Changes in maternal pregnane concentrations in mares with experimentally-induced, ascending placentitis

The objectives of this study were to compare via liquid chromatography-tandem mass spectrometry (LC-MS/MS) progesterone (P4), 5α-dihydroprogesterone (DHP), allopregnanolone, 3β-hydroxy-5α-pregnan-20-one (3β5P), 20α-hydroxy-5α-pregnan-3-one (20α5P), 5α-pregnan-3β,20α-diol (βα-diol), and 5α-pregnan-3β,20β-diol (ββ-diol) concentrations in plasma of mares with experimentally-induced, ascending placentitis compared to gestationally age-matched control mares. Placentitis was induced via intracervical inoculation of Streptococcus equi spp. zooepidemicus between 260 and 280 days of gestation. Placentitis mares were subdivided into those which aborted in less than eight days (n = 6; acute) and those that aborted at ≥ 8 days after inoculation (n = 9; chronic). Ten pregnant mares at similar gestational ages served as healthy controls. Pregnanes were measured for days (−8), −6, −4, −3, −2, −1, and 0 days preceding abortion in the treated mares, and for the matched days of gestation in the control mares by LC-MS/MS and by immunoassay for immunoreactive (ir) P4. In mares with chronic placentitis, concentrations of DHP and its downstream metabolites (allopregnanolone, 3β5P, 20α5P, βα-diol) increased at 2-8 days prior to abortion compared to control mares. Of these pregnanes, 20α5P and βα-diol increased at eight days prior to abortion and demonstrated the largest increase (approximately 3 to 4×) in mares with chronic placentitis compared to control mares. Concentrations of P4 determined by LC-MS/MS were at or below the limit of detection (0.5 ng/mL) for control mares and did not increase significantly in mares with chronic placentitis. Immunoreactive-P4 was increased at two days prior to abortion in mares with chronic placentitis but was not different from controls in mares with acute placentitis. In mares with acute placentitis, concentrations of DHP, allopregnanolone, 3β5P, 20α5P, and βα-diol decreased within 0-3 days prior to abortion. In mares with chronic placentitis, the patterns of increased pregnanes metabolized by the placenta was similar to changes in maternal pregnanes noted in normal mares beyond Day 300 of gestation and likely represent the effects of fetal stress and adrenal activation on pregnane metabolism by the fetus and placenta. Decreases in these same pregnanes in mares with acute cases likely reflect extreme fetal or placental compromise.