Phagocytosis and dynamic RES scintigraphy: an evaluation of commercial colloids in rats

Reticuloendothelial system (RES) phagocytic function or capacity can be measured by clearance studies with test substances. This study in the rat examines three commercially available 99Tcm-labelled test substances, and different methods of calculating RES capacity. Albures size 500 nm and sulphur colloid size 600 nm used for liver scintigraphic imaging and Nanocoll size 50 nm used for bone-marrow scintigraphic imaging were tested. The rats were examined under a gamma camera and the uptake by the liver, clearance from the heart and clearance from blood samples were recorded. Different amounts of substances were tested. The final uptake in different organs was recorded after the animals were sacrificed. Colloid size and stability were tested with a polycarbonate filter. (Nanocoll was found to have at least 10 times smaller colloid diameter, leading to a thousand times more particles per milligram). RES function was calculated as the uptake rate or clearance rate, k. In a logarithmic plot, the relationship between uptake or clearance and time was found to be linear between 90 and 300s and calculations of k in this interval are recommended. k-values selected from blood sample curves were 11 +/- 5 S.E.M. lower than those calculated from heart clearance curves or liver uptake curves. Increasing amounts of Nanocoll caused a decrease in uptake rate k. Albures and sulphur colloid could not be given in amounts that caused any change in k. Only Nanocoll could be given in sufficiently large amounts (above the critical dose) to challenge RES and avoid complete extraction from the circulation during first passage through RE organs. Nanocoll seems suitable for use in tests of RES function and the optimal amount was 0.03 mg per rat (0.6 X 10(12) particles).