RESUMO
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 expression in isolated brain microvessels and in cultured calf vascular cells derived from brain microvessels and aorta. We report here that GLUT4 mRNA can be detected in rat forebrain and its microvasculature using high stringency hybridization of poly(A)+ RNA isolated from these sources. This mRNA is identical to that found in adipose cells from rat. Immunoblot analysis of isolate brain microvessels reveals that GLUT4 protein is also present. Peptide preadsorption studies and absence of our antibody reaction to human red cells suggest these findings are specific. Immunohistochemical staining of cultured calf vascular cells reveals that GLUT4 is expressed in brain endothelial cells but not pericytes, nor in aortic endothelium or smooth muscle cells. The sensitivity of the methods required to detect GLUT4 in brain and comparison to its abundance in low density microsomes from rat adipose cells indicate that GLUT4 is expressed in relatively low abundance in brain microvascular endothelium. No significant differences are observed in steady state levels of GLUT4 mRNA in brain from streptozotocin diabetic compared to control rats. This last finding supports the concept of tissue-specific regulation of GLUT4. We conclude that brain microvascular endothelium specifically expressed GLUT4 while other vascular cells do not.
