Are claudin-5 tight-junction proteins in the blood-brain barrier porous?

The capillaries of the brain are particularly special,as they are not simply conduits for blood,but are primarily responsible to ensure that the neurons function in a strictly regulated homeostatic interstitium.Brain endothelial cells(BECs)express a plethora of ion channels on its luminal and abluminal surfaces,namely:potassium(K^+)channels(i.e.,Kir2 and Kv1),chloride(Cl^)/bicarbonate(HCO3^–)channels,as well as a number of ion-solute exchangers(Redzic et al.,2011).