In vivo effect of growth hormone on DNA synthesis and expression of milk protein genes in the rabbit mammary gland.

The aim of this 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 extent injections of growth hormone increased DNA synthesis in the mammary 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 hormone failed to induce the synthesis of casein proteins. Lactogenic hormones act through binding to receptors in target tissues. Prolactin receptors were shown to be abundant in the rabbit mammary glands but no specific binding sites for 125I-labelled GH have been found in membranes isolated from mammary glands of pregnant or lactating rabbits. The specificity of hormone binding was examined using unlabelled hormones as competitive inhibitors of 125I-labelled prolactin. Bovine and recombinant bovine growth hormone did not displace prolactin from its receptors, thus excluding the possibility of action of GH through lactogenic receptors. Our results support the hypothesis that GH may act directly on the mammary gland and independently from prolactin; however, the mechanism of its action is still unknown.