The Effect of Progesterone on Biosynthetic Pathways in Mammary Tissue

Abstract

The hypothesis that progesterone is the sole hormone suppressing lactation during pregnancy in the rat has been tested in several ways in these experiments. At the dose frequently administered to rats (6 mg/24 hr) this steroid prevented the appearance of casein in the mammary gland, and greatly diminished the incorporation of 32PO4-2 into RNA of mammary tissue of ovariectomized rats. One effect of progesterone was to prevent the formation of RNA of both subunits of the ribosomes. Tissue from ovariectomized animals treated with this steroid had polysome profiles very similar to that obtained from mammary tissue from sham-operated pregnant rats. Functionally, polysomes from progesterone-treated ovariectomized rats had a low capacity to incorporate amino acids which corresponded closely with the capacity residing in polysomes from sham-operated pregnant subjects; animals in full lactation provided polysomes which were 2 to 3 times more active. Chronic infusion of low levels (12–48 μg/hr) of progesterone into ovariectomized rats depressed casein biosynthesis and, to a lesser degree, the biosynthesis of RNA in the mammary gland. Thus progesterone at physiological levels, if given constantly, has the capacity to prevent the onset of lactation in the rat. (Endocrinology91: 1011, 1972)