Serum and tissue pregnanes and pregnenes after dexamethasone treatment of cows in late gestation.

Dexamethasone (DEX) initiates parturition by inducing progesterone withdrawal and affecting placental steroidogenesis, but the effects of DEX in fetal and maternal tissue steroid synthetic capacity remains poorly investigated. Blood was collected from cows at 270 days of gestation before DEX or saline (SAL) treatment, and blood and tissues were collected at slaughter 38h later. Steroid concentrations were determined by liquid chromatography tandem mass spectrometry to detect multiple steroids induding 5 alpha-reduced pregnane metabolites of progesterone. The activities of 3 beta-hydroxysteroid dehydrogenase (3 beta HSD) in cotyledonary and luteal microsomes and mitochondria and cotyledonary microsomal 5 alpha-reductase were assessed. Quantitative PCR was used to further assess transcripts encoding enzymes and factors supporting steroidogenesis in cotyledonary and luteal tissues. Serum progesterone, pregnenolone, 5 alpha-dihydroprogesterone (DHP) and allopregnanolone (3 alpha DHP) concentrations (all <5 ng/mL before treatment) decreased in cows after DEX. However, the 20 alpha-hydroxylated metabolite of DHP, 20 alpha DHP, was higher before treatment (approximate to 100ng/mL) than at slaughter but not affected by DEX. Serum, cotyledonary and luteal progesterone was lower in DEX- than SAL-treated cows. Progesterone was >100-fold higher in luteal than cotyledonary tissues, and serum and luteal concentrations were highly correlated in DEX-treated cows. 3 beta HSD activity was >5-fold higher in luteal than cotyledonary tissue, microsomes had more 3 beta HSD than mitochondria in luteal tissue but equal in cotyledonary sub-cellular fractions. DEX did not affect either luteal or cotyledonary 3 beta HSD activity but luteal steroidogenic enzyme transcripts were lower in DEX-treated cows. DEX induced functional luteal regression and progesterone withdrawal before any changes in placental pregnene/pregnane synthesis and/or metabolism were detectable.