Uptake mechanism of interstitially injected 99mTc-labeled antimony trisulphide colloid in the popliteal lymph node of rabbits.

The right popliteal lymph node was studied in rabbits by well counting, histoautoradiography and electron microscopy, after interstitial injection of 99mTc-labeled antimony trisulphide (Sb2S3) colloid into the right hind pad. The highest radioactivity concentration (96.8%/g) was measured 6 hr following injection. At 24 hr, the concentration had dropped to nearly half of the maximum (51.5%/g). At each time, only a single or a few lymph node sectors were found to contain 99mTc. Initially, the radioactivity distribution pattern in the draining lymph node was stripy. Beginning at 15 min p.i., there was a progressive change from stripy to focal radioactivity distribution pattern. Until 6 hr after injection, the bulk of radioactivity was trapped by macrophages in the lumen and wall of the lymph node sinus system, predominantly in medullary sinuses. Surprisingly, at 24 hr the majority of labeled cells were eosinophilic polymorphonuclear leukocytes located in medullary sinuses and medullary cords. Up to 24 hr p.i., no accumulation of radioactivity could be detected in the cortical and paracortical lymph node parenchyma. In conclusion, interstitially injected 99mTc-Sb2S3 colloid is not homogeneously but sectorially distributed in the draining lymph node. Moreover, both macrophages and eosinophilic polymorphonuclear leukocytes are involved in the filtration process.