Glucose transport and recycling determined by means of two tracers and multicompartmental analysis

Glucose labeled with 3H and 14C was injected into cows, and blood samples were taken from 1 to 720 min for measurement of plasma glucose concentration and the specific activities of each tracer. Experiments were done on two cows when nonlactating, lactating, and exhibiting spontaneous ketotic hypoglycemia. Three exponential terms were required to fit the data for each of the tracers in all six experiments. The data were consistent with two 3 compartment models of the glucose system proposed previously on the basis of 14C data only: compartments 1 and 2 represent the glucose pool, while compartment 3 represents derivatives and precurors of glucose. Constraint tests were applied to kinetic parameters in the models to test the hypothesis that [3H]glucose does not recycle. When the parameter representing the recycling of [3H]glucose was set to zero in either model, systematic deviations were found between the predicted and observed data. When the recycling parameters for [3H]glucose and [14C]glucose were allowed to adjust independently of one another, while all other parameters in the model were constrained to be the same for both [3H]glucose and [14C]glucose, a good fit was obtained. These findings are not consistent with the hypothesis that [3H]glucose does not recycle. Recycling of [14C]glucose or [3H]glucose was estimated in terms of the return transport rate back into the glucose pool (compartments 1 or 2) divided by the transport rate from the glucose pool to compartments 3. It was 22 to 53% of [4C]glucose in the normal cows but only 12 and 13% in the ketotic hypoglycemia cows.