THE UNIVERSITY OF MANITOBA STUDIES ON OVINE PLACINTAL ...

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production (casein synthesis) by rabbit mammary gland
(Josímovich and MacLaren, 1962¡ Josimovich and Brande, 1964;
Fríesen, 1966¡ Turkington, 1921) and simian, mamnary
gland (Beck, 19721 . It also stimulates the pigeon crop-
sac analogous to the action of pituitary prolactin (Josimovich
and MacLaren, L962i Forsyth, 1970; Li, Ig72'). The lactogenic
potency of hPL had been assessed by various assays. In
pigeon crop-s¿g assay, hpt has 109 the potency of sheep
prolactin (Forsyth, 1970; Li, 19721 , but 50-lO0S of the .
activity of sheep prolactin in stimulating casein synthesis
in rabbit, and mouse ma¡nmary gland explants (Forsyth, 19Zt;
Turkington, L971; Kteinberg et al, I97I). hpL and prolactin
compete for the same receptors in rabbit manunary gland ce1I
membranes, and hpt is 30-50t as potent as hpRL (Shiu et at,
1973), but their reLative roles in ¡narnmary development are
stíll unclear. Since hpl, is present at tern in concentrations
35 times greater than those of hpRL (Hwang et al , 1971 ;
FrÍesen, L97I), it is possible that hpL may be more important.
Leader (1975) has postulatêd that the primary action of hpl,
Ís to stjmulate the development of the mammary gland during
pregnancy, hrithout causing milk secretion due to its blockade
of prolactin receptorst at birth, withdúawal of hpl, (hence
withdrav,¡al of the blocking effect on proLactin receptors)
then results in milk secretion under the active stimul"us
of prolactin of pituitary origin. Although Leaderrs hypothesis
is interesting, unfortunatefy there is no report of lactogenic
receptors in hurnan rnammary gl"and.