HETEROGENEITY OF THE PROLACTIN RECEPTOR IN THE RAT MAMMARY-GLAND AND LIVER DURING VARIOUS PHYSIOLOGICAL STATES

This work was undertaken to determine variations in the I-125-labelled ovine prolactin (oPRL) binding in rat liver and mammary gland membranes, and to study the molecular forms of prolactin receptor in different physiological situations. Prolactin binding was determined using I-125-labelled oPRL in the 100 000 g pellet. I-125-Labelled oPRL, was cross-linked to receptors in membranes from rat liver and mammary gland and subjected to SDS-PAGE under reducing conditions, followed by autoradiography of dried gels. In the mammary gland, the specific binding of oPRL to membranes, expressed as mean+/-S.E.M. fmol/mg protein increased from 1.36+/-0.24 on the day of dioestrus to 3.26+/-0.60 on the day of oestrus. It remained very low during pregnancy but increased during lactation to reach 4.78+/-0.99. In the Liver, the specific binding of oPRL to membranes was higher than in the mammary gland on the days of dioestrus 1, dioestrus 2 and oestrus, but not on the day of pro-oestrus. It increased until day 14 of pregnancy when the specific binding of I-125-labelled oPRL was 17.01+/-0.30. Cross-linking revealed different molecular forms in the mammary glands and the liver. In the mammary gland we observed four prolactin-binding form of 80, 50, 30 and 16 kDa, all of which were specific for prolactin. The 80, 50 and 40 kDa forms were also able to bind to a Concanavalin A-Sepharose column, indicating that these binding form were glycosylated while the smaller one (16 kDa) appeared to be unglycosylated. The 40 kDa prolactin receptor was seen at all stages studied: the oestrous cycle (dioestrus, pro-oestrus and oestrus), pregnancy (days 8, 14 and 22) and lactation. The 50 kDa form was observed in the mammary gland during the day of pro-oestrus and gestation. It was also observed in ovariectomized rats treated with oestradiol (OE(2)), suggesting that OE(2) could be one of the factors involved in the induction of this receptor form. The 16 kDa form of the receptor was found in the mammary gland only during lactation. This form, while unglycosylated, bound specifically to prolactin, suggesting that it might play a specific role during lactation. In the liver, two forms were shown, a major one of 40 kDa and a minor one of 80 kDa. No variation in receptor molecular weight was found in the liver during the physiological stages studied.