The dynamic steroid landscape of equine pregnancy mapped by mass spectrometry.

This study examines the changing concentrations of different classes of steroids in the blood plasma of pregnant horses over time to gain a deeper understanding of horse reproductive biology and fetal development.

Methodology

The researchers used a method called liquid chromatography-tandem mass spectrometry (LC-MS/MS) for this study because of its ability to perform a thorough analysis of textural variations in steroids:

• The study included nine mares, which were used to observe and map patterns in steroid variations over the course of gestation.
• The mares were subjected to regular blood tests until they gave birth, which allowed for an extensive data set capturing the full gestational period.

Key Findings

The study revealed the involvement of different groups of steroids at various stages of the pregnancy, mirroring significant physiological and developmental shifts:

• During the period of weeks 6-20, which accompanies the stimulation of primary corpora lutea by equine chorionic gonadotropic (eCG) and the formation of secondary luteal structures, there was an observable increase in the levels of progesterone, 17OH-progesterone, and androstenedione (all Δ4 steroids).
• The 5α-reduced metabolite of progesterone, dihydroprogesterone (DHP) was present at concentrations less than half that of progesterone until around week 12. As progesterone levels began to decline, DHP concentrations continued to rise, exceeding progesterone concentrations by week 16. This trend marks the luteo-placental shift, where pregnane synthesis transitioned from primarily ovarian to placental.
• The period of fetal gonad growth was marked by increasing levels of dehydroepiandrosterone and pregnenolone (Δ5 steroids) from week 14, peaking at week 34, and then declining towards term.
• 5α-reduced metabolites of DHP, including allopregnanolone, became dominant in the steroid profile in late gestation. At different points, some of these exceeded DHP concentrations as much as ten times.

Implications

Comprehensive LC-MS/MS steroid analysis may offer an opportunity to gain a detailed understanding of the physiology and progress of equine pregnancies, including fetal development:

• Δ4 steroids, the study suggests, have a significant presence during the eCG-stimulated phase, while for the luteo-placental shift, the ratio of progesterone to DHP (the increasing 5α-pregnanes) is an important indicator.
• The study also outlined the role Δ5 steroids play in elucidating fetal gonadal growth, and noted the dominance of 5α-reduced metabolites of DHP in the steroid profile in mid-to-late gestation.