Changes in Messenger Ribonucleic Acid Concentrations and Plasma Levels of Growth Hormone during the Ovine Estrous Cycle and in Response to Exogenous Estradiol*

The role of GH during the ovine estrous cycle is unknown. In this study, pituitary GH mRNA and peripheral serum GH levels were measured in ewes representing the major phases of the estrous cycle. There was a significant rise (P less than 0.05) in both GH mRNA and serum GH concentrations during the follicular phase of the cycle, occurring near the peak of the LH/FSH surge. Subsequently, these parameters were also measured in untreated and estradiol-treated anestrous ewes. This estradiol treatment is known to elicit a characteristic gonadotropin surge within 24 h after insertion of the estradiol implants. The estradiol-treated ewes demonstrated a serum GH surge (P less than 0.05) approximately 22 h after implantation with estradiol. However, no change was observed in GH mRNA amounts in response to the treatment, suggesting, perhaps, a different mechanism for the GH surge in these ewes vs. the surge during the normal estrous cycle. Additionally, these results are consistent with the hypothesis that the effects of GH on ovarian follicular development and maturation, previously shown in rats and humans, may also be present in the sheep. Furthermore, this may represent part of an endocrine feedback system that involves metabolic effects on reproduction.