Estimation of glucose turnover and recycling in rabbits using various 3H, 14Cglucose labels

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-14Cglucose 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-14Cglucose are equivalent and greater than those obtained with 3-3H,U-14C- and 6-3H,U-14Cglucose. Although the means of the glucose replacement rates and percent carbon recycling obtained using 3-3H,U-14C- and 6-3H,U-14Cglucose 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-14Cglucose 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-14Cglucose 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-14Cglucose is the more suitable tracer for determining the glucose replacement rate and carbon recycling in vivo.