Regulation of bovine mammary growth by peptide hormones: involvement of receptors, growth factors and binding proteins

Abstract

Involvement of the somatotropin (ST)/prolactin(PRL)/placental lactogen (PL) family and their mediators in regulating mammary growth in cattle was evaluated using both in vitro and in vivo models. Mitogenic activity of bovine mammary epithelial cells (BMEC) was not increased by ST, PRL or PL in vitro. Direct infusion of ST but not PRL into mammary glands of late pregnant beef cows increased mammary DNA. Exogenous treatment of steroid-primed dairy heifers with PL increased mammary DNA while PRL treatment resulted in lactogenesis but no detectable increase in mammary DNA. Insulin-like growth factor-I (IGF-I) directly increased BMEC DNA synthesis in vitro and infusion of IGF-I into mammary glands of late pregnant beef cows increased mammary growth. Epidermal growth factor and transforming growth factor-alpha increased the mitogenic response to IGF-I and are likely involved in paracrine regulation of mammary growth. IGF binding proteins are produced and secreted by BMEC and may be autocrine regulators of growth. Collectively, these data indicate that lactogenic hormone receptors are not involved in regulation of mammary growth in cattle. Increased mammary growth following ST and PL treatment was apparently mediated through local production or uptake of IGFs and/or alteration of IGF binding protein concentrations. Presently, it is not clear whether ST and PL act through local as well as distant somatotrophic receptors to alter availability and uptake of IGFs.