Characterization of a voltage-gated K+ channel that accelerates the rod response to dim light

In this study a K+ current, IKx, in isolated salamander rod photoreceptors was characterized and its role in shaping small photovoltages was examined. IKx is a standing outward current of about 40 pA at -30 mV that deactivates slowly when the cell is hyperpolarized (tau max = 0.25 s). The voltage and time dependence of IKx are similar to that of M-current, but IKx can be distinguished from M-current because it is not suppressed by acetylcholine and is "blocked" by external Ba2+ in a surprising manner: the activation range of IKx is shifted strongly in the positive direction. Using current-clamp recordings and a computer simulation of the photo-response, we show that IKx figures prominently in setting the dark resting potential and accelerates the voltage response to small photocurrents.