 to 90% of the 3HOH liberated from injected [2-3H]glucose is formed by catabolism in the periphery and the rest by recycling in the liver between glucose and glucose 6-P. Detritiation of other labels by hepatic recycling accounts for a very small fraction of the total 3HOH yield.