This study questioned whether and to what extent diabetes-induced changes in the cerebral vasculature increase the risk of hemorrhage and augment ischemic injury. There were 3 important findings in this study. First, infarct size was smaller and localization was different in the diabetic rats. Second, hemorrhagic transformation was increased in this group. Third, despite a very short duration of mild diabetes, tortuosity of cerebral vessels was significantly enhanced. These results provide important and timely information regarding the potential mechanisms that may contribute to increased stroke risk and worse outcome in diabetes.
In recent years, the role of hyperglycemia in the pathophysiology and outcome of acute ischemic stroke has gained significant attention. Numerous studies including the NINDS tPA stroke trial showed that elevated admission blood sugar was a significant predictor of poor clinical outcome and HT [16
]. The relative role of acute versus chronic hyperglycemia in the pathogenesis of this poor outcome has been debated and has not been determined unequivocally [10
]. However, based on the findings that short-term mortality is higher and final infarct size is larger in nondiabetic patients who present with hyperglycemia at admission, several studies have suggested that hyperglycemia and not necessarily diabetes aggravates ischemic injury and stroke outcome [18
]. In experimental models, hyperglycemia has been shown to increase infarct size and a limited number of studies also demonstrated augmented HT in hyperglycemic ischemia/reperfusion injury [6
]. However, these studies employed hyperglycemia induced by glucose injection prior to or at the time of MCA and blood glucose levels were above 300 mg/dl. The intriguing finding of the current study is that infarct size is smaller but secondary hemorrhage is larger in diabetic GK rats that present with moderate elevations in blood glucose (180–250 mg/dl). Consistent with smaller infarct size, diabetic rats had less edema. Moreover, the infarct is subcortical as opposed to subcortical and cortical localization in control rats. These results suggest a difference in the pathophysiology of ischemic injury in the diabetic state. It is well established that diabetes promotes ischemic preconditioning in the myocardium [20
]. It is also known that ischemic preconditioning is neuroprotective [21
]. It is possible that in our model, diabetes induced-changes in the cerebrovasculature result in ischemic preconditioning and thereby prevent infarct expansion but promote BBB breakdown and hemorrhage emphasizing the importance of the neurovascular unit.
During focal cerebral ischemia, disruption of the BBB complex, which consists of the endothelial cell, its tight junctions, the basal lamina and the astrocytic foot processes results in damage to the entire neurovascular unit [23
]. If the ischemia is prolonged, breakdown of BBB increases cerebral injury by the development of vasogenic edema and secondary hemorrhage known as HT [11
]. Matrix metalloproteinases (MMPs) are a class of zinc-dependent endopeptidases that contribute to the breakdown of BBB. Several laboratories have demonstrated that MMP activity, especially gelatinases MMP-2 and MMP-9, is increased after focal cerebral ischemia and contributes to the development of HT [23
]. MMPs are also up-regulated in diabetes. We have also found that MMP activity is increased in MCAs of GK rats and this increase is associated with enhanced pathological remodeling after 10–12 weeks of diabetes [9
]. The animals used in the current study had mild diabetes only for 4–5 weeks. While we cannot rule out the possibility that in-breeding may cause alterations in vessel structure, reactivity and mechanics that may contribute to the changes we observe in this colony of diabetic rats, our data provide evidence that despite the short duration of diabetes, there was increased MMP-2 activity and tortuosity suggesting early cerebrovascular remodeling. This remodeling response may contribute to increased hemorrhage in diabetes.
Increased tortuosity is commonly measured in evaluation of coronary vessels. This is an indicator of ischemia-induced vascular restructuring as it increases vessel surface area. The current study found that the TI of cerebral vessels was increased in diabetes. These findings suggest that there is microvascular remodeling and potential neovascularization in the GK rat, which may contribute to decreased infarct size. Neovascularization in diabetes is very complex and regulated in a temporal and tissue specific manner. It is well established that hyperglycemia-mediated oxidative damage to microvascular endothelial cells triggers a cascade of events that lead to changes in vascular proliferative retinopathy and formation of new blood vessels on the surface of the retina [29
]. These immature vessels then break and leak worsening the retinal damage. Although we do not have direct evidence, it is highly possible that similar to retina, in the cerebrovasculature newly formed or remodeled vessels cannot resist the impact of ischemia/reperfusion injury and increased bleeding occurs. Preliminary evaluation of baseline BBB permeability in the absence of ischemic injury by Evans blue extravasation method was not sensitive to detect permeability differences between control and diabetic animals. Interestingly, edema after MCAO was significantly less in GK rats most likely due to smaller infarct size. It has to be recognized that the current study has several limitations. First, cerebral blood flow was measured to ensure that similar degree of blood flow is achieved in control versus diabetic rats during MCAO and after reperfusion but evaluation of blood flow with more reliable methods are needed to assess whether changes observed in vessel structure are associated with alterations in baseline blood flow as well as collateral flow during MCAO. Second, HT was defined as the presence of visible bleeding in coronal brain sections and more quantitative methods are needed. Similarly, vascular remodeling was assessed by visualization of the vascular tree, measurement of tortuosity and morphometry of histogel and not pressure-fixed vessels. Whether increased tortuosity is due to remodeling of existing vessels or due to neovascularization or both remains to be determined. Lastly, the effect(s) glycemic control on the severity of ischemic injury and outcome remain to be determined. Nevertheless our findings are very important and timely.