Calcium-activated potassium current clamps the dark potential of vertebrate rods

Vertebrate photoreceptors respond to light with a graded hyperpolarization from a membrane potential in the dark of approximately -35 mV. The present work investigates the physiological role of the Ca2+-activated K+ current in the photovoltage generation in mechanically isolated rods from salamander retina. Membrane current or voltage in isolated rods was recorded from light- and dark-adapted rods under voltage- or current-clamp conditions, respectively. In light-adapted rods of the salamander, selective blockade of Ca2+-activated K+ channels by means of charybdotoxin depolarized the plasma membrane of current-clamped rods by approximately 30 mV, from a resting potential of approximately -35 mV. A similar depolarization was observed if external Ca2+ (1 mM) was substituted with Ba2+ or Sr2+. Under control conditions, the injection of currents of increasing amplitude (up to -100 pA, to mimic the current entering the rod outer segment) could not depolarize the membrane potential beyond a saturating value of approximately -20 mV. However, in the presence of charybdotoxin, rods depolarized up to +20 mV. In experiments with dark-adapted current-clamped rods, charybdotoxin perfusion lead to transient depolarizations up to 0 mV and steady-state depolarizations of approximately 5 mV above the dark resting potential. Finally, the recovery phase of the voltage response to a flash of light in the presence of charybdotoxin showed a transient overshoot of the membrane potential. It was concluded that Ca2+-activated K+ current is necessary for clamping the rod photovoltage to values close to the dark potential, thus allowing faithful single photon detection and correct synaptic transmission.