Monocarboxylic acid transporters, MCT1 and MCT2, in cortical astrocytes invitro and in vivo

We used sequence-specific antibodies to characterize two monocarboxylic aci d transporters, MCT1 and MCT2, in astrocytes. Both proteins are expressed i n primary cultures of cortical astrocytes, as indicated by immunoblotting a nd immunofluorescence. Both MCT1 and MCT2 are present in small, punctate st ructures in the cytoplasm and at the cell membrane. Cells showing very low levels of labeling for glial fibrillary acidic protein (GFAP) also label mo re dimly for MCT2, but not for MCT1. In vivo, double-label immunofluorescen ce studies coupled with confocal microscopy indicate that MCT1 and MCT2 are rare in astrocytes in the cortex. However, they are specifically labeled i n astrocytes of the glial limiting membrane and in white matter tracts. Bot h transporters are also present in the microvasculature. Comparison of labe ling for MCT1 and MCT2 with markers of the blood-brain barrier shows that t he transporters are not always limited to the astrocytic endfeet in vivo. O ur results suggest that the level of expression of monocarboxylic acid tran sporters MCT1 and MCT2 by cortical astrocytes in vivo is significantly lowe r than in vitro but that astrocytes in some other regions of the brain can express one or both proteins in significant amounts.