Effects of transient loss of shear stress on blood-brain barrier endothelium: role of nitric oxide and IL-6.

Loss of blood-brain barrier (BBB) function may contribute to post-ischemic cerebral injury by yet unknown mechanisms. Ischemia is associated with anoxia, aglycemia and loss of flow (i.e. shearing forces). We tested the hypothesis that loss of shear stress alone does not acutely affect BBB function due to a protective cascade of mechanisms involving cytokines and nitric oxide (NO). To determine the relative contribution of shear stress on BBB integrity we used a dynamic in vitro BBB model based on co-culture of rat brain microvascular endothelial cells (RBMEC) and astrocytes. Trans-endothelial electrical resistance (TEER), IL-6 release and NO levels were measured from the lumenal and ablumenal compartments throughout the experiment. Flow-exposed RBMEC were challenged with 1 h of normoxic-normoglycemic flow cessation (NNFC) followed by reperfusion for 2 to 24 h. NNFC caused a progressive drop in nitric oxide production during flow cessation followed by a time-dependent increase in ablumenal IL-6 associated with a prolonged NO increase during reperfusion. The nitric oxide synthetase (NOS) inhibitor L-NAME (10 microM) abrogated all effects of NNFC, including changes in NO and cytokine production. BBB permeability did not increase during or after NNFC/reperfusion, but was increased by treatment with L-NAME or when the effects of IL-6 were blocked. Flow adapted RBMEC and astrocytes respond to NNFC/reperfusion by overproduction of IL-6, possibly secondary to increased production of NO during the reperfusion. Maintenance of BBB function during and following NNFC appears to depend on intact NO signaling and IL-6 release.