Hormonal regulation of casein synthesis at the end of pregnancy.

Ovariectomy or ovariohysterectomy on day 18 of pregnancy augmented mammary beta-casein content 28 h later. Progesterone injected immediately and 12 h after ovariectomy showed a clear inhibitory effect on casein synthesis. Estrogen induced a significant increase in mammary beta-casein content when injected 12 h after surgery. Treatment with CB-154 to prevent prolactin release did not affect the increase of casein induced by ovariectomy. When CB-154 was injected to ovariohysterectomized pregnant rats, significant reduction of casein synthesis was obtained. According to these findings, rat placental lactogen in the absence of prolactin and progesterone induces beta-casein synthesis. Therefore prolactin, ovarian and placental hormones interplay at the end of pregnancy for full expression of the mammary gland genome.