Light Enhances the Turnover of Phosphatidylinositol in Rat Retinas

Light stimulation of isolated rat retinas is shown to enhance the turnover of phosphatidylinositol (PI) as demonstrated by a light-dependent increase in ³H]inositol incorporation and concurrent hydrolysis of existing PI. Studies with rat retinas incubated with ³H]inositol and then microdissected at the level of the outer plexiform layer into photoreceptor cell and inner retina layers indicated that the light-enhanced incorporation of ³H]inositol was associated with the photoreceptor cell layer. The rate of PI hydrolysis in retinas prelabeled in vivo with ³H]inositol was higher in light than in dark incubations and was higher in the photoreceptor cell layer than within the inner retina. Within the photoreceptor cell layer, PI turnover involved 2%/min of the total PI contentin dark and 6–8%/min in light. In contrast to what has been reported for stimulus-enhanced turnover of PI in some tissues, this light-enhanced turnover of PI in the retina was not associated with detectable reductions in PI content. Parallel studies of sodium (²²Na) uptake demonstrated that the photoreceptor cells remained functional during these incubations as they retained the capacity to restrict the entry of ²²Na in light but not in dark.