THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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explanation is that there are receptors ln the ovary wh.ich are able
to bind the specific common sequence of ovine placenta.l lactogen
and oPRL but the affìnity for these two hormones are substantially
dìfferent, hence the inhibition curves are different.
The present results differ .ln some respects from those
of Bolander et a] (1976). In their report, opL bound to pregnant
rabbit adrenal , ìiver, ovary, marnmary g'tand, uterus, kidney, brain,
and fat, and oPRL displaced l25I-opL in a parallel nanner. In our
studies, significant binding of opl to adrenal, manmary gland,
kidney, and brain 100,000 x g mìcrosomal fractions was not observed,
and oPRL faiìed to d.isplace ì25I-opl in a paraiìeì manner .in 1.lver,
adipose tissue, ovary, uterus, and fetaì l.iver. These differences
stress the importance of specìes differences that have been noted
in hormone binding studies.
lindj¡S of oPRL toSheep Tissues
Ovine prolact.in js act.ive in promoting casein synthesis
and rnammary g'land growth .in experimental animals (Handwerger et al ,
'1974; Chan et al ,1976), and yet we could not demonstrate greater
than 3% specific bind.ing in ovine mammary glands as well as other
ovine tissues studied. Indeed the only tissue to exhibit considerable
binding of oPRL is the uterus of the pregnant ewe in which the
specific binding increased from less than 3* to more than l4%/mg
protein near term. At the present stage, one can only speculate that