The cation selectivity and voltage dependence of the light‐activated potassium conductance in scallop distal photoreceptor.

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Abstract
Light-dependent voltage and current responses were measured from the distal hyperpolarizing photoreceptors of the scallop (P. irradians) retina. In normal external solution, the hyperpolarizing receptor potential was caused by a light-dependent K+ outward current. The magnitude of the hyperpolarizing receptor potential and the light-dependent outward current, measured at the resting potential, was graded with light intensity. In normal external solution, during prolonged illumination the light-dependent K+ outward current was characterized by an early peak and a subsequent plateau. Current responses to brief light flashes were reduced progressively during background illumination. In the absence of external Na+ ions, the reversal potential for the receptor potential changed 58 mV per 10-fold change in the extracellular K+ concentration. The estimated internal K+ concentration was 385 mM. The hyperpolarizing receptor potential produced by prolonged bright illumination consists of an early peak which decays to a plateau. This decay was determined by a decrease in the light-dependent K+ conductance during maintained illumination. The light-dependent conductance pathway passed outward currents better than inward K+ currents. The light-dependent K+ conductance was estimated to increase e-fold per 23-34 mV depolarization at the peak and during the plateau of the light response. The light-dependent conductance pathway was highly selective for K+ ions. The selectivity sequence for monovalent cations was Tl+, K+ > Rb+ > NH4 > Cs+, Li+, Na+. External Cs and tetraethylammonium blocked inward but not outward K+ currents through the light-dependent K+ conductance pathway. K+ ions probably move through an aqueous pore which is controlled by light.