| Abstract: | The glucose replacement rate, percent carbon recycling, mean glucose transit time, and the glucose mass were determined in fasted unanesthetized rabbits after administration of 2-3H,U-14C]-, 3-3H,U-14C]-, 5-3H,U-14C]- or 6-3H,U-14C]glucose using the procedures of Katz et al. (10). The glucose replacement rates and carbon recycling determined with 2-3H,U-14C] and 5-3H,U-14C]glucose are equivalent and greater than those obtained with 3-3H,U-14C]- and 6-3H,U-14C]glucose. Although the means of the glucose replacement rates and percent carbon recycling obtained using 3-3H,U-14C]- and 6-3H,U-14C]glucose are similar, greater variation resulted using the former tracer. Comparisons of detritiation rates and percent carbon recycling using 2-3H,U-14C]- and 6-3H,U-14C]glucose suggest that about 10% of tritium is lost from carbon 2 via futile cycling at the glucose 6-phosphate level. Similarly, comparisons of 5-3H,U-14C]- and 6-3H,U-14C]glucose metabolism suggest that about 10% of tritium lost from carbon 5 occurs via futile cycling at the fructose diphosphate level and/or via the transaldolase reaction. Our results indicate that 6-3H,U-14C]glucose is the more suitable tracer for determining the glucose replacement rate and carbon recycling in vivo. |
