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Large variation in steroid concentrations and insulin-like growth factor binding proteins exists among individual small antral follicles collected from within cows at random stages of the estrous cycle¹

A. J. Roberts^*,2, M. J. Al-Hassan^,3, P. M. Fricke^,4 and S. E. Echternkamp

^* USDA-ARS, Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301-4016; and USDA-ARS, Roman L. Hruska US Meat Animal Research Center, Clay Center, NE 68933-0166

² Corresponding author: andy{at}larrl.ars.usda.gov

Variation in the biochemical status of individual small (5 mm diameter) antral follicles within the ovaries of a cow at any given time likely influences the capacity for undergoing recruitment, selection, and establishing dominance. The objectives of this study were to provide insight into the magnitude of variation in follicular fluid concentrations of steroids and activities of IGFBP that exists among individual small antral follicles within and between cows, and to determine the relationships between follicular fluid IGFBP and steroid concentrations in these follicles. A total of 108 small antral follicles were collected from 6 cows at random stages of the estrous cycle, with 10 to 26 follicles/cow. Concentrations of steroids (ng/mL of follicular fluid) in the overall population of follicles ranged from 0.1 (lowest detectable limit) to 51 for estradiol (E₂), 4 to 1,149 for progesterone (P₄), and 5 to 504 for androstenedione (A₄). Concentrations of E₂ and A₄ were associated positively (r = 0.2; P < 0.02), but E₂ (r = –0.4) and A₄ (r = –0.4) were associated negatively, with P₄. The proportion of variation in steroid concentrations accounted for by differences among animals (P < 0.05) was small for E₂ (12%), moderate for P₄ (43%), and greatest for A₄ (74%). Least differences between minimum and maximum concentrations of steroids observed in follicles from within a cow were 21-, 5.5-, and 3.5-fold for E₂, P₄, and A₄, respectively, whereas the greatest differences between minimum and maximum concentrations were 505-, 108-, and 26-fold for E₂, P₄, and A₄, respectively. Ranges of IGFBP concentrations (arbitrary densitometer units) detected in fluid from a sub-sample of 43 follicles were 1.18 to 4.50 for IGFBP-3, 0.54 to 4.68 for IGFBP-2, 0.07 to 2.56 for IGFBP-4, and 0.01 to 6.71 for IGFBP-5. Concentrations of E₂ were correlated negatively with each IGFBP (r = –0.4 to –0.8; P < 0.05) except IGFBP-3. In contrast, concentrations of A₄ were correlated positively with IGFBP-3 (r = 0.4; P < 0.05) but were not correlated with other IGFBP. Concentrations of P₄ were correlated positively (r > 0.4; P < 0.05) with IGFBP-4 and -5. The results indicate that steroid concentrations and IGFBP activities vary substantially among small antral follicles collected from within and among individual animals and that increasing production of E₂, the hallmark of a developing follicle, was associated with reduced activity of all IGFBP except IGFBP-3, thereby implicating these IGFBP in the regulation of follicular recruitment.

Key Words: bovine • estradiol • follicle • insulin-like growth factor binding protein • ovary • progesterone