Technetium-99m labelled LDL as a tracer for quantitative LDL scintigraphy.

The purpose of this study was to determine whether the hepatic uptake of dialysed technetium-99m labelled low-density lipoprotein (Tc-LDL) reflects the hepatic LDL receptor activity and to what extent the non-LDL receptor-dependent Tc-LDL uptake by non-parenchymal cells relates to the diagnostic utility of quantitative Tc-LDL scintigraphy of the liver. New Zealand White rabbits and Watanabe Heritable Hyperlipidaemic rabbits, which were sacrificed 24 h after simultaneous injection of Tc-LDL and iodine-125 labelled LDL, were clearly discriminated by their hepatic Tc-LDL uptake according to their genetically different hepatic LDL receptor activity. Yet the hepatic Tc-LDL uptake exceeded the I-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 I-LDL. In contrast, Tc activity was trapped in the liver. Analysis of biliary Tc activity provided evidence for the excretion of Tc-labelled apolipoprotein B. The amount of biliary excreted protein-bound Tc was linked to total hepatic Tc-LDL uptake and presumably reflected LDL receptor-mediated apolipoprotein excretion. Collagenase liver perfusion in Sprague-Dawley rats 90 min following simultaneous injection of Tc- and I-LDL and subsequent cell separation by gradient centrifugation revealed that Tc-LDL and I-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 Tc-LDL uptake. Thus, we have been able to show that Tc-LDL scintigraphy using dialysed Tc-LDL can quantitate different genetically determined hepatic LDL receptor activities. The finding of the biliary excretion of Tc-labelled apolipoprotein B deserves further attention for its possible diagnostic utility. [ABSTRACT FROM AUTHOR]