title: The cation selectivity and voltage dependence of the light-activated potassium conductance in scallop distal photoreceptor.
abstract: Light-dependent voltage and current responses were measured from the distal hyperpolarizing photoreceptors of the scallop (Pecten 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 T1+, K+ greater than Rb+ greater than NH4 greater than Cs+, Li+, Na+. External caesium and tetraethylammonium blocked inward but not outward K+ currents through the light-dependent K+ conductance pathway. The data suggest that K+ ions move through an aqueous pore which is controlled by light.
mesh_terms: D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002414:Cations, Monovalent / Q000494:pharmacology*; D002463:Cell Membrane Permeability / Q000187:drug effects; D002586:Cesium / Q000494:pharmacology; D066298:In Vitro Techniques; D008027:Light; D008564:Membrane Potentials / Q000187:drug effects; D008974:Mollusca / Q000502:physiology*; D010786:Photoreceptor Cells / Q000187:drug effects / Q000502:physiology*; D011188:Potassium / Q000502:physiology*; D012160:Retina / Q000502:physiology; D019789:Tetraethylammonium; D013757:Tetraethylammonium Compounds / Q000494:pharmacology; D013997:Time Factors
chemical_list: D002414:Cations, Monovalent; D013757:Tetraethylammonium Compounds; D002586:Cesium; D019789:Tetraethylammonium; D011188:Potassium