Follicular Dynamics in Insulin Resistant Mares

Obesity and insulin resistance have been linked to prolonged interovulatory period, aberrations in the estrous cycle, and continuous reproductive activity during the non-breeding season. EMS has been determined to influence the intrafollicular environment of mare ovaries. In humans, insulin resistance has been linked to polycystic ovaries as part of Polycystic Ovarian Syndrome (PCOS). A study was conducted to determine 1) the impact of insulin resistance on follicle growth and size at ovulation, and 2) whether predicted ovulatory follicles respond to hCG administration in Insulin-resistant (IR) mares. Mares were selected for the study based on insulin sensitivity and separated into an IR group (n=6) and a IS group (n=6); their ovaries and uterus were examined via ultrasound at regular intervals during a spontaneous cycle and a PGF2a shortened synchronized cycle. The dominant follicles (F1) had similar size and F1 size at ovulation between groups. Insulin-resistant mares had more subordinate follicles than Insulin-sensitive (IS) mares (P < 0.05). The second largest follicle (F2) of IR mares was larger in diameter (P < 0.05) than the F2 of IS mares which may signify a lack of dominance by the largest follicle. Administration of Human Chorionic Gonadotropin (hCG) injections induced ovulation before 48 h in 2 of 4 IR mares while inducing ovulation on all (n=4) IS mares although difference in time to ovulation after hCG administration did not differ statistically. Results observed in this study may provide caution to practitioners working with insulin resistant mares with regards to numbers and sizes of secondary follicles and the effectiveness of hCG for induction of ovulation. The results of this study may support information that the mare could be used as a model to study human ovarian pathologies.