Distribution of monocarboxylate transporters MCT1 and MCT2 in rat retina

Transport of lactic acid and other monocarboxylates such as pyruvate and th e ketone bodies through cellular membranes is facilitated by specific trans port proteins. We used chicken polyclonal antibodies to the monocarboxylate transporters-1 and -2 to determine their cellular and subcellular distribu tions in rat retina, and we compared these distributions to those of the gl ucose transporters-1 and -3. Monocarboxylate transporter-1 was most highly expressed by the apical processes of retinal pigment epithelium that surrou nd the outer segments of the photoreceptor cells. In contrast to glucose tr ansporter-1, monocarboxylate transporter-1 was not detected on the basal me mbranes of pigment epithelium. The luminal and abluminal endothelial plasma membranes in retina also exhibited heavy labeling by antibody to monocarbo xylate transporter-1. In addition, this transporter was associated with the Muller cell microvilli, the plasma membranes of the rod inner segments, an d all retinal layers between the inner and external limiting membranes. Mon ocarboxylate transporter-2 was found to be abundantly expressed on the inne r (basal) plasma membrane of Muller cells and by glial cell processes surro unding retinal microvessels. This transporter was also present in the plexi form and nuclear layers but was not detected beyond the external limiting m embrane. Recent studies have shown that lactic acid transport is of particular impor tance at endothelial and epithelial barriers where membranes of adjoining c ells are linked by tight junctions. Our results suggest that monocarboxylat e transporter-1 functions to transport lactate between the retina and the b lood, both at the retinal endothelium and the pigment epithelium. The locat ion of monocarboxylate transporter-2 on glial foot processes surrounding re tinal vessels suggests that this transporter is also important in blood-ret inal lactate exchange. In addition, the abundance of these transporters in Muller cells and synaptic (plexiform) layers suggests that they function in lactate exchange between neurons and glia, supporting the notion that lact ate plays a key role in neural metabolism. (C) 1999 IBRO. Published by Else vier Science Ltd.