Proopiomelanocortin, prolactin and growth hormone messenger ribonucleic acid levels in the fetal sheep pituitary during late gestation.

We have measured the relative levels of proopiomelanocortin (POMC), prolactin (PRL) and growth hormone (GH) messenger ribonucleic acid (mRNA) in the anterior and neurointermediate lobes of the fetal pituitary during the last 2-3 weeks of gestation. The mean POMC mRNA/18S RNA ratio in the fetal anterior pituitary was significantly greater (p < 0.02) at 130-136 days (0.90 +/- 0.08; N = 9) than at 141-143 days of gestation (0.67 +/- 0.07; N = 6). In contrast, the mean PRL mRNA/18S RNA ratio increased significantly (p < 0.02) between 130 and 136 days (0.31 +/- 0.05; N = 9) when compared with 141-143 days of gestation (0.58 +/- 0.10; N = 6). There was no significant difference, however, between the mean GH mRNA/18S RNA ratio in fetal anterior pituitaries at 130-136 days (0.95 +/- 0.04; N = 9) when compared with 141-143 days of gestation (1.08 +/- 0.14; N = 6). The POMC mRNA/18S RNA ratio in the neurointermediate lobes was seven-, five- and tenfold higher than anterior pituitaries at 130-134, 135-136 and 141-143 days of gestation, respectively. We hypothesize that elevated circulating cortisol levels after 140 days of gestation act in the slow time domain (i.e. over days) to suppress POMC gene expression and that the increase in fetal pituitary PRL mRNA levels may be a consequence of oestrogen stimulation in late gestation.