An extended Ca2+‐hypothesis of visual transduction with a role for cyclic GMP

Abstract

A model is described having the following features: Light induces Ca²⁺ release from vertebrate rod outer segments discs via pores composed of multimeric rhodopsin. Cytoplasmic Ca²⁺ reversibly blocks Na⁺ channels of the surface membrane, with the time course of development and amplitude of the response to light being influenced by restrictions on intradiscal Ca²⁺ diffusion. The falling phase of response reflects a decline in cytoplasmic [Ca²⁺] due to a Ca²⁺-binding protein controlled by cyclic GMP so that its binding capacity is increased by the reduction in cytoplasmic [cyclic GMP] which follows rhodopsin bleaching.