The effects of epidermal growth factor on membrane potential. Rapid hyperpolarization followed by persistent fluctuations

The effects of epidermal growth factor (EGF) on membrane potential were investigated in suspensions of the following three cell types endowed with a large complement of specific receptors: EGFR-T17 (a clone of mouse NIH-3T3 fibroblasts overexpressing EGF receptors); A431 and KB (two human carcinoma lines). In all these lines EGF induced a rapid and marked hyperpolarization constituted by an initial peak (in all three cell lines) and a subsequent sustained plateau phase, concomitant with the well-known increase of [Ca ²⁺](i). The time course and phorbol ester inhibitability of the membrane potential effects were the same as for the [Ca ²⁺](i) response. Experiments with Na ⁺-free and chloride-free media excluded a major role of the latter ions in the EGF-induced hyperpolarization. In contrast, experiments with high K ⁺ media, with the monovalent cation ionophore gramicidin and with Ca ²⁺-free media together with either a Ca ²⁺ ionophore (ionomycin, in A431 and EGFR-T17), or an agonist (bradykininin A431) addressed to a receptor coupled to phosphoinositide hydrolysis, were consistent with the involvement of Ca ²⁺-activated K ⁺ channels. The EGF-induced hyperpolarization was completely blocked by the K ⁺ channel blocker, quinidine, and unaffected by a variety of other drugs. Patch clamping of individual EGFR-T17 cells confirmed the initial hyperpolarization (from ~ -30 mV, the resting potential, to -60, -80 mV) was due to the activation of an outward current. This initial hyperpolarization was followed by fluctuations (period ~ 1 min) persisting as long as the cells could be analyzed. Thus, the changes of membrane potential appear to be not only novel members of the group of early events triggered by EGF in target cells but also long-lasting effects of the growth factor, which continue for unexpectedly long periods of time after EGF application.