Graduate Thesis Or Dissertation
 

The uptake and retention of cesium-134 in Microtus canicaudus Miller

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  • A study was performed to assess the uptake and retention patterns of cesium-134 in the gray-tailed vole, Microtus canicaudus Miller. To furnish a realistic source of contaminated food for the voles, wheat seedlings were raised in hydroponic solutions to provide radiocesium labeled wheat clippings. The hydroponic system was designed to provide optimal conditions for cesium uptake by the plants. In nutrient solutions containing about 10 and 20 μCi radiocesium/- 4 liters, eight-day-old wheat clippings accumulated 18% and 12% of the radioactivity, respectively. The labeled wheat clippings were fed to voles as a supplement to their normal diet. Under a chronic feeding schedule, the average radioactivity of each serving was 0.04 μCi. Under a single feeding schedule, the average activity was about 0.05 μCi/serving. Radiocesium was also administered to voles via a single, intraperitoneal (IP) injection of - 0.9 μCi in 4.0 ml normal saline. In the chronic feeding study, a fluctuating, whole-body equilibrium was achieved. The data give the apparent indication that equilibrium could have been reached as early as the first several feedings. The retention of radiocesium by M. canicaudus under the three modes of administration, i.e., chronic feeding, single feeding and IP injection was analyzed assuming exponential elimination rates. "Final" components of radiocesium elimination were arbitrarily assessed from retention curves drawn on semi-logarithmic graph paper. The y-intercepts (a) and biological half-lives (Tb) derived from linear regressions of the respective final components were: a = 8.0 ± 8.6%, Tb = 5.8 ± 2.4 days in the chronic feeding study; a = 52.6 ± 25.5%, Tb = 1.5 ± 0.5 days in the single feeding study and a = 13.9 ± 7.1%, Tb = 2.2 ± 0.2 days in the IP administration study. The Tb for the chronically fed animals was comparatively longer than the Tb for the single administration mode animals. This suggested that the longer period of equilibration resulted in a larger portion of the body burden being deposited in metabolically less active tissues. From tissue distribution studies, it was found that cesium was fairly uniform in its dispersion throughout the body. Skeletal muscle, due mainly to its large mass, will apparently become the major repository for radiocesium. This confirms an observation widely found in the literature. In the single feeding study, careful assessment of the radioactivity transfer from nutrient solution to the wheat seedlings and the wheat clippings to the voles yielded trophic transfers of 8% (after 5 days exposure) and 53.3% (after 0.5 days exposure), respectively. Clearly, even under optimal conditions for cesium uptake by the wheat seedlings, the "bottleneck" of transfer was from substrate to plant. This finding, in conjunction with the relatively rapid turnover of radiocesium in the vole, makes it unlikely that a vole foraging only occasionally in a contaminated region will develop any significant body burden. However, the results of the chronic feeding study indicate that a vole foraging consistently in such an area may indeed achieve a substantial body burden since larger fractions of radiocesium might then appear in metabolically less active tissues.
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