EFFECTS OF PROGESTERONE AND PROLACTIN ON THE SECRETORY ACTIVITY AND THE NUCLEIC ACID CONTENT OF THE MAMMARY GLAND OF PREGNANT RABBITS

Progesterone injected daily at different stages of pregnancy in the rabbit, reduces or temporarily blocks mammary secretion measured on the 24th day either by the incorporation of [14C] glucose into lactose or by the [12C] lactose content of mammary tissues. Progesterone does not affect the number of mammary cells, as determined by the DNA content of the mammary gland, while the quantity of RNA is only reduced if the hormone is injected from the beginning of pregnancy. These results suggest that progesterone has an inhibitory effect on secretory activity, which mainly takes place during the first two thirds of pregnancy.From day 19, twice daily administration of 12.5 IU of prolactin stimulates milk secretion and increases the content of nucleic acids measured on the 24th day. Progesterone (5 mg twice daily) does not prevent these lactogenic effects, nor does it modify the increase in uncleic acids caused by prolactin treatment. Despite the continued presence of progesterone, prolactin has the same qualitative properties in pregnant rabbits deprived of their gonads or pituitary as it has in intact rabbits. Hence prolactin plays an essential role in the mammogenesis and lactogenesis occurring during pregnancy in the rabbit.In addition, our studies show that ovarian secretions other than progesterone are effective under normal conditions of pregnancy, since the administration of progesterone in ovariectomized pregnant rabbit does not allow normal mammary growth despite the maintenance of pregnancy.