Specific 125I-Prolactin Binding in the Mouse Mammary Gland during Pregnancy and Lactation

Abstract

The level of specific ¹²⁵I-prolactin binding to mammary tissues was lowered by a large amount of endogenous prolactin. Weaning or preincubation of tissues minimized this effect on underestimation of the number of binding sites. Scatchard analysis showed that specific ¹²⁵I-prolactin bindings to lactating mammary tissues were correlated highly with the number of binding sites, poorly to the dissociation constant (Kd) for the prolactin binding. Amounts of specific ¹²⁵I-prolactin bindings increased slowly as pregnancy progressed but still remained small, increased significantly shortly after parturition and were maintained at high levels until the end of lactation. Both the lactose content and the RNA/DNA ratio changed similarly to the change in the specific ¹²⁵Iprolactin binding during pregnancy and at lactogenesis. Contrarily serum concentration of lactogenic hormones from placenta and pituitary increased rapidly from 55ng/ml on day 6 to 528ng/ml on day 8 of pregnancy and were maintained at higher levels than 135ng/ml through the latter half of pregnancy. The present results suggest that the increase in prolactin receptors accompanying parturition is one of the triggers for lactogenesis. Jpn.