Steroid concentrations in follicular fluid aspirated repeatedly from transitional and cyclic mares

The objectives of the present study were to determine follicular progestero ne (P-4) and estradiol-17 beta (E-2) in transitional mares and to compare f ollicular steroid concentrations between transitional and cyclic mares. Fol licles > 8 mm were aspirated under transvaginal ultrasound-guidance 4 times at 3 to 4 day intervals (T1-T4) in Norwegian pony mares during vernal tran sition. During the breeding season, follicular aspirations were conducted i n each mare on Day 6, Day 14 and Day 18 after ovulation of 3 separate estro us cycles (Day of ovulation = Day 0). Plasma and follicular fluids were ana lyzed for P-4 and E-2 with ELISA and RIA, respectively. Plasma P-4 concentr ations remained below 1 ng/mL throughout T1-T4, while the follicular P-4 co ncentrations increased significantly to cyclic levels after the first trans itional aspiration. Plasma E-2 concentrations similarly remained at low lev els during the course of the transitional aspirations, while the follicular E-2 concentrations increased gradually over the 4 aspirations to cyclic co ncentrations. The mares ovulated on average 9.8+/-1.6 (mean+/-SEM) days aft er the last transitional aspiration, and 16.6+/-0.2, 11.3+/-1.5 and 23.2+/- 4.4 days after aspirations conducted on Day 6, 14 and 18, respectively. The present study demonstrates that in the transitional mare newly developing follicles exhibit biosynthesis of P-4 and E-2. Furthermore, an increase in follicular steroid concentrations is not necessarily reflected in the perip heral steroid concentrations. (C) 2000 by Elsevier Science Inc.