Blood-brain barrier glucose transporter: Effects of hypo- and hyperglycemia revisited

The transport of glucose across the blood-brain barrier (BBB) is mediated b y the high molecular mass (55-kDa) isoform of the GLUT1 glucose transporter protein. In this study we have utilized the tritiated, impermeant photolab el 2-N-[4-(1-azi-2,2,2-trifluoroethyl)[2-H-3]propyl],3-bis(D-mannose-4-ylox y)-2-propylamine to develop a technique to specifically measure the concent ration of GLUT1 glucose transporters on the luminal surface of the endothel ial cells of the BBB, We have combined this methodology with measurements o f BBB glucose transport and immunoblot analysis of isolated brain microvess els for labeled luminal GLUT1 and total GLUT1 to reevaluate the effects of chronic hypoglycemia and diabetic hyperglycemia on transendothelial glucose transport in the rat. Hypoglycemia was induced with continuous-release ins ulin pellets (6 U/day) for a 12- to 14-day duration; diabetes was induced b y streptozotocin (65 mg/kg i.p,) for a 14- to 21-day duration. Hypoglycemia resulted in 25-45% increases in regional BBB permeability-surface area (PA ) values for D-[C-14]glucose uptake, when measured at identical glucose con centration using the in situ brain perfusion technique. Similarily, there w as a 23 +/- 4% increase in total GLUT1/mg of microvessel protein and a 52 /- 13% increase in luminal GLUT1 in hypoglycemic animals, suggesting that b oth increased GLUT1 synthesis and a redistribution to favor luminal transpo rters account for the enhanced uptake. A corresponding (twofold) increase i n cortical GLUT1 mRNA was observed by in situ hybridization, In contrast, n o significant changes were observed in regional brain glucose uptake PA, to tal microvessel 55-kDa GLUT1, or luminal GLUT1 concentrations in hyperglyce mic rats. There was, however, a 30-40% increase in total cortical GLUT1 mRN A expression, with a 96% increase in the microvessels. Neither condition al tered the levels of GLUT3 mRNA or protein expression. These results show th at hypoglycemia, but not hyperglycemia, alters glucose transport activity a t the BBB and that these changes in transport activity result from both an overall increase in total BBB GLUT1 and an increased transporter concentrat ion at the luminal surface.