Monocarboxylic acid transporters MCT1 and MCT2 in retina

Monocarboxylates (MCTs) are thought to play an important role in the barrier systems of the eye and in the nutritional delivery systems that provide essential metabolic substrates to cells of the visual system. The monocarboxylate transporters MCT1 and MCT2 were localized in fixative-perfused rat retinas using polyclonal antibodies to MCT carboxyl terminal peptides. Light and electron microscopy indicated that MCT1 is abundantly expressed in retina. Retinal pigment epithelium was most strongly stained, particularly the apical processes of these cells which extend between the outer segments of the rods. Other strongly labeled structures (listed in declining order of labeling intensity) included the retinal capillary endothelium, the Müller cell microvilli, the outer plexiform layer, and the inner segments of the rods. Labeled cellular processes were visible between the external and inner limiting membranes. Endothelial cells in the choroid were not labeled. Although MCT2 was less abundant in retina than MCT1, significant MCT2 labeling was present in the outer and inner plexiform layers, the ganglion cell layer, the optic nerve fiber layer, and the inner limiting membrane. The latter was intensely stained as were the Müller cell foot processes surrounding retinal blood vessels. These findings are consistent with the hypothesis that glycogen bearing glial cells participate in metabolic coupling by converting glucose to lactate and by providing this nutrient to neurons via lactic acid efflux.