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The present paper describes a method of membrane preparation from ewe mammary gland using a sucrose cushion (1.3 M) to select smooth membranes; this results in a membrane preparation richer in PRL receptors than the microsomal preparation classically used. This method was used for the characterization and measurement of PRL and ovine placental lactogen (oPL) receptors in three organs of the ewe (mammary gland, liver, and adipose tissue). PRL receptors were measured by competition of iodinated human GH ([125I]hGH) with ovine PRL (oPRL). This hormone, which has both growth and lactogenic activities, appears to interact with PRL receptors with a higher affinity than oPRL itself and is a good probe for the determination of PRL receptors in the ewe. oPL receptors were measured by the specific binding of [125I]oPL. This hormone appears to bind exclusively to a somatogenic site in the ewe, since various GHs compete efficiently for binding, whereas oPRL is without effect. The evolution of PRL and oPL receptors was determined during pregnancy and lactation and at different periods after an estradiol and progesterone treatment, which provokes growth of the mammary gland and milk secretion. During pregnancy, PRL receptors increased in the mammary gland up to day 100. During the last trimester, receptor content remained stable, and a second increase occurred during early lactation. No additional significant changes were observed either for PRL receptors in liver or adipose tissue or for oPL receptors in any of the organs studied (mammary gland, liver, adipose tissue). Injections of large doses of estradiol and progesterone to nonpregnant ewes were able to reproduce effectively the pattern of PRL receptors observed during pregnancy, but had no effect on oPL receptor levels. These studies demonstrate the independence of PRL and PL receptor sites in the ewe and suggest a different hormonal regulation for each type of receptor.

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