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 (t1/2 = 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.