Pro-opiomelanocortin mRNA levels fall in the fetal sheep pituitary before birth.

In order to clarify the corticotrophic capacity of the fetal sheep anterior pituitary in late gestation, we have measured the relative levels of messenger RNA for the ACTH precursor molecule pro-opiomelanocortin (POMC) in individual fetal sheep anterior pituitaries collected between 100 and 144 days of gestation. The mean relative POMC mRNA:poly(A)+ RNA ratio of the pituitary glands collected between 100 and 135 days (1.35 +/- 0.15) was significantly greater than the mean relative POMC mRNA:poly(A)+ RNA ratio of the pituitaries collected between 141 and 144 days (0.81 +/- 0.09). Northern blot analysis showed that a single band of RNA hybridized with the human POMC cDNA probe in adult and fetal sheep pituitaries. Our results do not contradict the hypothesis that an increase in basal ACTH concentrations after 140 days of gestation could reflect a change in the post-translational processing of POMC in the fetal sheep anterior pituitary.