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

Abstract

Milk secretion can be induced in vivo in the pseudopregnant rabbit by the injection of prolactin. When progesterone was injected simultaneously with prolactin, secretion was suppressed. The steroid depressed the increased lactose synthetase (EC 2.4.1.22) activity and the lactose synthesis which can be induced by prolactin in mammary tissue from pseudopregnant rabbits. However, the galactosyl transferase activity was only slightly decreased. The modification induced by prolactin in the polyribosome profile and the in- creasein the ratio ofmembrane-bound to free polyribosomes was also completelyabolished by progesterone. Measurement of the nucleic acid content of the mammary gland showed that the administration of progesterone suppressed the increase in RNA content which is induced by prolactin. These results suggest that progesterone inhibits the lactogenic action of prolactin without affecting its mammogenic role.