Background
Preexisting hyperglycemia is associated with enhanced injury in the postischemic brain, including a significantly higher incidence and severity of cerebral infarction and edema formation. Because elevated glucose is known to have a significant effect on vascular function, including myogenic tone and reactivity, we hypothesized that elevated glucose influences stroke outcome through a direct effect on cerebral blood flow (CBF) and the extent of postischemic reperfusion.
Methods
Male Wistar rats that were either normoglycemic (n=19; glucose=158±12 mg/dL) or hyperglycemic (n=17; glucose=348±41 mg/dL; p<0. 01) by injection of 50 mg/kg streptozocin (STZ) for 3 days, underwent middle cerebral artery occlusion (MCAO) for 1 h followed by 30 min. of reperfusion. Neurologic deficit score (out of 32), infarction score (0–4) after TTC staining, and percent of CBF recovery were compared between groups. In separate groups of animals that did not undergo MCAO, microvessels were isolated and PKCβII activity determined by Western analysis of phospho-PKCβII. Lastly, the percent of myogenic tone was determined in isolated and pressurized parenchymal arterioles.
Results
Hyperglycemic animals had significantly worsened stroke outcome compared to normoglycemic controls, including neurologic deficit score (32. 0 ± 0.0 vs. 4.5 ± 1.5; p<0.01), infarct score (3.5 ± 0.5 vs. 1.5 ± 0.4; p<0.01) and extent of CBF recovery (58 ± 12% vs. 87 ± 13%; p<0.05). Further, high serum glucose negatively correlated with decreased reperfusion after ischemia (r= 0.5; p=0.02). In cerebral microvessels, PKCβII activity was significantly elevated in hyperglycemic animals compared to controls (Western ratio = 2.7 ± 0.3 vs. 3.8 ± 0.8; p<0.01) that was associated with increased myogenic tone (% tone at 40 mmHg= 38 ± 2% vs. 54 ± 8%; p<0.01).
Conclusions
These results demonstrate that preexisting hyperglycemia significantly worsens stroke outcome, including enhanced infarction and neurologic deficit, possibly through a direct effect on the vasculature that diminishes postischemic reperfusion. Increased PKCβII activity in cerebral microvessels is likely due to enhanced flux of glucose through the glycolytic pathway that increases diacylglycerol accumulation, the primary activator of PKC. This pathway has been shown to be a major contributor to diabetic vascular complications. In addition, since PKC activity is known to underlie myogenic activity, it is possible that its enhanced activation under hyperglycemic conditions contributes to the greater myogenic tone in those animals, an effect that would serve to diminish CBF reperfusion and worsen stroke outcome. In conclusion, it appears that the cerebrovasculature is a potential therapeutic target for protection during hyperglycemic stroke, possibly by preventing the glucose-induced increase in PKCβII activation and/or enhanced myogenic tone.
Footnotes
Acknowledgements
Supported by NIH NS40071 and Totman Medical Research Trust.