Steroid metabolism by the bovine uterine endometrium and conceptus

Abstract

A comparison was undertaken of the in vitro ability of two uterine-specific tissues, viz endometrium and conceptus, to metabolize neutral steroids. Tissues were obtained at slaughter from 55 cows on Days (D) 16, 19, 23, and 27 postbreeding. D 16 and D 19 uteri which did not contain any products of conception were designated nonpregnant (NP); those bearing membranes, pregnant (P). Endometrial (500 mg) and conceptus (intact or halved) tissues (n=227) were incubated in minimal essential media (MEM) with 10 ng isotopically labeled progesterone (P4) or androstenedione (A). Extent of metabolism and qualitative appraisal of steroid metabolites were determined after extraction and separation through chromatographic columns before and after chemical conversion procedures. Identification and quantification of metabolites were verified by recrystallization to constant specific activity with authentic radioinert parent compounds. Both substrates were metabolized extensively by tissue with more metabolism occurring in 3 than in 1 h (P<0.001). Endometrial tissue metabolized similar amounts of P4/mg tissue on D 16 and D 19 irrespective of reproductive status (P vs. NP) or the location of tissue within the uterus. However, D 27 endometrial tissue metabolized more P4 (P<0.01) than that from D 16 or D 19 (59.1 ± 2.6% vs. 44.0 ± 2.3%) of pregnancy. Major metabolites produced by endometrial tissue were hydroxylated 5α-reduced steroids, most notably 5α-pregnane-3ξ,20ξ-diol from P4 and 5α-androstane-3ξ,17ξ-diol and 5β-pregnane-3α,20α-diol from P4 and 3α-hydroxy-5β-androstan-17-one and from A. In contrast, conceptus tissue metabolized substrates extensively to 5β-reduced steroids. Identified were 3α-hydroxy-5β-pregnan-20-one and 5β-androstane-3α,17α-diol from A. In addition, metabolism of A but not P4 resulted in production of small quantities (<200 pg) of estrone and estradiol-17β. The implications of the differential types of metabolites and extents of metabolism of P4 and A are discussed in relation to reproductive status of the cow.