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.

Abstract

Objectives of this study were to determine if concentrations of steroids, insulin-like growth factor -I (IGF-I), and IGF binding proteins (IGFBP) 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. Animals underwent bilateral ovariectomy when the dominant follicle was first identified (days 4-6; estrus = day 0; early; n = 5) or when it stopped growing (days 8-12; late; n = 8). All follicles were classified as dominant (DF), large (LG; > = 6 mm in diameter, excluding 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 compared with DF in early cows. Levels of IGFBP-3 in follicular fluid were unaffected by follicle class. Numbers of specific 125I-hCG/LH binding 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 125I-hCG/LH binding sites in granulosa cells were similar for follicle sizes in early cows, but, in late cows, were greater (P < 0.01) in DF compared with SM follicles and were severalfold greater (P < 0.01) in late DF compared with early DF. Numbers of receptors for IGF-I in thecal cells were 2-fold greater (P < 0.05) in DP and LG compared with SM in late cows. Numbers of IGF-I receptors in granulosa cells were unaffected by size or growth of follicles, but were severalfold greater than in theca cells. Concentrations of estradiol were severalfold greater (P < 0.01) in DF compared with LG and SM in both early and late cows. Concentrations of androstenedione in early cows were greater (P < 0.05) in DF and SM compared with LG follicles. Concentrations of progesterone 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 follicles. 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 than DF or LG follicles. We conclude that low amounts of IGFBP-2 and increased thecal binding sites for hCG/LH appear to be related to establishment of the dominant follicle during the first follicular wave in cattle exhibiting regular estrous cycles during late lactation.

Insulin-like growth factors-I and -II and insulin-like growth factor-binding protein-2 in dominant equine follicles during spring transition and the ovulatory season

“…In other domestic species, changes in IGFBP concentrations are more closely associated with follicle growth and regression than are changes in levels of IGF-I and -II (Spicer & Echternkamp 1995, Stewart et al 1996, because only free IGF has biological activity. Furthermore, it has recently been shown that IGFBPs can directly inhibit steroidogenesis (Wright et al 2002).…”

“…Follicular concentrations of IGFBP-2, -4 and -5 decrease during follicular growth and increase during atresia in most species, including horses (Spicer & Echternkamp 1995, Gerard & Monget 1998. In contrast, levels of IGFBP-3 remain constant (Stewart et al 1996, Gerard & Monget 1998. In large, oestrogen-active equine follicles, the relative proportion of IGF:IGFBP was twofold greater than in other follicle types, indicating that the increase in bioavailable IGF may play a key role in promoting follicular maturation .…”

Follicle selection in cattle and horses: role of intrafollicular factors

“…4) in cattle (Beg et al 2002) and horses (Ginther et al 2002b). Despite the species similarities associated with the beginning of deviation, a marked species difference occurs after deviation begins; androgen concentrations increase in the dominant follicle and decrease in subordinate follicles in cattle (Stewart et al 1996, Singh et al 1998, Beg et al 2001, whereas concentrations do not increase in the dominant follicle but do in the subordinate follicles in horses (Donadeu & Ginther 2002). This species difference was further illustrated by the results of intrafollicular injection of IGF-I into the second LF (Fig.…”

Relative Roles of FSH and LH in Stimulation of Effective Follicular Responses in Cattle

“…However, it can be hypothesized that the follicle destined to become dominant maintains an ability to grow in the presence of declining FSH concentrations due to a number of possible mechanisms: i) increased bioavailability of insulin-like growth factor-I (IGF-I) in that follicle due to reduced total IGF-I binding protein activity (Mihm et al, 1997;Stewart et al, 1996;Canty et al, 2006); ii) expression of LH-receptor messenger RNA (Bao et al, 1997) and LH-receptors (Ireland and Roche, 1983) in granulosa cells of that follicle rendering it more responsive to LH than other follicles in the cohort. Further use of the models discussed in this paper along with identification of expression of key genes involved with these processes should provide further insights in the near future.…”