Choice of infusion-sampling mode for tracer studies of free fatty acid metabolism

To determine the preferred infusion-sampling mode for isotopic studies of free fatty acid (FFA) metabolism, tracer ([¹⁴C]palmitate) was infused into the left ventricle of five anesthetized dogs, and tracee ([³H]palmitate) was infused into three separate peripheral veins of each dog. The [¹⁴C]palmitate specific activity (SA) was lower in mixed venous than arterial blood, and [³H]palmitate SA was equal in both sites. The actual infusion rate of [³H]palmitate [2.15 ± 0.31 x 10⁵ disintegrations/min (dpm) · kg^-1 · min^-1] could be accurately predicted (2.14 ± 0.32 x 10⁵ dpm · kg^-1 · min^-1) using the known [¹⁴C]palmitate infusion rate and the arterial plasma [¹⁴C]-to-[³H]palmitate ratio. In contrast, the mixed venous [¹⁴C]-to-[³H]palmitate ratio resulted in overestimates (P < 0.05) of the actual [³H]palmitate infusion rate. In summary, venous tracer infusion with arterial blood sampling for FFA tracer studies provides the most accurate estimates of tracee rate of appearance.