The effects of equine somatotropin (eST) on follicular development and circulating plasma hormone profiles in cyclic mares treated during different stages of the estrous cycle

The effects of exogenous equine somatotropin (eST) administration on ovaria n activity and plasma hormone levels were evaluated on horse and pony mares . The objectives of this study were to determine the effects of eST on foll icular development and circulating concentrations of leutinizing hormone (L H), estradiol, progesterone, and insulin-like growth factor I (IGF-I) in cy clic horse and pony mares. Sixteen mares received daily injections (im) of eST at a concentration of 25 mu g/kg body weight on either Days 6 through 1 2 (Treatment A) or 13 through 19 (Treatment B) postovulation. In addition, contemporary mares were similarly given the carrier vehicle and served as c ontrols (Treatments C and D). Blood samples were collected at 24-hr interva ls and ultrasonographic evaluations were performed on the ovaries of each m are at 48-hr intervals beginning on the first day of treatment and ending e ither on the day of ovulation or 5 d postovulation. Circulating levels of i nsulin-like growth factor-I (IGF-I) were increased in treated mares by Day 3 post-treatment (P < 0.05). Also, mares in Treatment B exhibited a decreas e in plasma estradiol concentrations (P < 0.05) when compared with control mares on Days 1 through 5 postovulation of the post-treated estrous cycle. In addition, circulating leutinizing hormone levels were different for mare s in Treatment A compared with controls on Days -8 through -1 pre-ovulation (P < 0.05). All follicles present on the ovaries of each mare were measure d and placed into one of five categories based on their diameter. Neither t he mean number of follicles per size category greater than or equal to 8 mm in diameter nor the mean follicular diameter within each size category dif fered among treatment and control mares. However, eST treatment significant ly increased the number of follicles less than or equal to 7 mm on the ovar ies of mares treated early in the estrous cycle when compared with control mares on Days 3 and 7 post-treatment and at the onset of standing estrus. ( C) Elsevier Science Inc. 1999.