Primate rod and cone photoreceptors may differ in glucose accessibility.

PURPOSE

Glucose is crucial for the function of retinal photoreceptors, other retinal neurons, and glial cells. Exogenous glucose can be extracted from the retinal and choroidal circulation, and endogenous glucose may be generated from breakdown of intracellular glycogen stores. Because glucose deprivation is a critical component of retinal ischemia, the authors sought to determine the sites of glucose entry into and generation within the retina.

METHODS

The localization of the glucose transporter, GluT-1, and the brain and muscle isozymes of glycogen phosphorylase, GlyP, was studied by immunohistochemistry of adult human and monkey retinas.

RESULTS

Brain glycogen phosphorylase (B-GlyP) immunoreactivity was found in cone, but not rod, photoreceptors. There was immunostaining of foveal and peripheral cones throughout the cytoplasm from the outer segment to the synaptic pedicle. Short wavelength ("blue") cones were positive for B-GlyP. Diffuse staining of the inner and outer plexiform and the nerve fiber layers did not resemble the distinct morphology of Müller cells. Immunoreactivity to muscle GlyP (M-GlyP) was confined to selected synaptic layers of the inner plexiform layer in monkey retina. Staining with antibody to GluT-1 demonstrated diffuse reactivity throughout the retina, including the blood-retinal barrier cells, retinal pigment epithelium, and vascular endothelium. Ultrastructural immunohistochemistry showed staining of rod and cone inner and outer segments.

CONCLUSIONS

These immunohistochemical studies indicate that rod and cone photoreceptors have the biochemical capability to transport exogenous glucose from the circulation. Only cones appear capable of using endogenous glycogen stores. These findings imply that cones could be more resistant to acute reductions in circulating glucose during hypoglycemia. However, during hypoxic insult, glycogenolysis and anaerobic glycolysis could result in increased production of intracellular lactic acid, potentially predisposing the cone to acidotic damage.