Effects of adaptation level and hypoglycemia on function of the cat retina during hypoxemia

Acute hypoxemia (low P(a)O₂) leads to changes in oxygen consumption and electrical responses of the outer retina of cats, but inner retinal ERG components and ganglion cell responses have been shown to be quite resistant to hypoxemia. The purpose of this study was to determine whether the resistance of the inner retina depends on (1) the stimulus conditions, specifically the degree of light adaptation; and (2) the ability of the photoreceptors to increase glycolysis during hypoxemia. To address these issues, recordings of single ganglion cell action potentials and of the b-wave and scotopic threshold response (STR) of the electroretinogram (ERG) were made from the eyes of anesthetized cats during hypoxemia alone and hypoxemia plus hypoglycemia. Ganglion cells appeared to be equally resistant to hypoxemia at high and low backgrounds (3.3 to 9.7 log equivalent quanta(555 nm)-deg^-2-s^-1), and the STR, recorded with dim stimuli during dark adaptation, when photoreceptor oxygen consumption is most susceptible to hypoxemia, was unchanged until P(a)O₂ was below 30 mm Hg. The amplitude of the b-wave was similarly resistant to hypoxemia when the animal was normoglycemic. During hypoglycemia, however, both the b-wave and the STR became more sensitive to hypoxemia, beginning to change at P(a)O₂s as high as 50 mm Hg when blood glucose was 40-50 mg/dl. It is argued that hypoglycemia limits or prevents the increased glycolytic ATP production that would ordinarily occur when the photoreceptor oxygen supply decreases, and that increased photoreceptor glycolysis is essential in the protection of the retina against mild hypoxemia.