A study of the effects of corticotrophin-releasing factor and dexamethasone on the secretion of the pituitary hormone corticotrophin (ACTH) in cultured ovine pituitary cells

Abstract

It was the aim of this study to determine the ways in which the secretion of the pituitary hormone ACTH was affected by corticotrophin-releasing factor ("CRF") and dexamethasone ("DEX") in cultured ovine pituitary cells.

Only recently has the isolation, sequence and characterisation of ovine hypothalamic CRF been reported. The characterisation of a CRF that stimulated the secretion of corticotrophin-like and B-endorphin like immunoreactivities in vitro and in vivo was certainly a major discovery, and indeed remarkable for it was, among other things, the identification of the chemical that triggers the body’s stress reactions. Its identification has, in fact, led to a much better understanding of the endocrine response to stress. As described by Fink (1981) one more factor, possibly the most vital, has been isolated.

It was Vale, Speiss, Rivier and Rivier who first reported the molecular structure of the releasing factor, in 1981 at the Salk Institute. Direct evidence for an ACTH releasing factor was initially reported by Guillemin and Rosenberg and Saffran and Schally in 1955. Although it was the first of the pituitary releasing factors to be described its early history was marked by controversy. Attempts to isolate the CRF in its pure state were hampered by the complexity of assays, interference by vasopressin and similar ACTH releasing substances, the loss of activity during purification, and other reasons (Chang, Huang, Arimura, Redding, Coy, Saffran, Kong, Hamilton, Cohn and Schally 1981). The breakthrough by Vale and his team in 1981 with their purification, sequence analysis and total synthesis of the 41 residue peptide was therefore an important advance.

Since the determination of the primary structure of CRF and its subsequent synthesis, it has been found to exhibit high potency and intrinsic activity to stimulate ACTH release both in vivo and in vitro in a number of species including man, rat, sheep and cattle. CRF is thus well documented as the principal neuroregulator of the secretion of ACTH and other proopiomelanocortin products of the anterior pituitary gland (Rivier, Rivier and Vale 1984).

It is also well established that glucocorticoids exert inhibitory feedback effects in the physiological control of ACTH secretion (Yates and Maran 1974; Fortier 1966). They are potent inhibitors of ACTH release (Vale and Rivier 1977; Raymond, Lepine, Lissitsky, Cote, Coy and Labrie 1979; Kraicer and Milligan 1970) in both man and experimental animals, (Fleischer and Rawls 1970). The site of their blocking action however, whether central or at the pituitary itself, has been the subject of controversy.

The relatively recent appearance of CRF, and its importance in the clinical sphere, (already it has been established as a useful tool of endocrine evaluation in patients with hypothalamic, pituitary disorders Orth, DeBold, De Cherney, Jackson, Alexander, Rivier, Rivier, Speiss and Vale 1982; Muller, Stalla and Vonwerder 1983), make CRF an interesting hormone to investigate.

Dexamethasone was selected for its well known affects on the cells in question and static culture chosen because of its value as a model for in vivo events.

Despite the established facts about a number of aspects of this study, the combination of experimental variables i.e. the selection of in vitro static culture techniques as opposed to in vivo studies, and the use of ovine cells rather than the commonly utilized rat pituitary cells, made it a unique investigation.

At the outset of this project, therefore, the general effects of CRF and dexamethasone in cultured rat adenohypophyseal cells were well documented but little was known of their activity in cultured ovine anterior pituitary cells.