Metabolism of beta-methyl[1-¹¹C]heptadecanoic acid in canine myocardium

To assess the contributions of metabolism to CO₂ and back-diffusion of nonmetabolized tracer to total clearance of beta-methyl[1-¹¹C]heptadecanoic acid ([1-¹¹C]BMHDA) from myocardium and its distribution into lipid pools, 10-15 mCi of [1-¹¹C]BMHDA were rapidly infused into the circumflex coronary artery of seven open-chest dogs. Externally detected clearance of myocardial ¹¹C-activity was protracted (t 1 2 = 41 ± 18 min) with over 50% of extracted tracer retained in tissue after 20 min. Efflux of extracted [1-¹¹C]BMHDA between 5 and 20 min was comprised of both ¹¹CO₂ (16.5 ± 15.2%) and nonmetabolized [¹¹C]BMHDA (24.6 ± 11.5%). At 20 min, ¹¹C-activity in lipid was distributed primarily between triglyceride (47 ± 16% of total) and phospholipid (39 ± 17%) pools. Partial oxidation to ¹¹CO₂ and partitioning into more than one lipid pool are likely to hinder evaluation of fatty acid metabolism based on residue detection of [1-¹¹C]BMHDA with tomography.