Electrophysiological measurements of the membrane potentials of mouse mammary epithelial cells in primary culture revealed the presence of spontaneous-oscillating-hyperpolarizing potentials in cells incubated with epidermal growth factor. The hyperpolarizing potentials were 5-20 mV in amplitude and about 10 sec in duration. The peak height of the response was reduced by hyperpolarization, and the input membrane resistance decreased during the response. The response was probably due to activation of K+ channels. The latency period for the epidermal growth factor induction of the hyperpolarizing potential was approximately 3 hr. In contrast, insulin induced spontaneous-depolarizing potentials that were about 5 mV in amplitude and 1 sec in duration. The depolarizing potentials were attributed to activity of ion channels, since the peak height was dependent on the membrane potential and the depolarizing potential was accompanied by a decrease of input membrane resistance. The time lag for the induction of the depolarizing potential was 6-12 hr. Other hormones involved in mammary cell differentiation, such as cortisol and prolactin, neither induced the depolarizing potentials nor changed the induction of depolarizing potential by insulin. In addition, other growth factors, such as nerve growth factor and fibroblast growth factor, elicited no electrical activity.The growth and differentiation of the mammary epithelium are regulated by a variety of peptide and steroid hormones (reviewed in refs. 1-3). In primary culture the synergistic actions of insulin, cortisol, and prolactin enhance the synthesis of milk components of mammary epithelial cells (4-7). In contrast, epidermal growth factor (EGF) stimulates proliferation of the mammary epithelium, both in vitro (6-8) and in vivo (9). Thus, mammary epithelial cells are a useful system in which to study the basic mechanisms of hormone action on cell growth and differentiation. (14) and fibroblasts (15). Alteration of ion-transport properties of the cell membrane has been suggested to be a key step in the regulation of cell growth. Thus, it is possible that some polypeptide hormones affect the permeability of ion channels in the cell membrane and that the movements of ions through such channels have important roles in the processes of cell proliferation and differentiation.To determine whether the movements of ions across the membrane of mammary epithelial cells are coupled with hormone actions, we examined the effects of these hormones and growth factors on the membrane potential and electrical activity of these cells by electrophysiological techniques. In this paper, we report that EGF and insulin induce two different membrane potential changes by modulating the activity of ion channels in cultured mammary epithelial cells. MATERIALS AND METHODSMaterials were obtained from the following sources: medium 199 (Hanks' salts) from GIBCO; bovine serum albumin from Miles-Yeda (Rehovot, Israel); bovine prolactin from the Hormone Distribution Program, National Institute ...

Epidermal growth factor (EGF) induces a hyperpolarizing response of 5-20 mV amplitude in mouse mammary epithelial cells in culture. The amplitude of the hyperpolarizing response was reduced by more than 60% within several minutes after addition of blockers of voltage and/or Ca2+-dependent K+ channels such as tetraethylammonium (7 mM) or quinine (0.29 mM). Both nifedipine (0.15 mM), a blocker of the CaZ+ channel, and ruthenium red (2 mM), an inhibitor of the Ca *+-binding site, also reduced the amplitude of the hyperpolarizing response by more than 60%. The Ca 2+ ionophore, A23187 (3.8 PM), induced a large hyperpolarization, which was 25-40 mV and lasted about 3 min. These data suggest that activity of the Caz+-dependent K+ channel was involved in the EGF-induced hyperpolarizing response of the mammary epithelial cells.

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