Fate and residues of [4-14C]estradiol-17 beta after intramuscular injection into Holstein steer calves.

Radiocarbon-labeled estra-1,3,5(10)-triene-3,17 beta-diol [4-14C-estradiol-17 beta; beta-estradiol] was suspended in commercial peanut oil and administered to each of three Holstein steer calves (142 to 170 kg body weight) by deep injection into neck muscle via 2.0 ml peanut oil carrier. The dosages were equivalent to .27 to .29 mg beta-estradiol/kg body weight. After dosing, radiocarbon was rapidly and almost totally eliminated in urine and feces. Most of the administered radiocarbon had been eliminated after 2 d, and by 11 d after treatment no radiocarbon was detectable in urine or feces of any calf. Of the total dose, 42.1 +/- 3.8% was excreted in urine and 57.7 +/- 5.2% was eliminated in feces. Analysis of noninjection site tissue samples (blood, brain, fat, kidney, liver and muscle) collected at sacrifice 14 d after treatment showed that none contained detectable radiocarbon residues, with the sensitivity limit for most tissues being 6 ppb beta-estradiol equivalent. Certain sections of muscle taken from the injection site area did contain detectable radiocarbon residues (as much as 69 ppb beta-estradiol equivalent), but total injection site area residues in each calf comprised less than .07% of the total administered dose. Radiocarbon in urine consisted primarily of alpha-estradiol, with much lesser amounts of estrone. Both compounds occurred as nonconjugates and as glucuronides. beta-Estradiol was not detected in urine. Radiocarbon in feces included primarily alpha-estradiol but also beta-estradiol and estrone, each in non-conjugated form. The fact that most of the intramuscular-administered beta-estradiol was eliminated in the feces strongly suggests a major role for biliary excretion in the disposition of this steroid by steer calves.