Long-term hypoxia enhances ACTH response to arginine vasopressin but not corticotropin-releasing hormone in the near-term ovine fetus

Abstract

This study tested the hypothesis that long-term hypoxia (LTH) results in enhanced fetal corticotrope sensitivity to the ACTH secretagogues, corticotropin-releasing hormone (CRH), and AVP. Ewes were maintained at high altitude (3,820 m) from 40 to 130–131 days of gestation. Upon return to the laboratory, hypoxia was maintained by maternal nitrogen infusion. Vascular catheters were placed in both LTH ( n = 4) and normoxic controls ( n = 4). Each fetus received a 15-min infusion of either saline, 100 ng/kg of ovine CRH, or 20 ng/kg of AVP/min over 3 consecutive days in a randomized order. Fetal blood samples were collected at 0, 15, 30, 60, and 90 min after the start of infusion and analyzed for ACTH1-39, ACTH precursors, and cortisol. Anterior pituitaries were collected from additional noninstrumented fetuses for analysis of vasopressin receptor 1b (V1b) mRNA and protein. Basal plasma concentrations of both ACTH1-39and ACTH precursors were higher in LTH fetuses and were not altered by saline infusion. In response to CRH, ACTH1-39increased in both groups and was higher in the LTH group compared with control ( P < 0.05). When analyzed as sum of ACTH1-39released (Δ0–90 min) above basal, CRH released equal amounts of ACTH1-39in both groups. In LTH fetuses, AVP evoked a greater ACTH1-39release ( P < 0.05) when analyzed as an increased sum of ACTH1-39(Δ0–90 min) above basal. Both CRH and AVP elicited a release of ACTH precursors with no differences observed between LTH and control. AVP and CRH elicited significant increases in cortisol, which were higher in response to AVP than CRH. V1b mRNA and protein were elevated in the anterior pituitary of LTH fetuses compared with control. LTH significantly increases pituitary sensitivity to AVP. This enhanced sensitivity may be a mechanism of our previously observed enhanced corticotrope function.