Hypothalamo-pituitary disconnection of the late-gestation ovine fetus results in profound changes in cortisol secretion that are not reflected in commensurate changes in adrenocorticotropin secretion

A prepartum increase in fetal glucocorticoid concentrations is essential for the perinatal transition to extrauterine life for many mammalian species. In the case of the sheep, this increase in cortisol is also the trigger for parturition, and depends upon an intact hypothalamo-pituitary unit. Fetal sheep that have undergone hypothalamo-pituitary disconnection (HPD) fail to have a prepartum cortisol surge or initiate labor, despite apparently normal fetal ACTH concentrations in late gestation. We have investigated whether a defect exists in the regulation of pulsatile neurohormone secretion in the pituitary-adrenal axis of the HPD sheep fetus, by comparing immunoreactive (ir) ACTH and cortisol secretory dynamics in intact and HPD fetuses at 126 and 145 days of gestation (normal gestation length, 147 days). The fetal surgery was conducted at 115 days of gestation. Blood samples were collected at 5-min intervals for 2 h on each experimental day, and the resulting irACTH and cortisol concentrations were analyzed by multiple-parameter deconvolution and cross-correlation analysis. Basal irACTH secretion was less (P < 0.01) in HPD fetuses than intact fetuses at 126 days, but it had recovered by 145 days. There were no differences in irACTH half-life or the number or duration of irACTH secretory bursts between the two group~ of fetuses or the two gestational ages (GAs). The size of the irACTtl secretory bursts was not affected by the operation, but it increased with GA to a similar extent in both groups of fetuses (P < 0.01). In keeping with the observations for irACTH secretion, there was no effect of age or the operation on cortisol half-life or on the number or duration of cortisol secretory bursts. In contrast, there were dramatic age-related increases (P < 0.01) in the basal cortisol secretion rate and the size of the cortisol secretory bursts in the intact, but not the HPD, fetuses. Cross-correlation analysis revealed a significant (P < 0.01) concordance between irACTH and cortisol secretion in only the intact fetuses at 126 days; this was not apparent in the intact fetuses at 145 days, or in the young or old HPD fetuses. These findings confirm a major defect in cortisol secretion in the late-gestation HPD fetus but suggest that this is not caused by defects in irACTH secretion. Together with other observations, these data suggest that ACTH may not be the sole, or primary, regulator of adrenal cortisol secretion in the late-gestation ovine fetus.