Control of Ca²⁺ in rod outer segment disks by light and cyclic GMP

Abstract

Photons absorbed in vertebrate rods and cones probably cause electrochemical changes at the photoreceptor plasma membrane by changing the cytoplasmic concentration of a diffusible transmitter substance, reducing the Na⁺ current flowing into the outer segment of the cell in the dark, to produce the observed membrane hyperpolarization that is the initial excitatory response^1–4. Cyclic GMP has been proposed as the transmitter because a light-activated cyclic GMP phosphodiesterase (PDE) has been found in rod disk membranes^5,6 and because intracellularly injected cyclic GMP⁷ reduces rod membrane potentials. Free Ca²⁺ has also been proposed because increasing external [Ca²⁺] quickly and reversibly reduces the dark current and divalent cationophores increase the Ca²⁺ sensitivity⁸. Ca²⁺ efflux from rod outer segments (ROS) of intact retinas occurs simultaneously with light responses^9,10. Vesicles prepared from ROS disk membranes become more permeable on illumination, releasing trapped ions or molecules^11–13, but intact outer segment disks have not previously been found to store sufficient Ca²⁺ in darkness and to release enough in light to meet the theoretical requirements for control of the dark current by varying cytoplasmic Ca²⁺ (refs 14–18). We now report experiments that show the required Ca²⁺ storage and release from rod disk membranes suspended in media containing high-energy phosphate esters and electrolytes approximating the cytoplasmic composition of live rod cells. Cyclic GMP stimulates Ca²⁺ uptake by ROS disks in such media.