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The feasibility of using 123I-heptadecanoic acid (HDA) as a metabolic tracer was studied. Different administration routes of HDA were compared. An intracoronary bolus injection was given to calves (n=3), and an intravenous injection was given to patients (n=4). In addition, we examined the influence of 4-h halothane anesthesia in calves and in patients the impact of an insulin (1.5 IU/kg)+ glucose (1.5 g/kg) infusion on the myocardial kinetics of HDA. Data were accumulated with a scintillation probe in calves (t=50 min) and a gamma camera in patients (t=70 min). In calves after an intracoronary bolus injection of HDA the myocardial time-activity curve could be described by two exponentials. The mean elimination halftime of the initial phase (ta 1/2) was 7.3 min and that of the second phase (tb 1/2) was 35 min. The ratio of the size of the initial and second component at to was 0.93. Halothane anesthesia prolonged the elimination half-times and reduced the component ratio. The biphasic behavior of the myocardial time-activity curve was maintained in patients after intravenous administration of HDA under basal conditions (initial ta 1/2=8.4 min). However, during infusion of insulin+glucose the decline in the myocardial activity was prolonged and monoexponential. This data shows that insulin glucose, interfering with fatty acid metabolism, influences the myocardial washout of HDA, and thus support its use as a metabolic tracer.

The purpose of this study was to determine whether the hepatic uptake of dialysed technetium-99m labelled low-density lipoprotein (99mTc-LDL) reflects the hepatic LDL receptor activity and to what extent the non-LDL receptor-dependent 99mTc-LDL uptake by non-parenchymal cells relates to the diagnostic utility of quantitative 99mTc-LDL scintigraphy of the liver. New Zealand White rabbits and Watanabe Heritable Hyperlipidaemic rabbits, which were sacrificed 24 h after simultaneous injection of 99mTc-LDL and iodine-125 labelled LDL, were clearly discriminated by their hepatic 99mTc-LDL uptake according to their genetically different hepatic LDL receptor activity. Yet the hepatic 99mTc-LDL uptake exceeded the 125I-LDL uptake in all animals. The different hepatic uptake of the tracers was elucidated in the isolated perfused rat liver and was due to rapid intracellular degradation and the release of low molecular catabolites of 125I-LDL. In contrast, 99mTc activity was trapped in the liver. Analysis of biliary 99mTc activity provided evidence for the excretion of 99mTc-labelled apolipoprotein B. The amount of biliary excreted protein-bound 99mTc was linked to total hepatic 99mTc-LDL uptake and presumably reflected LDL receptor-mediated apolipoprotein excretion. Collagenase liver perfusion in Sprague-Dawley rats 90 min following simultaneous injection of 99mTc- and 125I-LDL and subsequent cell separation by gradient centrifugation revealed that 99mTc-LDL and 125I-LDL had a comparably low uptake into non-parenchymal cells; thus its contribution can be neglected for scintigraphic purposes. Planar scintigraphy was performed in New Zealand White and Watanabe Heritable Hyperlipidaemic rabbits. The different hepatic LDL receptor activities of the two groups were reflected by the slope of the liver/heart ratios over 24 h post injection and correlated well with their respective cumulative hepatic 99mTc-LDL uptake. Thus, we have been able to show that 99mTc-LDL scintigraphy using dialysed 99mTc-LDL can quantitate different genetically determined hepatic LDL receptor activities. The finding of the biliary excretion of 99mTc-labelled apolipoprotein B deserves further attention for its possible diagnostic utility.

99mTc-DMPE was used for myocardial scintigraphy in ten patients with coronary artery disease. As in 201Tl studies regional activity of 99mTc-DMPE was reduced in infarcted myocardium. However, activity accumulation of 99mTc-DMPE in the heart was faint, while that in the liver was prominent. The activity ratio of heart to liver improved with time, whereas that of heart to lung decreased. The scintigraphic quality was considerably worse in 99mTc-DMPE studies than in those with 201Tl, due to high background activity. Also the visualization of the ribs and sternum interfered with the interpretation of the scintigrams. From these results it appears that 201Tl remains still the agent of choice for myocardial perfusion scintigraphy.