THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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binding of that. hormone to its target cell is necessary
(Roth, 1973). Thus, specific target ceIl membranes Ìqere
isolated f,rorn animaL tissues and used to assay for specific
hormones. The advantages of these assays are that they
are rel-atively simple to perform, quite sensitive (10 ng,/ml
without sen¡m and 5ong/ml for serum samples), and most importantly,
they are not species specific. For eXample, a radioreceptorassay
for proLactin (Shiu et a1, 1923) can be used to measure
prolactin ánd placental lactogens derived from many species,
\,\thereas the conventional_ radioimmunoassay (RIA) is generally
species specific. Of course, in any determination the
RRA cannot distinguish pituitary proractin (pRL) from placentaì
ìactogen or prÍmate pituitary growth hormone (GH|
Fortunâtely ,
however in most species, serum levels of pituitary prolactin
or growth hormone during pregnancy are low as compared to
placentar lactogen levels. To determine the exact contribution
of the pituit.ary hormone level to total activity it ís
necessary to enploy a specific radioimmunoassay. The
difference between totaL RRA and RIA-pituitary hormone
concentration represents serum concentration of placental
J.actogen.
Thus, by employing these thro radioreceptorassays
(RRA-PRL and RRA*GH), several ¡o¡-primate placental lactogens
have been detect.ed and isolated ín the past 3 to 4 years
(Shlu et a1, 1973¡ KeLly et al, 1924a and b; Robertson et aI,