Adrenocorticotrophic hormone (ACTH) stimulation of sheep fetal adrenal cortex can occur without increased expression of ACTH receptor (ACTH-R) mRNA

Abstract

In the present study, it was hypothesized that the adrenocorticotrophin hormone receptor (ACTH-R) would be up-regulated in the adrenal gland of the sheep fetus following infusion of physiological amounts of ACTH, as shown for adrenal cortical cells in culture. In chronically catheterized sheep, an intravenous infusion of ACTH_1–24 was given to 6 fetuses for 24 h at a rate of 0.5 g h^–1, starting on Day 126 or 127 of gestation (term ~147 days). Four control fetuses received an infusion of vehicle (saline). Total RNA was extracted from the fetal adrenal glands by the guanidinium thiocyanate method. Expression of specific mRNAs was determined by ribonuclease protection assay using cRNA probes directed against: ACTH-R; the steroid enzymes side-chain cleavage (P450scc), 3β-hydroxysteroid dehydrogenase (3β-HSD), 17 α-hydroxylase (P450c17) and 21β-hydroxylase (P450c21); and β-actin. Ratios of mRNA expression to β-actin mRNA expression (arbitrary units) were calculated to correct for differences in RNA quality between samples. The concentration (mean SEM) of immunoreactive cortisol in fetal plasma was greater after ACTH infusion than after vehicle infusion (47 3 v. 13 2 ng mL^–1 respectively; (P<0.001). Adrenal expression of P450scc and P450c21 mRNA increased after ACTH infusion (P<0.05), whereas expression of P450c17 and 3β-HSD mRNA was unchanged. There was no difference in ACTH-R mRNA expression between ACTH- and vehicle-infused fetuses (254 48 v. 305 76 arbitrary units respectively). It was concluded that ACTH is able to increase plasma cortisol concentrations in the sheep fetus by up-regulating cortisol synthesis in the adrenal gland, but that in vivo this does not require up-regulation of ACTH-R mRNA.