Ionic effects on the membrane potential of hyperpolarizing photoreceptor in scallop retina

Publisher Website
Google Scholar
Abstract
The effects of different external ionic conditions and of metabolic inhibitors on the membrane potential of hyperpolarizing photoreceptors in the retina of the scallop Pecten irradians were examined in the presence and absence of light. Changes in extracellular K+ have a greater effect on membrane potential in the light than in darkness. Receptor potential increased in amplitude when [K]o was reduced and decreased when [K]o was elevated. It was hyperpolarizing when [K]o was less than the estimated value for [K]i and depolarizing when this condition was reversed. The complete replacement of [Na]o caused a significant hyperpolarization of membrane potential in darkness, it had a much smaller hyperpolarizing effect on the peak of the receptor potential. The ratio of Na+ to K+ permeabilities (PNa/PK) decreased during bright illumination. PK was 7 times that for PNa in the dark but was 57 times greater than PNa in light. The metabolic inhibitors DNP [dinitrophenol] and NaCN caused membrane potential in the dark to hyperpolarize. This hyperpolarization was associated with a decrease in the PNa/PK ratio similar to that during illumination. High [Ca+]o caused membrane potential in the dark to hyperpolarize. This hyperpolarization was associated with an increase in membrane conductance. Results indicated that hyperpolarizing receptor potential of the distal photoreceptor was produced by a light-evoked increase in K+ permeability.