Infusion and sampling site effects on two-pool model estimates of leucine metabolism

To assess the effect of site of isotope infusion on estimates of leucine metabolism infusions of alpha-[4,5-3H]ketoisocaproate (KIC) and [U-14C]leucine were made into the left or right ventricles of sheep and pigs. Blood was sampled from the opposite ventricle. In both species, left ventricular infusions resulted in significantly lower specific radioactivities (SA) of [14C]leucine and [3H]KIC. [14C]KIC SA was found to be insensitive to infusion and sampling sites. [14C]KIC was in addition found to be equal to the SA of [14C]leucine only during the left heart infusions. Therefore [14C]KIC SA was used as the only estimate for [14C]SA in the equations for the two-pool model. This model eliminated the influence of site of infusion and blood sampling on the estimates for leucine entry and reduced the impact on the estimates for proteolysis and oxidation. This two-pool model could not compensate for the underestimation of transamination reactions occurring during the traditional venous isotope infusion and arterial blood sampling.