THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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Our studies on the blnding of opl showed that
oPL binds effectively to ovine adipose tissues, and ovine
growth hormone competes with opl in a parallel manner in displacing
the binding ot l2sl-opl to its receptor site. Thus, these studies
suggest strong'ly that structurally the molecuìe of opL is simi rar
to oGH because they both compete with each other for binding to
the growth hormone receptors in ovine tissues. Furthermore,
sheep pituitary growth hormone ciuring pregnancy is not e.levated,
whereas the substrate (g'lucose is the maìn source of substrates
for the fetus) requirements by the conceptus progressively increases
throughout gestation making it plausibìe that opl acts as the
"growth hormone" of pregnancy. If opL acts as the ,'growth hormone"
of pregnancy in the sheep, it might induce a tonic effect on
maternal metabolism by mobiìizfng FFA release fron adipose tissues
for gluconeogenesis in the maternal liver which ensures the fetus
a steady source of various fue1s, of which glucose is the principaì
one. Insulin is a fìuctuating modifier of the effect of opl on
the maternal organísm. Feastlng increases effectÍve insuìin and
restores naterna'l substrates , whereas fastìng results in a
decreased effect of insu'l in and induces prfmary cataboìic effects
and giuconeogenesis enhanced by opL to ensure an adequate supp.ly
of metabolic nutrieñts for the fetus.