COMPARATIVE RESPONSE OF THE IMMATURE AND MATURE OVINE FETUS TO CORTICOTROPIN-RELEASING HORMONE (CRH)

The aim of this study was to address the possibility that the low conc entrations of adrenocorticotrophin (ACTH) seen in the ovine fetus betw een 90 and 120 days of gestation could be attributed to an alteration in the sensitivity or responsiveness of the fetal pituitary to cortico trophin-releasing hormone (CRH), a key regulator of ACTH secretion. Ch ronically cannulated ovine fetuses at Days 104-108 (n=11, representing fetuses from this 90-120-day period) and Days 138-142 (n=6) of pregna ncy received graded doses of ovine CRH (0.8, 1.6, 3.8 and 7.6 mu g h(- 1) for 60 min each, given consecutively and in ascending order) or iso tonic saline (n=4 at both age groups studied). Arterial blood samples were taken concurrently for analysis of plasma immunoreactive CRH, ACT H and cortisol throughout the infusion to assess the pituitary-adrenal response. Regression lines describing the relationship between log.P- CRH and log.P-ACTH were calculated for both age groups studied. A sign ificant (P<0.001) rightward shift in the log.P-CRH/log.P-ACTH regressi on 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 t han the mature Day 138-142 fetus. This decreased responsiveness could explain the low concentrations of endogenous ACTH seen during the 90-1 20-day period.