Restriction of fetal growth has a differential impact on fetal prolactin and prolactin receptor mRNA expression

Prolactin is present in the fetal circulation and prolactin receptors are e xpressed in a wide range of fetal tissues. The factors which regulate the s ynthesis and secretion of prolactin, and the expression of its receptors be fore birth, are poorly understood. We have investigated whether experimenta l restriction of placental growth in the sheep has an impact on the prolact in axis in the growth restricted fetus. The majority of uterine endometrial caruncles were removed before pregnancy in 10 ewes (placental restriction; PR group). Placental, fetal liver and kidney weights were reduced in the P R compared to the control group (n =10). The ratio of fetal prolactin mRNA: 18S rRNA was significantly lower (P <0.01) in the PR group (1.83 +/- 0.45, n = 6) than in the control group (4.11 +/- 0.54, n = 6), The ratio of prola ctin mRNA: 18S rRNA in the fetal pituitary was positively correlated with f etal and with placental weight. Using stepwise linear regression, it was de termined that the level of fetal prolactin mRNA:18S rRNA expression was bes t described (as judged by the maximum adjusted R-2) by prolactin mRNA, 18 S rRNA = -3.0378 + 0.17 PO2+2.772 glucose (adjusted R-2 = 0.765, F = 17.53, P < 0.001), Fetal plasma prolactin concentrations were significantly reduce d (P <0.05) in the PR group compared to control animals between 109 and 141 days gestation. Fetal prolactin receptor (PRLR) mRNA transcripts encoding long (PRLR1) and short forms (PRLR2) of PRLR were present in the liver and kidney of animals in the PR and control groups at 140-141 days gestation. P R did not alter the levels of PRLR1 or PRLR2 mRNA in the fetal liver or kid ney. The suppression of the synthesis and secretion of prolactin in the gro wth restricted fetus may limit the action of prolactin on the growth and me tabolism of key fetal organs during suboptimal intrauterine conditions.