IMMUNOLOCALIZATION OF GLUT-1 GLUCOSE-TRANSPORTER IN RAT SKELETAL-MUSCLE AND IN NORMAL AND HYPOXIC CARDIAC TISSUE

We compared the expression and cell-type localization of GLUT-1 mRNA and protein between cardiac and skeletal muscle of normal rats. Also, since we recently showed that cardiac GLUT- 1 is upregulated in rats exposed to hypobaric hypoxia, we examined the cellular localization of GLUT-I in cardiac tissue of normal and hypoxic rats. Confocal light microscopy and double immunofluorescent labeling revealed intense localization of GLUT-1 around neurofilament immunoreactivity within gastrocnemius muscle consistent with the previously described localization of large amounts of GLUT-1 in perineurial sheaths of skeletal muscle. However, using the same methods, we were unable to visualize GLUT- 1 adjacent to nerve fibers in numerous sections of right or left ventricles or atria. Compared with skeletal myoctes, however, GLUT-1 immunofluorescence among cardiomyocytes was much more intense, particularly along the plasma membrane and especially intercalated discs. GLUT-1 immunofluorescence was also seen within the walls of arterioles within the heart. The predominant localization of GLUT-1 expression to cardiomyocytes in heart tissue was confirmed by in situ mRNA hybridization to digoxigenin-conjugated GLUT-1 cDNA. Northern blot analysis demonstrated that GLUT-1 mRNA was increased severalfold in the cardiac tissues compared with skeletal muscle. Although we detected GLUT-1 protein by immunoblotting of detergent extracts of the heart, we could not detect GLUT-1 in similar extracts of skeletal muscle. The cell type distribution of GLUT-1 in hearts of hypoxic rats was not different by immunohistochemistry from normals. These data indicate that 1) the cell-type distribution of GLUT-1 in the heart differs markedly from that in skeletal muscle. GLUT-1 in cardiac tissue, unlike skeletal muscle, is predominantly expressed within myocytes. 2) Cardiac GLUT-I is not located along nerve fibers. 3) GLUT- 1 mRNA and protein levels in cardiac tissue are considerably greater than in skeletal muscle. 4) The hypoxia-induced increase in cardiac GLUT-1 that we previously reported must occur within cardiomyocytes.