Comparative response of the immature and mature ovine fetus to corticotrophin-releasing hormone (CRH)

Abstract

The aim of this study was to address the possibility that the low concentrations of adrenocorticotrophin (ACTH) seen in the ovine fetus between 90 and 120 days of gestation could be attributed to an alteration in the sensitivity or responsiveness of the fetal pituitary to corticotrophin-releasing hormone (CRH), a key regulator of ACTH secretion. Chronically cannulated ovine fetuses at Days 104-108 (n = 11, representing fetuses from this 90-120-day period) and Days 138-142 (n = 6) of pregnancy received graded doses of ovine CRH (0.8, 1.6, 3.8 and 7.6 micrograms h-1 for 60 min each, given consecutively and in ascending order) or isotonic saline (n = 4 at both age groups studied). Arterial blood samples were taken concurrently for analysis of plasma immunoreactive CRH, ACTH and cortisol throughout the infusion to assess the pituitary-adrenal response. Regression lines describing the relationship between log.PCRH and log.PACTH were calculated for both age group studied. A significant (P < 0.001) rightward shift in the log.PCRH/log.PACTH regression line for the Day 104-108 group was found, suggesting that the ovine fetus at this age is less sensitive or responsive to exogenous oCRH than the mature Day 138-142 fetus. This decreased responsiveness could explain the low concentrations of endogenous ACTH seen during the 90-120-day period.