Epidermal growth factor-activated calcium and potassium channels

The earliest responses to activation of the epidermal growth factor (EGF) receptor include a transient increase in calcium influx and a transient membrane hyperpolarization. The underlying mechanisms are, however, not well understood as yet. In the present study, we have applied patch clamp recording in the cell-attached and the outside-out mode, and fluorimetric cytosolic Ca²⁺ determinations, to identify the nature of the ion channels involved, to characterize their properties at the level of single channels, and to unravel their mechanism of activation. We provide evidence that activation of the EGF receptor results initially in the activation of voltage-independent Ca²⁺ channels that can be defined as direct receptor-operated channels. This in turn causes the activation of Ca²⁺-dependent K⁺ channels, which results in a (delayed) membrane hyperpolarization and then leads to the activation of a second class of Ca²⁺ channels that are sensitive to hyperpolarization. An autocatalytic generation of further hyperpolarization and Ca²⁺ influx is the predicted outcome of this ionic cascade. Based on the observed inhibitory effects of protein kinase C activation on the activity of Ca²⁺-dependent K⁺ channels, we propose that protein kinase C is involved in the negative regulation of this cascade, which explains the transient nature of these responses.