Binding of Mouse Choriomammotropin to Prolactin Receptors in the Mouse Mammary Gland

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The interaction between mouse choriomammotropin and mouse mammary glands was examined by radioreceptor assays using ovine prolactin (NIH-P-S9) iodinated by lactoperoxidase as a tracer. Mouse pituitary extracts and placental extracts were subjected to 10% acrylamide gel electrophoresis. Gels were cut into 2-mm segments after electrophoresis, and stored in 1ml 0.05M phosphate buffer (pH 7.4) containing 0.05M NaC1 overnight for elution. Lactating mammary tissues from D strain mice were incubated for 120 min in 1ml Medium 199 containing 6 ng of 125I-prolactin and 0.1ml of each eluate. Pituitary extracts displaced 125I-prolactin only at the position which coincides with the prolactin band. Displacement was observed at two positions of the gel when placental extracts were used. Relative mobilities (Rm) were 0.21 and 0.71, respectively. The slowly migrating component of choriomammotropin inhibited the binding of 125I-prolactin more strongly than the rapidly migrating one. Neither of them was identified as a distinct band in stained gels. The molecular weight of ovine prolactin, mouse pituitary prolactin and the slowly migrating component of mouse choriomammotropin was estimated to be 23000 using disc electrophoresis but the ion charges of these hormones were considerably different.
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