LEVELS OF INSULIN-LIKE GROWTH-FACTOR (IGF) BINDING-PROTEINS, LUTEINIZING-HORMONE AND IGF-I RECEPTORS, AND STEROIDS IN DOMINANT FOLLICLES DURING THE FIRST FOLLICULAR WAVE IN CATTLE EXHIBITING REGULAR ESTROUS CYCLES

Objectives of this study were to determine if concentrations of steroi ds, insulin-like growth factor-I (IGF-I), and IGF binding proteins (IG FBP) in follicular fluid and numbers of LH and IGF-I receptors change during growth of the dominant follicle. Ovarian follicular development was monitored daily via ultrasound in lactating Holstein cows. Animal s underwent bilateral ovariectomy when the dominant follicle was first identified (days 4-6; estrus = day 0; early; n = 5) or when it stoppe d growing (days 8-12; late; n = 8). All follicles were classified as d ominant (DF), large (LG; greater than or equal to 6 mm in diameter, ex cluding DF) or small (SM; < 6 mm), follicular fluid was aspirated, and theca and granulosa cells were collected. Levels of IGFBP-2, assessed via ligand blotting, were greater (P < 0.05) in LG and SM follicles c ompared with DF in early cows. Levels of IGFBP-3 in follicular fluid w ere unaffected by follicle class. Numbers of specific I-125-hCG/LH bin ding sites in thecal cells were greater (P < 0.01) in DF compared with LG and SM follicles of both early and late cows. Numbers of specific I-125-hCG/LH binding sites in granulosa cells were similar for follicl e sizes in early cows, but, in late cows, were greater (P < 0.01) in D F compared with SM follicles and were severalfold greater (P < 0.01) i n late DF compared with early DF. Numbers of receptors for IGF-I in th ecal cells were 2-fold greater (P < 0.05) in DF and LG compared with S M in late cows. Numbers of IGF-I receptors in granulosa cells were una ffected by size or growth of follicles, but were severalfold greater t han in theca cells. Concentrations of estradiol were severalfold great er (P < 0.01) in DF compared with LG and SM in both early and late cow s. Concentrations of androstenedione in early cows were greater (P < 0 .05) in DF and SM compared with LG follicles. Concentrations of proges terone and IGF-I did not differ (P > 0.10) among follicle classes, but both were greater (P < 0.10) in late LG compared with early LG follic les. Concentrations of IGF-II in follicular fluid did not differ (P > 0.10) between early and late cows but were greater (P < 0.10) in SM th an DF or LG follicles. We conclude that low amounts of IGFBP-2 and inc reased thecal binding sites for hCG/LH appear to be related to establi shment of the dominant follicle during the first follicular wave in ca ttle exhibiting regular estrous cycles during late lactation.