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Depletion Patterns of Radioactivity and Tissue Residues in Beef Cattle after the Withdrawal of Oral¹⁴C-Diethylstilbestrol

United States Department of Agriculture, Beltsville, Maryland 20705

Abstract

Seven beef heifers and eight beef steers were orally administered monoethyl-1 ¹⁴C-diethylstilbestrol (¹⁴C-DES) and then one heifer and one steer were slaughtered on each of the following days after the withdrawal of DES: .75, 1.5, 3, 5, 7, 9 and 14. One steer was slaughtered 30 days after withdrawal. Total recovery of the administered radioactivity averaged 95.5%. Approximately 99.5% of the recovered radioactivity was excreted within 5 days after withdrawal of DES; however, a low level of radioactivity above background was measured in excreta up to 12 days after withdrawal. Semilog plots of the radioactivity in liver tissue, urine and feces versus withdrawal time were quadratic. A similar quadratic trend was noted for bile, but the quadratic function was not significant. Radioactivity above background was measurable up to 5 days after withdrawal in kidney tissue, 7 days in liver tissue, 9 days in bile and 11 to 12 days in urine and feces. DES residues in liver tissue were not measurable by gas chromatography (GLC) techniques after withdrawal periods of 3 days and longer. The percentage of total radioactivity in liver tissue presumptively identified by radioisotope dilution techniques as being associated with DES or its glucuronide conjugate decreased with increasing withdrawal time: from approximately 77% at .75 days to 4% at 7 days. The proportion of total radioactivity in liver tissue extracted by the routine GLC method decreased from 39% at .75 days to 2% at 7 days. The remainder of the radioactivity was not identified. Approximately 75% of the presumptively identified radioactivity was associated with conjugated DES and 25% was associated with free DES. Combining the quadratic depletion curve for radioactivity in liver tissue with the change in percentage of presumptively identified radioactivity resulted in a linear semilog plot for depletion from liver with a half-life of 17 hours. The biological half-life of the more slowly depleted unidentified fraction of radioactivity in liver tissue was 5.5 days.

¹ ARS, Nutrition Institute, Ruminant Nutrition Laboratory.

² The authors gratefully acknowledge the advice and assistance of Mr. R. M. Simpson and Drs. J. E. Spaulding and A. J. Malanoski of U.S.D.A., Animal and Plant Health Inspection Service in setting up the GLC procedure for tissue analysis of DES and for conducting duplicate analysis of tissues and the assistance of Mr. E. E. Williams and Mr. D. F. Hucht with sample preparation and radiocarbon analysis.

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