Short communication: Intramammary infusion of IGF-I increases bromodeoxyuridine labeling in mammary epithelial cells of prepubertal heifers

When dairy heifers are fed to gain more than 900 g of body weight/d, they have less mammary parenchymal DNA at puberty but more insulin-like growth factor-I (IGF-I) in serum. This negative relationship between serum IGF-I concentration and mammary epithelial cell proliferation is in disagreement with the extensively reported role of IGF-I as a stimulator of mammary epithelial cell proliferation. Despite the large body of evidence suggesting that an increase in IGF-I concentration should lead to an increase in mammary epithelial cell proliferation of prepubertal heifers, it had not been previously tested. Our objective was to determine if intramammary infusions of IGF-I would stimulate mammogenesis in prepubertal heifers in vivo. After 7 d of treatment, bromodeoxyuridine was infused intravenously and heifers were slaughtered 3 h later. Samples from 3 regions of the mammary parenchyma were collected, fixed, sliced, and incubated with bromodeoxyuridine monoclonal antibody to identify cells in the S-phase of the cell cycle. Intramammary infusion of IGF-I increased the percentage of epithelial cells in the S-phase by 52% (6.4 vs. 4.2%, ± 0.3%). Proliferation was similar in all 3 parenchymal regions, and the response to IGF-I was similar in each region. We conclude that local IGF-I increases proliferation of mammary parenchymal epithelial cells in prepubertal heifers