Effect of Antalarmin, a Novel Corticotropin-Releasing Hormone Antagonist, on the Dynamic Function of the Preterm Ovine Fetal Hypothalamo-Pituitary-Adrenal Axis

This study describes the effect of antalarmin on basal and stimulated activity of the hypothalamo-pituitary-adrenal (HPA) axis function in the late gestation ovine fetus. Fetuses received antalarmin (15 mg/h i.v.) or vehicle (cremophor El 50% in ethanol) from day 130 gestational age. Antalarmin infusion did not significantly affect immunoreactive corticotropin (ir-ACTH) concentrations, although there was a tendency for ir-ACTH to be lower and cortisol concentrations were lower in the antalarmin-treated fetuses (p < 0.01). The ir-ACTH response to corticotropin-releasing hormone (CRH) challenge was attenuated (p < 0.05) in the antalarmin-treated fetuses, but neither antalarmin- nor vehicle-treated fetuses had significant cortisol responses to CRH. The ir-ACTH response to hypoxia was diminished (p < 0.05) in the antalarmin-treated fetuses while the cortisol responses of antalarmin- and vehicle-treated fetuses were indistinguishable. Deconvolution analysis revealed no effect of antalarmin treatment on ir-ACTH secretory dynamics. In contrast, antalarmin decreased (p < 0.05) basal, mean and integrated cortisol. The plasma cortisol responses of antalarmin- and vehicle-treated fetuses to exogenous ACTH<sub>1–24</sub> were indistinguishable. These data indicate that, while antalarmin inhibits CRH- and stress-induced ir-ACTH secretion, basal ir-ACTH secretion may be less affected by antalarmin treatment. Paradoxically, cortisol secretion is impaired by antalarmin infusion, although adrenal responsiveness to ACTH is not impaired. These results confirm a role for CRH in stress-induced ACTH secretion in the ovine fetus, though its role in the regulation of basal ACTH and cortisol secretion is unclear.