Studies of the Secretion of Corticotropin-Releasing Factor and Arginine Vasopressin into the Hypophysial-Portal Circulation of the Conscious Sheep

Abstract

Studies were undertaken to characterize the secretion of corticotropin-releasing factor (CRF) and arginine vasopressin (AVP) into the hypophysial-portal circulation of the conscious sheep. In addition, we examined the temporal relationship between the secretion of these two hypothalamic peptides and the secretion of three pro-opiomelanocortin peptides – adrenocorticotropic hormone (ACTH), ir-β-endorphin, and ir-α-melanocyte-stimulating hormone – and cortisol and determined the effects of an audiovisual emotional stimulus and insulin-induced hypoglycemia on the entire hypothalamic-pituitary-adrenal axis. In the basal state, the secretion of CRF, AVP, the three pro-opiomelanocortin peptides, and cortisol was pulsatile in nature, and three CRF and AVP pulse patterns were observed: a concordant increase in CRF and AVP, an isolated rise in CRF, and an isolated increase in AVP. In 4 of the 5 animals, a 3-min audiovisual stress (barking dog) rapidly increased the plasma levels of all the measured substances, although the magnitude and duration of the effect differed markedly between the animals. Insulin-induced hypoglycemia markedly increased AVP and, to a lesser extent, CRF concentrations in portal plasma and thereby altered the CRF:AVP molar ratio. Although pituitary-adrenal activation was closely correlated with the increased hypothalamic activity, a strict 1:1 concordance between CRF/AVP secretion and ACTH secretion was not seen. The anesthetic ketamine selectively increased portal AVP concentrations to levels which exceeded those attained during hypoglycemia and rapidly activated the pituitary-adrenal axis. We conclude the following: (1) CRF and AVP are secreted by the hypothalamus in a pulsatile fashion; (2) ACTH secretion can be stimulated by increases in either CRF or AVP; (3) the absence of a strict 1:1 concordance between hypothalamic CRF/AVP release and pituitary ACTH secretion during stress may be partly due to the release of additional hypothalamic ACTH secretagogues; (4) the ability of both audiovisual stimuli and insulin-induced hypoglycemia to augment CRF and AVP secretion indicates that the paraventricular hypothalamus may be activated by a variety of neural inputs, and (5) the marked alteration of the CRF:AVP molar ratio during stress suggests that AVP may be an important ACTH secretagogue in vivo in the sheep.