External assessment of myocardial metabolism with [11C]palmitate in rabbit hearts

To determine whether the rate of fatty acid utilization, reflecting myocardial oxygen consumption under steady-state conditions, can be assessed noninvasively, we assayed positron emission from [11C]palmitate (100 muCi) in isolated rabbit hearts and after intra-atrial injection in vivo. In 11 isolated hearts, clearance of 11C-radioactivity during a monoexponential portion of residue detection curves correlated with tension-time index (TTI)(r = 0.86) and peak dP/dt (r = 0.89). Among 9 hearts prelabeled with [14C]palmitate, 14CO2 production from neutral lipid stores correlated with TTI (r = 0.83). In vivo the decline of 11C-radioactivity was 1st order and constant when physiological conditions were maintained constant (slope of the semilog plot = 0.099 +/- 0.002 (SE) in counts/min) (n = 2 animals, 7 injections). Decline of activity changed predictably when oxygen requirements were increased by administration of methoxamine. Thus, residue detection of [11C]palmitate 1) permits external assessment of global myocardial metabolism in vitro, and 2) reflects effects of interventions that alter the rate of substrate utilization in vivo, and should, therefore, permit comparison of rates of regional myocardial metabolism in patients with detection of radioactivity by positron-emission transaxial tomography.