Light stimulates in vivo inositol lipid turnover in frog retinal pigment epithelial cells at the onset of shedding and phagocytosis of photoreceptor membranes.

We have developed an experimental model to study in vivo inositol lipid metabolism in frog retinal pigment epithelial (RPE) cells, including the effect of light on phospholipase C-mediated hydrolysis of phosphatidylinositol 4,5-bisphosphate. RPE cells were rapidly isolated after either brief light or dark periods. Light and electron microscopy showed complete detachment of the retina from the RPE cells, and that the RPE cell suspensions were devoid of photoreceptor cell outer segments. Frog tissues were labeled in vivo for 20 hr by intravitreal injection of [3H]inositol (4 microCi, 4 microliters per eye) within a 24-hr constant illumination period. Following 1 hr of darkness (priming period), frogs were intravitreally injected with LiCl (0.5 M, 4 microliters per eye) 15 min before the onset of either 30-min light stimulation or an additional 30 min of darkness (controls). In order to preserve endogenous inositol phosphate pools present after dark and light exposure, the RPE cells were harvested in the shortest time possible, at low temperatures (18-20 degrees C), and in the presence of 10 mM LiCl. Total [3H]inositol-labeled water-soluble products (inositol plus inositol phosphates) were increased by 86% after 30 min of light. Inositol trisphosphate (IP3) showed the highest accumulation (a 5.5-fold increase), followed by inositol bisphosphate (1.9-fold increase) and inositol monophosphate (1.4-fold increase). Free [3H]inositol also accumulated (2.8-fold increase), reflecting only a partial inhibition of phosphomonoesterase by LiCl. These changes were paralleled by a 12% decrease in 3H-labeled phosphatidylinositol with no significant difference in the labeling of polyphosphoinositides.(ABSTRACT TRUNCATED AT 250 WORDS)