Glucose is the primary energy source for mammalian brain and its uptake and utilization involve all of the glucose transporters (GLUTs) in a regulated, cell-specific manner. The goals of this project are to understand the individual roles each transporter plays in normal neural function and cerebral metabolism and how this may be altered in pathological situations such as diabetes and/or stroke. We have reinvestigated the effects of chronic hyperglycemia and hypoglycemia on glucose uptake across the blood-brain barrier (BBB) in adult rats. Using a combination of direct in vivo regional glucose transport measurements, Western blot analysis, in situ hybridization and in vivo photoaffinity labeling, we have concluded that chronic (14 day) hyperglycemia induced in streptozotocin diabetes does not result in a measurable decrease in either the level or function of the 55 kDa GLUT1 glucose transporter, the mRNA for which is paradoxically increased. In contrast, chronic hypoglycemia induced by subcutaneous insulin pellets (2 U/day; 12 days), resulted in a generalized increase in BBB GLUT1 mRNA and protein, and a concomitant 20- 36% increase in transport activity. Photoaffinity labeling revealed that this increase in transport activity was due to an absolute increase in transporters, not to a subcellular redistribution of microvascular transporters. We are currently investigating the effects of cerebral ischemia on GLUT expression in 3 animal models; two combine unilateral carotid artery ligation with a period of systemic hypoxia (H/I), in either the immature rat or the diabetic, db/db, mouse, (in collaboration with Dr. Vannucci), and a third model of focal ischemia induced by middle cerebral artery occlusion (MCAO) in the adult rat, (with Dr. Hallenbeck). In the neonatal brain, a severe H/I insult produces enhanced GLUT1 mRNA expression in microvessels in both contralateral ( 50%), undamaged, and ipsilateral (15-30%), damaged hemispheres, which is seen as a 20% increase in 55 kDa BBB GLUT1 protein at 4 hours of recovery, relative to control. GLUT3 mRNA is elevated in penumbral regions of the ipsilateral hemisphere, such as piriform cortex and amygdala, but is rapidly reduced in the damaged areas: cortex, hippocampus, thalamus. Whereas the contralateral hemisphere returns to normal by 24 hours of recovery, the ipsilateral hemisphere does not. Preliminary experiments designed to adapt this model to the adult mouse and to determine the appropriate hypoxic exposure times, revealed marked differences in the mortality and severity of damage between females/males and between control/diabetic animals. This data is currently being integrated with our observations of a 55 kDa GLUT1 reduction in BBB of males vs females, a reduced 45 kDa GLUT1 in db/db vs control, particularly in males and an elevation of GLUT3 in cortex of female db/db compared to all other groups. The potential relationship between these observed differences in the expression of GLUT1 and GLUT3 proteins to subsequent ischemic vulnerability is under investigation. In the MCAO model we are specifically assessing the temporal relationship between glucose transporter expression and apoptotic vs necrotic cell death in the area of the infarct.
