Potassium channels in the membrane of Hydrodictyon africanum.

The potential difference across the membrane of Hydrodictyon africanum was controlled by voltage clamping and positive and negative steps in the PD were applied. For positive steps in the PD to values less negative than a threshold value, there is a PD and time-dependent increase in the outward current which has an S-shaped time course. Following the cessation of these steps, the current reverses instantaneously and declines with a simple time course. These currents show a strong K dependence and are blocked by tetraethylammonium (TEA) and nonyltriethylammonium (C) ions, suggesting that they arise from the opening and then the closing of K channels. There is also a PD and time-dependent increase in the inward currents in response to negative steps in the membrane PD. The membrane properties have been described by three current-voltage curves, for the instantaneous current, for the steady-state current and for the current flow when the K channels are open. The response of the unclamped or free-running membrane PD to steps of constant current can be accounted for by the observed kinetics of the opening and closing of the K channels. [ABSTRACT FROM AUTHOR]