Quantitation of Platelets in the Microcirculation. Measurement of Indium-111 in Microthrombi Induced in Rabbits by Inflammatory Lesions and Related Phenomena

The use of platelets labeled with ¹¹¹In-oxine for in vivo studies of thrombosis was examined. Platelets were separated from the blood of rabbits, labeled in vitro with ¹¹¹In, and returned to the animals by intravenous injection. In some animals repeated blood samples showed that the ¹¹¹In platelets gradually declined in the blood reaching half their initial concentration in 48 hr. In other rabbits, inflammatory lesions were produced in the skin of the back by the intradermal injection of killed Escherichia coli, or endotoxin, producing a local Shwartzman reaction. One hour after the iv injection of 111In platelets the animal was killed, the inflammatory lesions were removed, and the radioactivity in them was determined. ¹¹¹Indium accumulated in E. coli lesions within 1 hr after the injection of bacteria. The maximum rate of accumulation occurred in lesions which were 3 -4 hr old, but some ¹¹¹In platelets continued to accumulate in lesions that were 8-24 hr of age. The local Shwartzman reaction produced extensive thrombosis. This was demonstrated by a large accumulation of ¹¹¹In-labeled platelets, and by the observation of numerous platelet-containing thrombi in histological sections of the lesions. The findings demonstrate that ¹¹¹In can be used to label platelets for the in vivo quantitation of thrombosis in inflammatory tissue.