Residue Levels of Endogenous Estrogens in Beef Tissues2

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Abstract
Endogenous estrogen concentrations in edible tissues of cows, bulls and steers were compared with those of steers with estrogen implants. Concentrations are expressed as pg estrogen/g of tissue, wet weight. In addition, depletion rates of estradiol-17β (E2β) estrone (E1) and estradiol-17α (E2α) from these tissues and blood plasma were determined. In muscle, the main estrogen was E2β, regardless of sexual status (nonpregnant cow, bull or steer); however, concentrations approached the lower limits of analytical sensitivity. In liver and kidney, E2β and E1 were equimolar (25 to 40 pg/g for each) in heifers and steers, whereas in kidney fat, concentrations of E1 exceeded those of E2β. Concentrations of E1 in fat were slightly higher in bulls than in cows or steers. The predominant estrogen in fat during pregnancy was E1, with concentrations 150 times greater than those of nonpregnant heifers or nonimplanted steers and 75 times the concentration found in steers with E2β implants. In kidney of pregnant cows, E1rose 40-fold and in liver 10-fold over that of implanted steers. Concentrations of E2α were low and depleted rapidly after withdrawal of an E2β implant. Tissue depletion studies of the three estrogens demonstrated that E1 disappeared from plasma, fat, liver and kidney more slowly than E2β or E2α Depletion of E2β from the tissue can be manipulated as shown by the faster rate of depletion in implanted steers than in nonimplanted steers. Because E1 is cleared from fat more slowly than E2, and E1 concentrations are higher, this estrogen-tissue combination should be used to monitor estrogen implants as anabolic agents.

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