What can we learn from sampling hypophysial portal blood?

Since it has become possible to sample hypophysial portal blood from sheep without totally compromising pituitary function, several important features of the secretion of hypothalamic hormones have been elucidated. The secretion of gonadotropin-releasing hormone (GnRH) has been detailed most thoroughly with the important observation that each pulsatile discharge of luteinizing hormone (LH) is the direct result of a large secretory episode of GnRH from the hypothalamus. There is high fidelity in the GnRH relationship in terms of frequency and amplitude. During the LH surge, additional factors such as an alteration in the degree of enzymic degradation of GnRH may be important physiological mechanisms. The secretion of factors that control the release of growth hormone (GH), prolactin and adrenocorticotropic hormone (ACTH) have also been studied. Hypothalamic factors controlling GH and ACTH release do not bear such an explicit relationship to the secretory episodes of pituitary hormone as seen with the GnRH/LH axis. The factor involved in the acute stress-induced release of prolactin has not yet been identified in sheep.