Background: Facilitative glucose transporters (GLUT) have unique kinetic characteristics and distributions suited to the functions of the tissues in which they reside. However, little is known about their individual roles in renal glucose metabolism, and previous investigations of renal GLUT expression have been extensive only with respect to their mRNA levels. We provide here a complete analysis of three GLUT isoforms along the nephron using a sensitive immunodetection method.
Experimental design: Normal rat kidneys were harvested, fixed in paraformaldehyde, and embedded in either paraffin or resin as required for immunogold labeling of individual GLUT Isoforms 1, 3, and 4. Samples were evaluated by light microscopy and selected regions analyzed by high resolution optical scanning with computer-assisted detection of immunogold-labeled GLUT at the subcellular level. We describe, compare, and related to the local patterns of glucose metabolism the cellular and subcellular expression patterns of these GLUT along the nephron.
Results: GLUT1 was most intensely labeled in the medullary thick ascending limbs of Henle, cortical collecting ducts, and inner medullary collecting ducts. In contrast, GLUT3 was most prominent in the inner medullary collecting ducts and GLUT4 in medullary thick ascending limbs of Henle. All three GLUT were detected in glomerular tufts, and GLUT1 was also detected in parietal epithelial cells. The predominant subcellular distributions in tubule cells were: basolateral and basolateral/cytoplasmic for GLUT1; basolateral and cytoplasmic for GLUT3; and perinuclear/cytoplasmic for GLUT4. GLUT 1 and 3 expressions were confirmed in specific regions by immunoblotting.
Conclusions: 1) GLUT 1, 3, and 4 are expressed in both glomeruli and renal tubules. 2) The unique GLUT expression patterns along the renal tubules suggests unique functional roles for these isoforms. 3) The renal cortex demonstrates lesser labeling intensity for the high-affinity GLUT compared with the medulla, where higher rates of glucose oxidation and glycolytic metabolism are paralleled by higher GLUT labeling intensities.