Dose-response study of intrafollicular injection of insulin-like growth factor-I on follicular fluid factors and follicle dominance in mares.

The research article discusses a study that examined how intrafollicular injections of insulin-like growth factor-I (IGF-I) affect concentrations of various substances in follicular fluid, contributing to follicular growth and ovulation in mares.

Study Overview and Methodology

• The study investigated IGF-I’s impact within follicle development and dominance in mares. Follicle deviation refers to the point when one follicle in the ovary begins to grow faster and becomes the dominant one, which will eventually ovulate.
• The procedures employed involved intrafollicular injection of IGF-I into the second-largest follicle at the anticipated start of deviation. This process was facilitated by transvaginal ultrasound.

Experiments and Findings

• Two experiments were conducted, with the first one employing a 500 mcg dosage of IGF-I, and the second one using doses of 250, 25, and 2.5 mcg. All injections were administered once.
• Results from both experiments showed that larger doses of IGF-I (500 and 250 mcg) spurred follicle growth, with higher rate of ovulation compared to control groups.
• These doses also blocked increases in IGF binding protein-2 and androstenedione (a precursor of estradiol and testosterone), while elevating concentrations of activin-A, inhibin-A, and vascular endothelial growth factor (VEGF) – all factors thought to influence follicle growth and function.
• However, it was noted that significant increase in estradiol concentration did not occur until Hour 48 post-injection of the 500-microg dose. The lower doses did not have this effect.
• In the second experiment, researchers outlined that decreasing doses of IGF-I led to progressively lower concentrations of free IGF-I in the second-largest follicle at Hour 24. However, this 2.5-microg dose led to higher IGF-I concentrations compared to control groups and similar to endogenous concentrations in dominant follicle (F1).
• Moreover, this lowest dose (2.5 microg) reduced the production of androstenedione while increasing activin-A, inhibin-A, and VEGF, mirroring the results observed in F1.

Conclusions

• In conclusion, the study results suggested that an intrafollicular dose of IGF-I does not stimulate the production of estradiol but suppresses androstenedione and boosts the creation of activin-A, inhibin-A, and VEGF during follicle selection.
• These findings might have implications in understanding the controls of follicle development and dominance, and could potentially inform the development or improvement of fertility treatments for mares.