Growth Hormone and IGF Research, 1998, 8, 439-446 The effects of ovine placental lactogen and bovine growth hormone on hepatic and mammary gene expression in lactating sheep N. S. Bassett 1, M. J. Currie 1, B. H. Breier 1, M. Klempt 1, S. H. Min 2, S. N. McCutcheon 2, D. D. S. MacKenzie 2 and P. D. Gluckrnan 1 1Research Centre for Developmental Medicine and Biology,Department of Paediatrics,University of Auckland, Auckland, NewZealand, 2Department of AnimalScience, Massey University,Palmerston North,NewZealand Summary The ability of ovine placental lactogen (oPL) to bind to the growth hormone receptor (GHR) raises the possibility that oPL may exert a growth hormone (GH)-Iike action on galactopoiesis. We have compared the effects of treating lactating ewes for 5 days with an equimolar dose (0.1 mg/kg/day, administered as two equal doses 12 hourly) of either bovine growth hormone (bGH) (n = 10), oPL (n = 10) or saline (n = 9) on hepatic and mammary GHR, insulin- like growth factor-I (IGF-I) and IGF-binding protein-3 (IGFBP-3) gene expression and hepatic GHR number. Hepatic GHR and IGFBP-3 mRNA were unaltered by bGH or oPL treatment. Hepatic IGF-I mRNAs increased following bGH (P < 0.05) but not oPL treatment. GHR gene expression was greater in liver compared to mammary gland extracts. There was no effect of either bGH or oPL treatment on mammary GHR, IGF-I or IGFBP-3 mRNA or hepatic GHR number. These studies confirm the galactopoietic effects of bGH in lactating ruminants and suggest that the mechanism of this action is not via increased hepatic GHR number or gene expression. In addition, the increase in hepatic but not mammary IGF-I mRNA with bGH treatment suggests an endocrine action of IGF-I on milk synthesis. These studies also demonstrate that an equimolar dose of oPL is not galactopoietic or somatogenic in the lactating ewe. 9 1998 Churchill Livingstone Key words: galactopoiesis, growth hormone, placental lactogen, gene expression. INTRODUCTION Placental lactogen (PL) is a hormone synthesized and secreted by the placenta during pregnancy. ] However, the physiological role and the mechanism of PL action are currently unclear. Previous ligand binding studies have suggested the presence of a specific PL receptor. 2,3 Despite these data, attempts to isolate, purify and/or sequence a specific PL receptor have remained unsuc- cessful. Recent data have suggested that PL and growth hormone (GH) may both exert their action via a single common receptor, 4 suggesting the possibility that PL may exert similar actions to GH. Certainly PL is as potent Correspondenceto: Dr N. S. Bassett,Research Centre for Developmental Medicine and Biology,Department of Paediatrics,University of Auckland, PrivateBag92019,Auckland, NewZealand. as GH in stimulating omithine decarboxylase activity 5 and amino acid transport 6 in post-natal tissues. Although the action of GH to stimulate galactopoiesis in ruminants is well documented, 7 it is unclear whether PL may also exert a similar action. Studies in the cow have demonstrated that bovine placental lactogen (bPL) is lactogenic in vivo and have suggested that bPL may also be a mammary mitogen. 8 However, although both ovine placental lactogen (oPL) and bPL share consider- able sequence homology 9,~~ it is uncIear whether oPL may exert similar lactogenic and/or somatogenic actions in the sheep. Studies across species have demonstrated that oPL stimulates lactogenesis in mammary explants from the pseudopregnant rabbit ]' and is somatogenic in the GH- deficient rat,'2 suggesting a role for oPL in tissue differen- tiation and growth. However, these effects appear to be 1096-6374/98/060439+08 $18.00/0 9 1998 Churchill Livingstone