Phosphoinositide Metabolism in the Retina: Localization to Horizontal Cells and Regulation by Light and Divalent Cations

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Abstract
Isolated retinas from Xenopus laevis incorporated greater amounts of [3H]inositol and 32Pi into phosphoinositides when incubated in light than did control retinas incubated in the dark. Inositol was primarily incorporated into phosphatidylinositol (83-86%), while phosphate labeled the polyphosphoinositides (72-79%). The incorporation of radioactive glycerol, serine, choline, or ethanolamine into retinal lipids was unaffected by light. Following incubation with [3H]inositol, the cell type involved in the light response was identified by light microscope and EM autoradiography to be the horizontal cell. These results are consistent with a classic phosphatidylinositol effect in the retina. An interesting feature of this response was that the stimulus (light) was received in the photoreceptor cell and the effect was manifest in the horizontal cell.