A placental lactogen or chorionic mammotropin (rCM) has been identified in the serum of pregnant rats by radioreceptor assay (RRA). Two peaks of activity were found, the first between days 11–13 and the second between days 17–21 of pregnancy. Gel filtration of day 12 serum or placental extracts revealed two peaks of lactogenic activity, the first eluting ahead of and the second appearing immediately after 125I-hPRL (human prolactin). On the other hand, when serum or placental extracts from day 17–21 pregnant rats were fractionated in the same manner, only a single peak of lactogenic activity was eluted after 125IhPRL. The placental concentration of rCM increased from 11 μg⁄g wet weight at day 12 to 74 μg⁄g at days 14–15, after which the concentration declined to 29 μg⁄g on day 21. Electrophoresis of a fraction from G-100 column of a day 12 or day 20 placental extract or serum with a Ve⁄Vo ratio of 2.08 revealed a single peak of lactogenic activity by RRA with an Rf = 0.43. Large MW rCM (Ve⁄Vo ratio of 1.35) from serum had an Rf = 0.52 whereas the large MW species from placental extracts had a mobility similar to that of small MW fraction. The half-time disappearance rate of serum rCM on day 12 is 19.5 min compared to 1.2 min on days 17–21 of pregnancy. (Endocrinology96: 1187, 1975)

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