Endocrine Effect of IGF-I on Mammary Growth in Prepubertal Heifers
Pubertal mammary growth in heifers is stimulated by exogenous GH (Sejrsen et al., 1986; Purup et al., 1993). The mechanism of action of GH on the mammary gland, however, is unclear, but evidence favours an indirect action via IGF-I. IGF-I is produced mainly in the liver, but also locally in other tissues, including the mammary gland. Therefore the in vivo effect of GH may be mediated by systemic as well as locally produced IGF-I. To elucidate this question we investigated the effect of sera from GH-treated and control (placebo-treated) heifers on mammary cell proliferation in vitro.