This is a review paper that includes original calculations and figures. The drone retina is composed of two essentially uniform populations of cells, the photoreceptors and the glial cells. The photoreceptors contain many mitochondria but no glycogen has been detected; the glial cells contain much glycogen and very few mitochondria. The oxygen consumption of the photoreceptors in the dark is 20 microliters min-1 per g of retinal tissue and in response to a single flash of light there is an extra consumption that reaches a maximum of 40 microliters min-1 per g. In addition, light stimulation of the photoreceptors leads to changes in the glycogen metabolism of the glial cells, and to movements of K+. Measurements with intracellular K+-sensitive micro-electrodes showed that during light stimulation with a series of flashes the K+ activity (alpha K) in the photoreceptors fell by an average of 27% while in the glial cells alpha K rose by an amount that is estimated to correspond to most of the quantity of K+ lost by the photoreceptors. The relative contributions to the clearance of extracellular K+ of extracellular diffusion, spatial buffering and possible net K+ uptake by glial cells are discussed.

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