THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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(1972) reported that when goat plasma samples taken during
pregnancy were exa¡rlined for prolactin by radioimmunoassay
and for total lactogenic act,ivity by a rabbit mammary gland
culture assay, high levels of Lactogenic activity were
detected in the second. and 1ast third of pregnancy when
the concentration of immunoreactive pituitary prolactin was
low or even absent. Co-culture experiments (Forsyt.h, :-g72l
demonstrated that this mam¡notropic substance (caprine
placental lactogen, cpL) is secret.ed by fetal cotyledons,
and could be detected in the maternal circulat,ion from
the 9th week (64 days) of gestation until term. Horúever,
it is not clear which cells of the placental cotylêdons
are secreting placental lactogen and also it is not clear
at v¡hat stages of pregnancy this hormone is first secretêd.
UsÍng radioreceptorassay for prolactín (RRA_PRL)
and RRA-GH, Kelly et at, (19?6) and Currie et. al (l9ZZ)
estimated the serum levels of caprine placental lactogen
(cPL) throughout pregnancy. In their studies, they
found that lactogenic and GH-l_ike act.ivities increased from
less than 100 ng,/ml (ovine proLactin and grob¡t.h hormone
equivalents) about 60 days after mating t,o reach peak
levels (400-1600 nglmt) b et¡.reen days ll0 and 130 of pregnancy.
The levels of both activities increased in essentially
the s arne fashion but during the last 15 days of pregmancy,
lactogenic activity decLined l-ess than Gu-like activity
This probably reflected the increased secretion of pituitary
prolêctin near parturition.