CHANGES IN RADIOACTIVE-TRACER DISTRIBUTION IN RATS AFTER 24 HOURS OF 45-DEGREES HIND-LIMB UNWEIGHTING

Introduction: Changes in radioactive tracer distribution Mere examined in rats after exposure to a simulated microgravity model of 45 degree s head down tilt (45HDT) or 45 degrees hind limb unweighting (45HU) fo r up to 24 h. Methods: Rats were randomly assigned to either 45HDT for (45HU) experimental groups or control groups for each time point of 0 .5 h, 1 h, 2 h, 4 h, 8 h, or 24 h. The 0.5-h through 8-h experimental rats were anesthetized and placed head-down on a ramp at 45 degrees, w hile control rats were placed in a prone position. Non-anesthetized ra ts in the 24-h experimental group were tail-suspended at 35 degrees, w hile control rats were allowed unrestrained movement. Technetium-label ed diethylenetriamine pentaacetate (99m)TcDTPA, physical half-life of 6.02 h, MW = 492 amu and indium-labeled diethylenetriamine pentaacetat e (99m)InDTPA. physical half-life of 3.5 d, MW = 545 amu were used to measure body organ distributions of the radioactive tracers at the 0.5 -h-8-h and 24h time points, respectively. Major organs were harvested after each lime period and measured for radioactive counts. Light and electron micrographs were examined. Results: Mean (99m)InDTPA counts f or the lungs, kidneys, and brains of the 24 h 45HU groups were signifi cantly higher than control counts. Light and electron microscopy demon strated the development of pulmonary edema in the alveolar septal area s after 2 h of 45HDT, and a shift in edema to the pulmonary airways an d pulmonary arteries alter 24 h of 45HU. Conclusions: Pulmonary edema development, accompanied by a significant increase in (99m)InDTPA lung , kidney, and brain counts in the 24-h 45HU groups, suggests vascular injury in the microcirculation of these organs.