THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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5.Relationships amonq opRL. oGH. opl and proqesterone in the Sheep
during Pregnancy
In '1953 Sgouris and Meites postulated that proìactin
nay act to inhibit its own secretion by the pituitary . Thìs
hypothesis has been verified subsequently by many studies (for
reviews, see Meites et al ,t972; Cjemens et al ,ì974; Tindal ,l974).
Recently, Nagasawa et al(1976) demonstrated that daîly injections
of human placental Iactoqen(hpL) significantly decreased both
pituitary and serum reveis of proractin in the intact femare rat
Their ¡esults suggest that hpl inhibits pituitary pro'lactin by
lncreasi ng hypothalamic prolactin-inh.ibitory factor(pIF) activity.
It is unìikely that hpl inhlbits prolactín secretion by acting
directly on the pituitary, since proìacfin released by the pitu.itary
into the medium was not affected by the addition of hpL. Nagasawa
et al suggest that the inhibltory mechanism of hpL on pitultary
prolactin secret.ion is simllar to the negative short-.loop of
prol acti n . The physioìogical significance of thèlr finding
on the interrelation between placentat lactogen and pituitary
prolactìn is not clear at present. .-
Interestingiy¡ the secretìon of opRL durinqì
pregnancy in sheep.is similar to rats (Amenomori et al ,1970;
Nagasawa et al ,.l972). Furthermore, the ìevels of opRL are
inversely related to serum oPL and progesterone levels.