Phosphatidylinositol synthesis and phosphorylation are enhanced by light in rat retinas

Incorporation of labeled precursors (glycerol, glucose, or 32Pi) into phosphatidylinositol (PI) was 2-3-fold higher in rat retinas incubated in light compared to those in dark. During brief (30 min) incubations with labeled glycerol, there was a selective increase in the radioactivity associated with PI and phosphatidic acid, whereas, upon longer (60 min) incubations, synthesis of other lipids was also enhanced in light. Accumulation of the precursors was similar in light or dark throughout 60 min of incubation. Phosphorylation of PI to triphosphoinositide (TPI) was also enhanced in light, and, in both light and dark, up to 40-50% of the total 32Pi incorporated was associated with TPI. A model is proposed for PI metabolism based on two pathways: 1) a PI cycle representing synthesis and turnover of PI, and 2) phosphorylation of PI to TPI and possible hydrolysis of TPI. Light stimulation was shown to enhance the synthesis of PI within the photoreceptor cell layer and truncated rods and to increase the phosphorylation of PI to TPI within the inner retina and synaptosomes. Parallel studies with retinas from Royal College of Surgeons rats with advanced photoreceptor cell degeneration and intact inner retina showed that loss of the photoreceptor cells did not affect the capacity for phosphorylation of PI to TPI, but was associated with reduced capacity for PI synthesis. These results provide evidence that light stimulation affects different pathways of PI metabolism within different cells of the retina.