Abstract
Our previous studies suggested that fresh water fishes do not discriminate against strontium relative to calcium when the isotopes are taken up from water in which they swim (1, 2). Unlike fresh water fishes. Boroughs et al. (3) found that marine Tilapia discriminate against strontium relative to calcium. Discrimination against strontium also appears to be well documented in small laboratory mammals and in man (4-6). However, Wasserman et al. (7) found that strontium discrimination in rats is not affected by dietary calcium levels. The present results, using double tracer technics, indicate that fresh water fishes display only a small degree of discrimination against strontium relative to calcium and essentially substantiate our previous data.
Methods and materials. Adult fresh water fishes, obtained from commercial sources, were of mixed sexes for zebra fish (Dunio) and white cloud mountain fish (Tanicthys) but guppies (Lebistes) were sexually mature males. Fish were placed in artificial pond water (8) containing both strontium-90 and calcium-45. The basic solution, adjusted to pH 7.0, contained 50 mg/liter of each of the following analytical reagent grade salts: sodium nitrate, potassium sulfate and magnesium sulfate. Solutions were modified by additions of calcium chloride, strontium chloride and sodium chloride as shown in results. Addition of strontium-90 and calcium-45 contributed approximately 0.25 mM/liter calcium(2). Small amouilts of these ions in the hasic solution are not included in the calculation. After 5 da57s in various solutions, fish were sacrificed and prepared for radioactive assay as previously described (9). Samples were held for 21 days to attain secular equilibrium Of stroritiuni-90 and its yttrium-90 daughter nuclide. Total beta activity (strontium-90 plus calcium-45) was determined with windowless gas flow counter.
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