34 H. Leibovich et al. Animal Reproduction Science 64 2000 33–47
suggest important roles for oPL in fetal growth and mammogenesis. Immunization of ewes against oPL may thus represent a novel practical technique for enhancing birth weights of lambs born to
prolific sheep, as well as milk production by both dairy and mutton ewes. © 2000 Elsevier Science B.V. All rights reserved.
Keywords: Sheep-placenta; Placental lactogen; Immunization; Lamb production; Milk production
1. Introduction
While the presence of high circulating concentrations of placental lactogen PL in preg- nant ewes has long been recognized, the biological role of this hormone has not been
clarified. Ovine oPL, a 198-amino acid non-glycosylated protein, is a member of the growth hormoneprolactin GHPRL gene family and manifests both somatogenic and lac-
togenic activities in heterologous systems Byatt et al., 1992; Anthony et al., 1995a,b. In contrast, in a homologous ruminant system, it displays agonistic activity through lacto-
genic receptors only Herman et al., 1999 or by heterodimerization of somatogenic and lactogenic receptors Herman et al., 2000. oPL is produced by chorionic binucleated cells
of the placenta and is released into both the maternal and fetal circulation, probably under differently regulated secretion patterns Schoknecht et al., 1991; Kappes et al., 1992. In
maternal plasma, oPL is first detected at 40–60 days of gestation, it peaks at days 120–140 when it can reach levels of up to 1 mgml, and then declines until parturition Gluckman
et al., 1979; Kappes et al., 1992. Fetal oPL plasma concentrations reach maximal levels by day 90 of pregnancy and are relatively low in the range of tens of nanograms per milliliter
Kappes et al., 1992.
Results from studies in which oPL levels were altered by feeding restrictions Butler et al., 1987; Bauer et al., 1995, by infusion of oPL into the maternal and fetal circulations
Oliver et al., 1995; Currie et al., 1996; Schoknecht et al., 1996, by infusion of antibodies to oPL to neutralize its activity Waters et al., 1985, by carnuclectomy Falconer et al.,
1985 or by fetectomy Rueda et al., 1995 have suggested albeit not conclusively, that oPL regulates fetal growth by stimulating repartitioning of maternal nutrients to the fetus, and
by stimulating the fetus to use the substrates Anthony et al., 1995b.
Active immunization against ‘self’ hormone molecules that can inhibit or potentiate en- dogenous hormonal activity has been utilized to investigate physiological roles of hormones
Holder and Carter, 1995; Meloen, 1995; Pell and Aston, 1995. In studies to immunomod- ulate GH activity, infusion of monoclonal antibodies against bovine GH Aston et al., 1987;
Bomford and Aston, 1990, or active immunization against specific peptide regions of oGH Pell and Aston, 1991 enhanced GH bioactivity and its physiological effects. In swine, vac-
cination against a short peptide corresponding to amino acids 54–95 of porcine GH improved growth performance Wang et al., 1996, and long-term treatment of Snell dwarf mice with
GH complexed to anti-GH antibodies stimulated growth rates Mockridge et al., 1998.
In nature, anti-oPL antibodies are not present in the ewe’s circulation. It can be hypothesized that the appearance of anti-oPL antibodies in the maternal circulation
before and during pregnancy may interfere with, or enhance the establishment of preg- nancy, fetal growth and development. Results of this study may therefore be indicative of
H. Leibovich et al. Animal Reproduction Science 64 2000 33–47 35
the oPL’s role during pregnancy. The aim of the present study was therefore to investi- gate the physiological role of oPL following active immunization of ewe-lambs against
recombinant oPL roPL prepared in our laboratory Sakal et al., 1997.
2. Materials and methods
