The perivascular pool of aquaporin-4 mediates the effect of osmotherapy in postischemic cerebral edema

Abstract

Objective: Osmotherapy with hypertonic saline ameliorates cerebral edema associated with experimental ischemic stroke. We tested the hypothesis that hypertonic saline exerts its antiedema effect by promoting an efflux of water from brain via the perivascular aquaporin-4 pool. We used mice with targeted disruption of the gene encoding alpha-syntrophin (alpha-Syn(-/-)) that lack the perivascular aquaporin-4 pool but retain the endothelial pool of this protein.

Design: Prospective laboratory animal study.

Setting: Research laboratory in a university teaching hospital.

Measurements and main results: Halothane-anesthetized adult male wildtype C57B/6 and alpha-Syn(-/-) mice were subjected to 90 min of transient middle cerebral artery occlusion and treated with either a continuous intravenous infusion of 0.9% saline or 3% hypertonic saline (1.5 mL/kg/hr) for 48 hr. In the first series of experiments (n = 59), increased brain water content analyzed by wet-to-dry ratios in the ischemic hemisphere of wildtype mice was attenuated after hypertonic saline (79.9% +/- 0.5%; mean +/- SEM) but not after 0.9% saline (82.3% +/- 1.0%) treatment. In contrast in alpha-Syn(-/-) mice, hypertonic saline had no effect on the postischemic edema (hypertonic saline: 80.3% +/- 0.7%; 0.9% saline: 80.3% +/- 0.4%). In the second series of experiments (n = 32), treatment with hypertonic saline attenuated postischemic blood-brain barrier disruption at 48 hr in wildtype mice but not in alpha-Syn(-/-) mice; alpha-Syn(-/-) deletion alone had no effect on blood-brain barrier integrity. In the third series of experiments (n = 34), alpha-Syn(-/-) mice treated with either hypertonic saline or 0.9% saline had smaller infarct volume as compared with their wildtype counterparts.

Conclusions: These data demonstrate that 1) osmotherapy with hypertonic saline exerts antiedema effects via the perivascular pool of aquaporin-4, 2) hypertonic saline attenuates blood-brain barrier disruption depending on the presence of perivascular aquaporin-4, and 3) deletion of the perivascular pool of aquaporin-4 alleviates tissue damage after stroke, in mice subjected to osmotherapy and in nontreated mice.

Figure 1

% Brain water content in wild type (WT) and α-Syn^−/− surgical shams (n = 4 each) (in each hemisphere) and the ipsilateral ischemic (I) and contralateral (C) nonischemic hemisphere of WT and α-Syn−/− mice treated with 0.9% saline (NS) or hypersaline saline (HS) for 48 hrs following 90-min middle cerebral artery occlusion (WT-NS: n = 12; WT-HS: n = 12; α-Syn^−/− -NS: n = 6; α-Syn^−/− HS: n = 6). *p < 0.05 vs. corresponding ipsilateral ischemic and contralateral nonischemic hemisphere.

Figure 2

Blood brain barrier disruption as estimated by Evans blue (EB) extravasation, expressed as the ratio of absorbance intensity in the ischemic hemisphere to that in the nonischemic hemisphere (EB extravasation index) in various treatment groups at 48 hrs following middle cerebral artery occlusion (n = 5 each). *p < 0.05 vs. wild type-0.9% saline (WT-NS). HS, hypertonic saline.

Figure 3

Triphenyltetrazolium chloride-determined infarct volume (mm3) in the cerebral cortex, caudoputamen complex, and the ipsilateral hemisphere at 48 hrs post-middle cerebral artery occlusion in various treatment groups (wild type-0.9% saline [WT-NS]: n = 5; WT-hypertonic saline [HS]: n =5; α-Syn^−/− -NS: n = 6; α-Syn^−/− -HS: n = 6). *p < 0.05 vs. WT-NS; †p < 0.05 vs. WT-HS. CP, caudoputamen.