Regional Variations in the Apparent Diffusion Coefficient and the Intracellular Distribution of Water in Rat Brain During Acute Focal Ischemia

Abstract

Background and Purpose-The apparent diffusion coefficient of water (ADC) rapidly drops in ischemic tissue after cerebral artery occlusion. This acute drop is thought to be caused by the loss of extracellular fluid and the gain of intracellular fluid. To test the latter possibility, changes in ADC and the size of several cellular compartments were assessed in 3 regions of rat brain at the end of 90 minutes of focal cerebral ischemia. Methods-One middle cerebral artery was permanently occluded in 8 Sprague-Dawley rats; sham occlusions were performed in 2 other rats. ADC maps were generated 90 minutes later, and the brains were immediately perfusion fixed. Three regions of interest (ROIs) were defined on the basis of ADC range. Various neuronal, astrocytic, and capillary compartments in each ROI were quantified with light and electron microscopy. Results-At the end of 90 minutes of ischemia, mean ADC was normal in the cortex of sham-operated rats and the contralateral cortex of ischemic rats (ROI-a), 25% lower in the ipsilateral frontoparietal cortex (ROI-b), and 45% lower in the ischemic lateral caudoputamen (ROI-c). At this time, the frequency of swollen astrocytic cell bodies and volume of swollen dendrites and astrocytic processes in neuropil were ROI-aϽROI-bϽROI-c. In ROI-b and ROI-c, 40% and 60% of the neurons, respectively, were shrunken; the shrunken neurons were Ϸ25% smaller in ROI-c than in ROI-b. In these areas, many capillary endothelial cells, pericytes, and perivascular foot processes were swollen. Conclusions-The initial lowering of ADC during focal ischemia probably is the result of not only the acute loss of extracellular fluid and concomitant swelling of various cellular compartments but also concurrent neuronal shrinkage.

In Vivo Excitotoxicity Induced by Ouabain, a Na+/K+-ATPase Inhibitor

“…This leads to swelling of some cellular compartments and to a net decrease of extracellular volume and an increase in intracellular volume. It is thought that these changes to a large extent underlie the decrease in ADC that occurs in the acute phase after the onset ischemia (Benveniste et al, 1992;Liu et al, 2001;Pfeuffer et al, 1998;van der Toorn et al, 1996). In our model, inhibition of Na + /K + -ATPase resulted in an acute 40% drop in water ADC, which could be attenuated, but not completely prevented, by pretreatment with the NMDA-receptor antagonist MK-801.…”

Bumetanide Inhibition of the Blood-Brain Barrier Na-K-Cl Cotransporter Reduces Edema Formation in the Rat Middle Cerebral Artery Occlusion Model of Stroke

“…In contrast, our present study examined events occurring during permanent MCAO without the complicating factors of reperfusion. We assessed brain edema via determination of ADC because a decrease in ADC values has been shown to be an excellent indicator of brain edema (Hasegawa et al, 1994;Knight et al, 1994;Liu et al, 2001a;Moseley et al, 1990). This method also allows comparison of specific ipsilateral and contralateral regions, providing an internal control for each experiment.…”

Hydrogen Peroxide Potentiates Volume-sensitive Excitatory Amino Acid Release via a Mechanism Involving Ca2+/Calmodulin-dependent Protein Kinase II

“…The potent VRAC blocker tamoxifen reduces infarct size after transient and global ischemia, although the exact mechanism of tamoxifen protection remains to be elucidated (79,80). Astrocytes are a plausible site for VRAC activation because, unlike neurons, they are highly susceptible to ischemia-induced cell swelling (12)(13)(14). However, swelling may not be the only factor responsible for pathological VRAC regulation.…”

“…These anion channels are also permeable to a variety of small organic osmolytes including amino acids, polyols, and methylamines (7,10,11). Although the molecular identity of VRACs and the precise mechanisms of their regulation are not known, VRAC activation is probably provoked by pronounced pathological cell swelling, primarily seen in astrocytes in ischemia (12)(13)(14).…”