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Influence of Large Doses of Oxytocin on Milk Ejection and Metabolic Rate of Rat Mammary Gland

Department of Biology, Zoological Institute, Laboratory of Endocrinology, Catholic University of Leuven, Naamsestraat 61, B-3000 Leuven, Belgium

ABSTRACT

Milk ejection activity and milk synthesis were studied in primiparous lactating Wistar rats (9 to 12 days postpartum) 24 h after a single intraperitoneal injection of 10 IU of oxytocin or 1 ml saline. In a first series of experiments litters were allowed to suckle during this period. Gain in litter weight was blocked completely in the group treated with oxytocin with a decrease in oxygen consumption of mammary gland as compared with saline controls. However, lactose, glucose, and total sugar content did not differ between groups. In a second series of experiments pups were taken away immediately after injections. Weaning 24 h decreased oxygen consumption of mammary gland in saline controls. Weaning together with injection of oxytocin caused an even more pronounced decrease. Lactose content of the weaned control group was lower than in the suckled control group and decreased after injection of oxytocin. In vitro milk ejection of small pieces of mammary gland also was completely inhibited in the oxytocin group. Prolactin administration to the oxytocin group increased both lactose and total sugar content of the gland. A pronounced increase in in vitro milk ejection activity in the oxytocin group followed prolactin treatment. Milk ejection in vivo, as measured by intra-mammary pressure, also was drastically inhibited by injection of oxytocin. In contrast to the in vitro experiments prolactin injections did not remove this inhibition, though lactose concentrations of mammary gland increased.