Concentrations of insulin-like growth factor-I in follicular fluid and blood plasma of mares during early and late oestrus

Abstract

To assess the relationship (s) among concentrations of insulin-like growth factor-I (IGF-I) in blood plasma and ovarian follicular fluid (FFL), and concentrations of steroids in FFL of individual preovulatory equine follicles, FFL and blood plasma were collected simultaneously from mares between Days 1 and 6 of oestrus. Follicles were placed into two groups: early oestrus (EE) (Day 1 or 2 of oestrus, n = 12) and late oestrus (LE) (Day 3–6 of oestrus, n = 10). LE follicles were larger and had greater ( P < 0.05) concentrations of oestradiol, progesterone and testosterone than EE follicles. Androstenedione and IGF-I did not differ between EE and LE follicles. Concentrations of IGF-I were greater in plasma than FFL (579 vs. 429 ng ml −1) and did not differ in plasma of EE and LE mares. The ratio of FFL: plasma IGF-I tended ( P < 0.15) to be greater in LE than EE follicles (86 vs. 69%). IGF-I levels in FFL correlated with follicular size ( r = 0.51, P < 0.02) but were not correlated ( P > 0.10) to any steroid in FFL. However, the ratio of FFL: plasma IGF-I was correlated with FFL oestradiol ( r = 0.75, P < 0.001) and FFL androstenedione ( r = 0.43, P < 0.05). These results support the hypothesis that changing permeability of follicles to plasma IGF-I may allow for increased IGF-I levels in follicles which, in turn, may enhance follicular differentiation.