Inhibition by progesterone of the lactogenic effect of prolactin in the pseudopregnant rabbit.

The effects of progesterone on lactose synthesis activity and changes in mammary gland cells were studied in pseudopregnant rabbits simultaneously treated with prolactin. The injection of progesterone alone on Days 15 and 17 of pseudopregnancy decreased the activity of lactose synthetase (LSA) and galactosyl transferase (GTA), while the administration of prolactin for 2-4 days increased their activities. Th e simultaneous administration of progesterone and prolactin decreased the increase in LSA observed with prolactin alone by 70% on the 4th day of treatment, and decreased GTA by 30%. Progesterone completely suppressed the polyribosome profile and the ratio of endoplasmic reticulum bound polyribosomes to free polyribosomes induced by prolactin. The increase in RNA content in the mammary gland induced by prolactin was also suppressed by progesterone. The results suggest that progesterone inhibits the lactogenic action of prolactin without interfe ring with its mammogenic role.