Lactogenic Effect of Bovine Placental Lactogen on Pregnant Rabbit but not Pregnant Heifer Mammary Gland Explants

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Abstract
The biological function of bovine placental lactogen is unknown. However, bovine placental lactogen is known to bind prolactin receptors in rabbit mammary gland. This hormone was therefore cultured with rabbit mammary gland explants to confirm that it is lactogenic in this species. Mammary explants from pregnant heifers were also cultured with bovine placental lactogen to determine if the same preparation of hormone possessed similar lactogenic potential in homologous species. Bovine placental lactogen was tested over a range of concentrations from 10 to 500 ng/ml and was about 50 to 80% as potent as bovine prolactin when cultured with mammary explants from 19-d pregnant virgin rabbits. Lactogenic response was assessed by both the incorporation of [14C]acetate into lipid and by the synthesis of casein. Bovine placental lactogen displayed negligible lactogenic activity when cultured with mammary explants from 6 to 7-mo pregnant heifers. Lactogenic response was assessed using the same criteria as used with the rabbit mammary explants; in addition, accumulation of .alpha.-lactalbumin in the explants was also measured. Although bovine placental lactogen was lactogenic in the rabbit, the same hormone preparation, was apparently not lactogenic in cattle. It is therefore vital to test for the biological activity of a hormone in a homologous system, because inappropriate conclusions may be drawn from the response obtained in a heterologous system.