In vitro metabolic competence of frog retina in the absence of exogenous glucose

Purpose. The frog (Rana pipiens) has been used extensively to study various aspects of retinal cell biology, biochemistry, and electrophysiology. We evaluated the metabolic competence and histological integrity of the frog retina in vitro as a function of the presence/absence of exogenous glucose in the medium. Methods. Frog neural retinas were incubated for 1-8 h at 23°C in modified Ringer's-HCO₃ medium, under aerobic (95% O₂/5% CO₂) or anaerobic (95% N₂/5% CO₂) atmosphere, in the presence or absence of 10 mM glucose. Cellular ATP levels, lactic acid production, de novo nonsaponifiable lipid biosynthesis, and histology were then assessed. Results. Control retinas (glucose, aerobic) maintained ATP levels comparable to freshly excised tissue (6.2 nmoles ATP/mg dry wt.), produced minimal lactate (40 nmoles/h/mg dry wt.), and exhibited normal histology. In the absence of any exogenous carbon source, aerobically incubated retinas maintained cellular ATP levels, incorporated [³H]acetate into nonsaponifiable lipids, and produced lactate in a manner comparable to controls; histology appeared normal. In the presence of 1mM iodoacetate, aerobic ATP levels rapidly plunged to low levels, either in the presence or absence of glucose. Under anaerobic conditions, lactate production increase ≈ 10-fold, while ATP levels were maintained at control (aerobic) levels; again, histology appeared normal. However, in the absence of both glucose and O₂, ATP levels plummeted within 1 h, with concomitant cytoplasmic vacuolization and pyknosis. Conclusions. The frog retina is remarkably resilient to anoxic and hypoglycemic stress. Aerobically, it can maintain ATP levels and sustain de novo lipid synthesis equally well in the presence or absence of exogenous glucose in vitro, apparently due to oxidative recruitment of an endogenous carbohydrate substrate (e.g., glycogen). A strong Pasteur response maintains retinal viability under anaerobic conditions. These features are atypical, when compared with mammalian retinas.