Effect of hyperglycemia on brain penetrating arterioles and cerebral blood flow before and after ischemia/reperfusion

Abstract

The effect of preexisiting hyperglycemia on cerebral blood flow (CBF) and brain penetrating arterioles before and after 2 h of ischemia and 30 min of reperfusion was determined. Male Wistar rats that were either hyperglycemic (50 mg/kg streptozotocin; n=24) or normoglycemic (n=24) were subjected to transient ischemia by filament occlusion or nonischemic. CBF was measured prior to ischemia using microspheres and during transient ischemia using laser Doppler. Edema was compared by wet/dry weights. Constriction to apamin, TRAM-34, and L-NNA, inhibitors of small- and intermediate-conductance calcium-activated potassium channels (SK and IK) and nitric oxide, were compared in penetrating arterioles from the ischemic hemisphere to investigate changes in basal tone and endothelium-dependent vasodilator responses. Preexisiting hyperglycemia did not affect CBF in non-ischemic animals or after transient ischemia; however, edema was significantly greater. Ischemia and reperfusion caused decreased basal tone in penetrating arterioles similarly in normoglycemic and hyperglycemic animals that was restored by apamin, and further increased by TRAM-34 and L-NNA. The restoration of tone in penetrating arterioles by apamin and TRAM-34 suggests that transient ischemia activates SK and IK channels in penetrating arterioles. This effect of ischemia was not different between normoglycemic and hyperglycemic animals, suggesting that it was related to ischemia and reperfusion rather than hyperglycemia.

Fig. 1

Relative changes in cerebral blood flow (CBF) during MCAO and for 30 min of reperfusion using laser Doppler flowmetry. Changes in flow in response to carotid occlusion (CCAo) and middle cerebral artery occlusion (MCAo) and after restoration of CBF to allow reperfusion of the middle cerebral artery (MCAr) and carotid artery (CCAr). There was no difference in CBF either basally or in response to occlusion between normoglycemic (closed circles) and hyperglycemic (open circles) animals. Reperfusion restored CBF to normal in both groups, with hyperglycemic animals tending to have higher flows after 30 min of reperfusion compared to normoglycemic (p=0.09)

Fig. 2

Brain water content in ipsilateral and contralateral hemispheres from normoglycemic and hyperglycemic animals after 2 h of ischemia and 30 min of reperfusion. MCAO caused vasogenic edema and increased water content in the ipsilateral hemisphere in both normoglycemic and hyperglycemic animals (**p<0.01 vs. contralateral)

Fig. 3

Ratio of ipsilateral to contralateral water content after 2 h of ischemia and 30 min of reperfusion in normoglycemic and hyperglycemic animals. Hyperglycemic animals had a significantly greater increase in water content compared to normoglycemic animals (**p<0.01 vs. normoglycemic)

Fig. 4

Percent tone in penetrating arterioles basally at 40 mmHg and after cumulative addition of apamin (300 nM), TRAM-34 (1.0 µM), and L-NNA (0.1 mM) prior to and after 2 h of ischemia and 30 min of reperfusion. Normoglycemic animals are shown in a, and hyperglycemic animals are shown in b. Penetrating arterioles had considerable basal tone that was increased with addition of apamin, TRAM-34, and L-NNA. There was no difference in tone between normoglycemic and hyperglycemic animals prior to MCAO. However, after MCAO, arterioles had diminished tone that was restored by apamin, suggesting that ischemia and reperfusion activate these channels. There was no difference in tone after MCAO between normoglycemic and hyperglycemic animals. *p<0.05 vs. nonischemic

Fig. 5

Percent constriction of penetrating arterioles after cumulative addition of apamin (300 nM), TRAM-34 (1.0 µM), and L-NNA (0.1 mM) prior to and after 2 h of ischemia and 30 min of reperfusion. Normoglycemic animals are shown in a, and hyperglycemic animals are shown in b. All vessels from all groups constricted to apamin, TRAM-34, and L-NNA, suggesting that basal EDHF and NO are produced in these vessels that mitigate tone. There was no difference in constriction between normoglycemic and hyperglycemic animals prior to MCAO; however, arterioles responded with greater constriction to apamin after MCAO. There was no difference in constriction after MCAO between normoglycemic and hyperglycemic animals. **p<0.01 vs. nonischemic