Changes in follicular fluid steroids, insulin-like growth factors (IGF) and IGF-binding protein concentration, and proteolytic activity during equine follicular development.
The objective of the present study was to evaluate changes in equine follicular fluid insulin-like growth factor binding protein (IGFBP) proteolytic activity as well as steroid, IGF, and IGFBP concentrations during follicular development in the mare. Mares (n = 14) were classified as either in the follicular phase (n = 8) or luteal phase (n = 6). Follicles (n = 92) were categorized as small (6 to 15 mm; n = 54), medium (16 to 25 mm; n = 23), or large (> 25 mm; n = 15), and follicular fluid was collected. Estradiol and androstenedione levels in follicular fluid were greater (P < 0.05), and IGFBP-3 concentrations tended to be greater (P < 0.10) in large than in small or medium follicles, whereas IGFBP-2, -4, and -5 levels were less (P < 0.05) in large than in small or medium follicles. Estradiol and androstenedione concentrations were negatively correlated (P < 0.01) with IGFBP-2, -4, and -5 but not IGFBP-3 concentrations. To evaluate proteolysis of IGFBP, follicular fluid was incubated with human 125I-labeled IGFBP-2, -3, and -5 and protein separated by 12% SDS-PAGE. Follicular fluid caused little or no proteolysis of 125I-lableled IGFBP-2 or -3, and the small amount of proteolysis of IGFBP-2 and -3 did not differ (P > 0.10) among follicle classes. However, more 125I-labeled IGFBP-5 was cleaved (P < 0.05) by follicular fluid from large follicles collected during the follicular phase than large follicles during the luteal phase, and small or medium follicles from follicular and luteal phase mares indicating that a protease to IGFBP-5 exists in estrogen-dominant equine follicles. This IGFBP-5 protease was inhibited by kallikrein/serine protease and metalloprotease inhibitors. We conclude that the tendency of estrogen-dominant follicles of mares to have greater levels of IGFBP-3 and lesser levels of IGFBP-2 does not appear to be due to differences in proteolysis, whereas changes in IGFBP-5 levels are likely due to changes in activity of a serine protease or metalloprotease. Changes in IGFBP may alter levels of bioavailable IGF that stimulate steroidogenesis and mitogenesis in developing mare follicles.
Bridges
T S
TS
Department of Animal Science, Oklahoma State University, Stillwater 74078, USA.
Davidson
T R
TR
Chamberlain
C S
CS
Geisert
R D
RD
Spicer
L J
LJ
D000735:Androstenedione / Q000378:metabolism; D000818:Animals; D010450:Endopeptidases / Q000378:metabolism; D004958:Estradiol / Q000378:metabolism; D005260:Female; D015571:Follicular Fluid / Q000378:metabolism* / Q000502:physiology; D006736:Horses / Q000378:metabolism / Q000502:physiology*; D018969:Insulin-Like Growth Factor Binding Proteins / Q000378:metabolism*; D006080:Ovarian Follicle / Q000378:metabolism / Q000502:physiology*; D010060:Ovulation; D013002:Somatomedins / Q000378:metabolism*
D018969:Insulin-Like Growth Factor Binding Proteins; D013002:Somatomedins; D000735:Androstenedione; D004958:Estradiol; D010450:Endopeptidases