NUCLEAR CARDIOLOGY Metabolic fate of radioiodinated heptadecanoic acid in the normal canine heart

To clarify the metabolic fate of radioiodinated heptadecanoic acid in myocardium, the time course and distribution of the radioactivity over '311-heptadecanoic acid, free radioiodide, and various lipids (with incorporated iodoheptadecanoic acid) were determined in normal canine myocardium. In 10 dogs seven biopsy specimens were taken over 30 min after injection of 1311-heptadecanoic acid. The radioactivity in the specimens increased until the fifth minute and decreased thereafter, with a half-time of 36 min. In the fifth minute, 61% of the radioactiviy was free iodide, and its curve paralleled the curve of the total radioactivity. As early as the first minute 13'I-heptadecanoic acid activity was reduced to 14% and decreased further. Activity of radioiodinated phospholipids, (mono, di, tri)glycerides, and cholesterol-esters remained constant after an initial increase. These results indicate that immediately after uptake, '3'I-heptadecanoic acid is either metabolized, liberating the radioiodide, or stored in lipids. Because the activity of radiolabeled lipids remained constant during the study period and because iodide activity paralleled the total activity in biopsy specimens, it is concluded that in normal myocardium, washout of free radioiodide determines the elimination rate as observed during a scintigraphic study. Thus the elimination rate cannot be related to the ,8-oxidation rate as previously supposed. Circulation 72, No. 3, 565-71, 1985. THE HEART uses a variety of substances like glucose, free fatty acids, and lactate as an energy source for its metabolism. Under normal physiologic conditions, 60% to 80% of energy-rich phosphate bonds are derived from oxidation of long-chain free fatty acids.' Therefore long-chain free fatty acids labeled with a radioisotope were proposed as indicators of cardiac metabolism in vivo. One of these radiolabeled free fatty acids was heptadecanoic acid (HDA) labeled with 123I in the omega position. It was found that after administration of radiolabeled HDA, regional elimination of the radioactivity differed under various pathophysiologic conditions of the myocardium. In ischemic myocardium the elimination rate was found to be lower than that in comparable normal myocardium.A-"0 In some patients with a recently sustained infarction, a higher elimination rate From the Departments of Cardiology and Nuclear Medicine, Free University Hospital, Amsterdam, The Netherlands, and Abteilung Nuklearmedizin, Universitiatsspital, Zurich, Switzerland. Address for correspondence: F. C. Visser, M.D., Department of Cardiology, Free University Hospital, De Boelelaan 1117, 1007 MB Amsterdam, The Netherlands. Received July 10, 1984; revision accepted May 16, 1985. Vol. 72, No. 3, September 1985 was found. 1 " Patients with congestive cardiomyopathy showed a diffuse, inhomogeneous pattern of elimination.'2 The rate of elimination was also influenced by simultaneous administration of glucose and insulin. '1 These results suggested a potential use of radioiodinated HDA as metabolic tracer. Two hypotheses have been developed to explain the observed differences in elimination rates. Both hypotheses are related to cardiac metabolism of radioiodinated free fatty acids. After uptake in the myocyte, radioiodinated long-chain free fatty acids are degraded by /3-oxidation'4 in the mitochondria. Radioiodine is split off and leaves the cell. In the first hypothesis the elimination rate is related to the /-oxidation rate of the radioiodinated fatty acids'5 in the mitochondria. In the second hypothesis the elimination rate is related to the diffusion rate of radioiodide from the myocardial mitochondria into the systemic circulation. 6 Because knowledge of the underlying mechanism is important for interpretation of scintigraphic data, this study was undertaken to ascertain which of the two hypotheses is correct. From normal canine myocardi565 by gest on A ril 1, 2017 http://ciajournals.org/ D ow nladed from