IN-VIVO EFFECT OF GROWTH-HORMONE ON DNA-SYNTHESIS AND EXPRESSION OF MILK PROTEIN GENES IN THE RABBIT MAMMARY-GLAND

The aim of tis work was to show whether growth hormone (GH) is able to directly induce growth and functional differentiation of the mammary gland. We have shown that i.m. injections of prolactin and to lesser e xtent injections of growth hormone increased DNA synthesis in the mamm ary gland of pregnant rabbits. Injections of pituitary and recombinant bovine growth hormone (GH), similarly to prolactin, could also induce the expression of milk protein genes - caseins alpha S1 and beta and whey acidic protein (WAP). However, in contrast to prolactin, growth h ormone failed to induce the synthesis of casein proteins. Lactogenic h ormones act through binding to receptors in target tissues. Prolactin receptors were shown to be abundant in the rabbit mammary glands but n o specific binding sires for I-125-labelled GH have been found in memb ranes isolated from mammary glands of pregnant or lactating rabbits. T he specificity of hormone binding was examined using unlabelled hormon es as competitive inhibitors of I-125-labelled prolactin. Bovine and r ecombinant bovine growth hormone did not displace prolactin from its r eceptors, thus excluding the possibility of action of GH through lacto genic receptors. Our results support the hypothesis that GH may act di rectly on the mammary gland and independantly from prolactin; however, the mechanism of its action is still unknown.