FOREBRAIN ENDOTHELIUM EXPRESSES GLUT4, THE INSULIN-RESPONSIVE GLUCOSE-TRANSPORTER

The presence of GLUT4, the insulin-responsive glucose transporter, in microvascular endothelium and the responsiveness of glucose transport at the blood-brain barrier to insulin have been matters of controversy . To address these issues, we examined GLUT4 mRNA and protein expressi on in isolated brain microvessels and in cultured calf vascular cells derived from brain microvessels and aorta. We report here that GLUT4 m RNA can be detected in rat forebrain and its microvasculature using hi gh stringency hybridization of poly(A)(+) RNA isolated from these sour ces. This mRNA is identical to that found in adipose cells from rat. I mmunoblot analysis of isolated brain microvessels reveals that GLUT4 p rotein is also present. Peptide preadsorption studies and absence of o ur antibody reaction to human red cells suggest these findings are spe cific. Immunohistochemical staining of cultured calf vascular cells re veals that GLUT4 is expressed in brain endothelial cells but not peric ytes, nor in aortic endothelium or smooth muscle cells. The sensitivit y of the methods required to detect GLUT4 in brain and comparison to i ts abundance in low density microsomes from rat adipose cells indicate that GLUT4 is expressed in relatively low abundance in brain microvas cular endothelium. No significant differences are observed in steady s tate levels of GLUT4 mRNA in brain from streptozotocin diabetic compar ed to control rats. This last finding supports the concept of tissue-s pecific regulation of GLUT4. We conclude that brain microvascular endo thelium specifically expresses GLUT4 while ether vascular cells do not .