CALCIUM GRADIENTS AND LIGHT-EVOKED CALCIUM CHANGES OUTSIDE RODS IN THE INTACT CAT RETINA

We have studied light-evoked changes in extracellular Ca2+ concentrati on ([Ca2+](o)) in the intact cat eye using ion-sensitive double-barrel ed microelectrodes. Two prominent changes in Ca2+ concentration were o bserved that differed in retinal location. There was a light-evoked in crease in [Ca2+],, accompanied by brief ON and OFF transients, which w as maximal in the inner plexiform layer and was not further studied. T here was an unexpected sustained light-evoked decrease in [Ca2+](o), o f relatively rapid onset and offset, which was maximal in the distalmo st region of the subretinal space (SRS). [Ca2+](o) in the SRS was 1.0 mM higher than in the vitreous humor during dark adaptation and this t ransretinal gradient disappeared during rod-saturating illumination. A fter correcting for the light-evoked increase in the volume of the SRS , an increase in the total Ca2+ content of the SRS during illumination was revealed, which presumably represents the Ca2+ released by rods. To explain the light-evoked [Ca2+](o) changes, we used the diffusion m odel described in the accompanying paper (Li et al., 1994b), with the addition of light-dependent sources of Ca2+ at the retina/retinal pigm ent epithelium (RPE) border and rod outer segments. We conclude that a drop in [Ca2+](o) around photoreceptors, which persists during illumi nation and reduces a transretinal Ca2+ gradient, is the combined effec t of the light-evoked SRS volume increase, Ca2+ release from photorece ptors, and an unidentified mechanism(s), which is presumably Ca2+ tran sport by the RPE. The relatively rapid onset and offset of the [Ca2+]( o) decrease remains unexplained. These steady-state shifts in [Ca2+](o ) should have significant effects on photoreceptor function, especiall y adaptation.