Distribution of fluorescent microspheres in vascular space and parenchymal organs of intact nude rats.

Purpose: To assess kinetics of elimination of different sized microspheres (MS) from the blood pool and tendency of their distribution in parenchymal organs of intact nude rats.

Materials And Methods: A mixture of 1 microm and 3 microm MS in phosphate-buffered saline was injected intravenously into eight rats under intraperitoneal anaesthesia. Blood samples were collected before, just after and in 2, 5 and 10 min after MS injection. Dynamics of MS elimination from blood pool was evaluated with flow cytometry. After euthanasia, histological sections were prepared and distributions of MS through the liver, spleen, kidney and lung were analysed with fluorescence microscopy and flow cytometry.

Results: The number of microspheres registered in the intravascular space showed a marked exponential decrease over time independent of MS size. Different amounts and proportions of 1 microm and 3 microm MS were revealed in lung, liver, spleen and kidneys of the rats. Most of 1 microm MS were localised in liver and spleen. In contrast, 3 microm MS were detected predominantly in lung.

Conclusion: 1 microm and 3 microm MS may be assumed as free circulating particles only for a short period of time after injection. Their elimination kinetics seems to be tightly linked to specific tissue properties such a pulmonary vasoconstriction and phagocytosis.