THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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the specific binding of oPL to 780 x g and 15'000 xg pellets was
less than 50% of the binding associated with the 100'000 x g
pellet. There seemed to be very little dìfference in oPL binding
to tissues obtained from non-pregnant, ear'ly pregnant(22-27 days
gestatìon), or late pregnant (130-'l 35 days gestation) ewes, with
the exceptìon of adipose tissue and uterus. In the latter two'
binding of oPL decreased approximate'ly 50% in late gestatìon when
compared t.rith the non-pregnant perìod. The exact reason for this
decrease is not cl ear.
The low binding of oPL to mar nary gland 100'000 x g
fractions (less than 3Í6) was unexpected. A potent PRL-I lke effect
of oPL has been demonstrated in the rabbit ( Chan et al ,1976;
Handwerger et al ,.l974) ,
in the pigeon crop sac bioassay
and receptor assays, and mouse maÍmary g'land explants (private
conmunications with Drs. Charles Nicoll, Isabeì Forsyth, and Fred
Leung, respective'ly). In bindìng studJes one must always remain
concerned that inappropríate conditions have led to a negative result
rather than that there is any absence of bindÍng sìtes,
Therefore, several attempts were made to optlm'lze
conditions for oPL-bindlng studies usìng mammary 91 and tìssue
preparations incìuding variations in temperature, time of incubation,
pH and buffer systems, and tissues at different gestational peniods.
In aì1 cases, less than 3% specific binding per mjìligram of protein
was obtained. If we assume that our studies are valid and that
there are few oPL sites in the marmary giand, questions arise