A model for isotopic sampling and administration in constant infusion experiments: a test with 13C-palmitate.

We have proposed and tested a model that demonstrates the importance of the site of isotopic sampling relative to the site of infusion for the constant infusion of labeled palmitate. Palmitate enrichment is significantly increased downstream from the infusion catheter. However, palmitate production is underestimated unless sampling is performed upstream from the catheter. There was no significant difference between upstream sampling from the femoral vein versus the right heart, whereas the downstream arterial sampling gave significantly higher enrichments compared to the femoral vein. We have also discussed the importance of sampling sites for other substrates and the constraints that this model places on the design and interpretation of whole body primed constant infusion experiments using labeled substrates.