Plasma progestagens in the mare, fetus and newborn foal.

This study used gas chromatography/mass spectrometry (GC/MS) to identify and measure plasma progestagens. The method included deuterated internal standards, e.g. [17,21,21,21-2H]-5 alpha-pregnane-3,20-dione, solid-phase extraction, derivatization (methoxime/t-butyldimethylsilyl) and GC/MS. Full-scan screening identified 3 5-pregnenes, 2 4-pregnenes and 7 5 alpha-pregnanes (no 5 beta-pregnanes). The selected ion mode was used for routine quantitation from calibration curves; response was linear (r greater than 0.98) from 2 to 2000 ng equivalents/ml (0.5 ng/ml method sensitivity) and intra- and inter-assay coefficients of variation were less than 20%. The predominant steroids in plasma from pregnant pony and light-horse mares near term were 20 alpha-hydroxy-5 alpha-pregnan-3-one (400-2100 ng/ml) and 5 alpha-pregnane-3 beta,20 alpha-diol (100-350 ng/ml). Four other mono- and dihydroxy-5 alpha-pregnanes were generally between 30 and 100 ng/ml. These pregnanes were detectable initially between 30 and 60 days gestation and increased gradually until a more rapid increase was observed 30 days pre-partum. Maximum concentrations occurred during the last 2-3 days, followed by a decline before parturition. The progesterone profile during early pregnancy was similar to that given in previous reports. However, progesterone was not detectable during mid- to late pregnancy, except in a few mares near term (0.5-1 ng/ml). 5 alpha-Pregnane-3,20-dione (5 alpha-DHP) concentrations were similar to those of progesterone during early pregnancy but remained elevated and relatively constant (30 ng/ml) to term. No 20 alpha- or 20 beta-hydroxy-4-pregnen-3-one was detected in mare plasma. In mares that aborted or delivered prematurely, these pregnanes showed a profile of decreasing concentrations. In the fetal artery (250-300 days) plasma concentrations (ng/ml) of the predominant steroids were: pregnenolone (422), 5-pregnene-3 beta,20 beta-diol (171), 3 beta-hydroxy-5 alpha-pregnan-20-one (277) and 5 alpha-pregnane-3 beta,20 beta-diol (221). In the fetal vein, the predominant steroid was 5 alpha-DHP (205); progesterone was low (12.7 ng/ml) but consistently found. Steroids in the uterine venous plasma were similar in type and concentrations to those found in the peripheral circulation; uterine arterial concentrations were generally 50% lower. These results suggest that fetal pregnenolone is converted rapidly to progesterone and/or 5 alpha-DHP by the placenta, with further 3 beta- and 20 beta-hydroxylation by the fetus. In contrast, 20 beta-hydroxylation predominated on the maternal side and so may not directly reflect the fetal-placental environment.(ABSTRACT TRUNCATED AT 400 WORDS)