Regional myocardial free fatty acid extraction in normal and ischemic myocardium

The rate constant for free fatty acid influx (k1) was studied in normal and ischemic myocardium. In 15 normal subjects and 30 patients with coronary artery disease, 201Tl and 15-(p-123I-iodophenyl)-pentadecanoic acid (IPPA) were administered during exercise under fasting conditions and at rest. In 10 patients, the study was repeated after percutaneous transluminal coronary angioplasty; in three patients, the study was repeated after infarction. The initial accumulation of IPPA, related to that of 201Tl (both background and crossover corrected), was used for determinations of the regional rate constant of IPPA influx into myocardial tissue (k1*). In normal subjects, no significant differences in k1* between major myocardial segments were found; the average value of k1* was 0.57 +/- 0.13/min (mean +/- SD) at rest and 0.42 +/- 0.06/min at exercise (average workload, 123 +/- 47 W). With increasing free fatty acid plasma concentration and perfusion, free fatty acid influx increased in a saturable fashion. The Michaelis-Menten constant (KM*) and the maximal velocity (Vmax*) for IPPA influx into myocardial tissue were estimated to be 470 nmol/g and 430 nmol/g.min, respectively. In ischemic areas, k1* was reduced to 57 +/- 18% of k1* value in nonaffected segments. The areas were larger than those showing reduced 201Tl uptake. Preinfarction and postinfarction studies showed that the size of 201Tl defects in postinfarction images corresponded with the size of the area with reduced k1* observed in preinfarction scintigrams. Revascularization led to an increase of 201Tl uptake and to normalization of k1*.