Binding of growth factors by cell-surface receptors is an essential means by which cell regulate normaltissue growth and differentiation. Exposure to growth factors is often transient, and our goal was to determine whether short-term exposure to insulin-like growth factor-1 (IGF-1) would lead to activation, assayed as cell proliferation, of mammary epithelial cells. The MAC-T cell line is an immortalized bovine mammary epithelial cell line, chosen as our model mammary cell line because of its known sensitivity to IGF-1. Using the Cytosensor Microphysiometer System, a biosensor capable of measuring extracellular acidification, we were able to measure activation of the cellsowing to IGF-1 addition in real time and found that peak acidification occurred in only 14 min. We show that this rapid response to IGF-1 is dose dependent and specific for IGF-1. A significant increase in [3H]thymidine incorporation by cells after a similar short-term exposure to IGF-1 suggests that the measured increase in extracellular acidification following IGF-1 addition is physiologically relevant. This technology offers a new, novel, and rapid means for the study of IGF-1 activity, as well as the screening of IGF-1 inhibitors, in mammary epithelial cells.
