Light-induced calcium fluxes from outer segment layer of vertebrate retinas

Abstract

Light affects the membrane potential of a vertebrate retinal rod or cone cell by transiently reducing its ‘dark current’, an inward flow of Na⁺ through the plasma membrane covering the light-absorbing outer segment^1,2. In rods most photons are absorbed by photopigment molecules in intracellular disk membranes that are physically separate from the plasma membrane^2–5. Photochemical events in the disks probably control the Na⁺ conductance of the plasma membrane through a chain of events that ultimately change the level of a diffusable substance in the cytoplasm of the outer segment. By combining reversibly with Na⁺ carriers or channels in the plasma membrane, this ‘internal transmitter’ substance would modulate the dark current and thus the rod's membrane potential⁶. There is evidence that the internal transmitter is free Ca²⁺ (refs 7–10). Ca²⁺ buffer EGTA has a desensitizing effect on rods, consistent with a free cyto-plasmic Ca²⁺ concentration of about 1 µM in darkness and release of 500–1,000 additional calcium ions for each absorbed photon¹⁰. However, there is insufficient proof that Ca²⁺ is released into the cytoplasm of rod outer segments by light and removed again in darkness. Reports that light reduces the total Ca²⁺ in outer segments have varied widely in their details^11–15. Small amounts of Ca²⁺, ∼1 mol per mol of photoisomerized rhodopsin, are released by strongly illuminated isolated rod disks¹⁶, but this is not enough to explain the large size of single photon responses^10,17. We have studied net movements of ²⁺ in the receptor layer of albino rat retinas during the responses of the rods to light with the aim of detecting release of internal ²⁺ by a rise in its extrusion into the interstitial space.