Ghrelin suppresses the GnRH-induced preovulatory gonadotropin surge in dairy heifers

Abstract Ghrelin, a known growth hormone (GH) secretagogue, alters gonadotropin secretion in many species. Our objectives were to study the effects of ghrelin, on GH, LH, FSH secretion, and on luteal function of the ensuing estrous cycle in cattle. The estrous cycles of eight heifers were synchronized with progesteron releasing intravaginal device, and ovulation was induced with GnRH. Eight animals were treated with 1.5 μg kg −1 bovine ghrelin (group Ghr, n = 4) or saline (group C, n = 4). Starting with the first ghrelin injection, 13 blood samples were collected over a 4-hour period for the determination of ghrelin, GH, LH, and FSH concentration. Progesterone levels were measured in samples collected every other day after estrus expression. Data were analyzed by repeated measures of ANOVA followed by Bonferroni post hoc testing and t test. In group Ghr, ghrelin concentration increased significantly 15 minutes after the first injection and remained in elevated levels until the 90th minute after the last injection. At the time of third ghrelin injection, GH was significantly higher in the Ghr group compared with C (17.1 ± 1.3 vs. 2.6 ± 0.3 ng mL −1 , P