Effect Of Locally Implanted Estrogen On Response Of Rats Lactating Mammary-Gland To Oxytocin

On the day following delivery (day 1 of lactation) one abdominal mammary gland was implanted with oestrogen and the contralateral gland received an empty needle. At 2, 5 or 10 days of lactation the rats were anaesthetized with pentobarbital and the nipples of both abdominal glands were cannulated and their pressures recorded by means of transducers coupled to an amplifier and recording system. The normal mammary glands of 5-day lactating rats responded to very low doses of oxytocin (Syntocinon®, Sandoz) (5× 10−8 mU) with a rhythmic elevation in pressure. However, saline infusion also evoked a small rise in intra-mammary pressure. Earlier (2 days) and later (10 days) in lactation the responses were smaller. Oestrogen decreases significantly the milk ejection response to oxytocin, and the effect was maximal at day 10 of lactation. Histological observations confirmed the diminished reaction of the gland to oxytocin, since the milk was retained in the alveoli of rats bearing a mammary-oestrogen implant. A paradoxical rise in pressure was detected in normal as well as in oestrogen-implanted glands when the lowest dose of oxytocin was injected in lactating rats which had previously received a high dose of oxytocin (50 mU or 500 mU). These results reinforce the hypothesis that oestrogen alters the milk ejection response to oxytocin and that the mechanism is probably related to changes in the contractility of the myoepithelial cells.