Effects of labor on pituitary expression of proopiomelanocortin, prohormone convertase (PC)-1, PC-2, and glucocorticoid receptor mRNA in fetal sheep

We hypothesized that the concurrent prepartum rise in adrenocorticotropic h ormone (ACTH) and cortisol in the plasma of fetal sheep might be attributab le to altered expression of pituitary endoproteases, prohormone convertase (PC)-1, and PC-2, or to changes in pituitary expression of glucocorticoid r eceptor (GR) that would influence negative feedback potential. We obtained pituitary tissue from fetal sheep during late pregnancy (d 100-d 145, term) and at precise times during the process of labor and used in situ hybridiz ation to localize and quantify mRNA levels. Proopiomelanocortin (POMC) mRNA was regionally distributed (pars intermedia > inferior pars distalis > sup erior pars distalis) and increased within the pars distalis during late pre gnancy and with labor, At term, levels of PC-1 and PC-2 mRNA were higher in the pars intermedia than pars distalis; PC-1 but not PC-2 in the pars dist alis increased with gestational age, although it did not change further at labor. GR mRNA levels in the pars distalis increased between d 135 and term , then decreased during labor. We suggest that the concomitant rise in plas ma ACTH and cortisol of fetal sheep during late gestation may be attributab le, in part, to increased expression of PC-1 leading to increased POMC proc essing. Furthermore, the negative feedback effects of cortisol on pituitary POMC synthesis and/or ACTH release during active parturition may be lessen ed by downregulation of anterior pituitary GR.