Hypothalamic Growth Hormone-Releasing Activity (GRA): Characterization by thein VitroRat Pituitary and Radioimmunoassay12

Abstract

Growth Hormone-Releasing Activity (GRA) in extracts derived from rat and porcine hypothalami was investigated by incubation with rat hemi-pituitaries in vitro. Growth Hormone (GH) in the medium was determined by double antibody radioimmunoassay. After gel nitration in G-2S fine beaded Sephadex, peak GRA was found in fractions corresponding closely in exclusion volume to ¹⁴C-Thyrotropin Releasing Hormone (TRH), Gonadotropin Releasing Hormone (Gn-RH), and Vasopressin. Immunoassay determinations of vasopressin in the most active rat fractions revealed concentrations of 0.76 and 8.6 milliunits/ml, insufficient to account for the observed stimulation of GH release. Control studies with Synthetic TRH and highly purified Gn-RH indicated that these neural hormones were not responsible for GH releasing activity. It is concluded that a growth hormone releasing substance (or substances) has been identified which shows an elution volume on Sephadex G-25 considerably greater than that described by Schally et al. (1). The probable basis for the discrepancy between the present results and those obtained previously by means of a biologic assay of ‘pituitary GH depletion’ is discussed. (Endocrinology89: 1419, 1971)