An Electrodiffusion Model for the Blood-Brain Barrier Permeability to Charged Molecules

The endothelial surface glycocalyx layer (SGL) and the basement membrane (BM) are two important components of the blood-brain barrier (BBB). They provide large resistance to solute transport across the BBB in addition to the tight junctions in the cleft between adjacent endothelial cells. Due to their glycosaminoglycan compositions, they carry negative charge under physiological conditions. To investigate the charge effect of the SGL and BM on the BBB permeability to charged solutes, we developed an electrodiffusion model for the transport of charged molecules across the BBB. In this model, constant charge densities were assumed in the SGL and in the BM. Both electrostatic and steric interaction and exclusion to charged molecules were considered within the SGL and the BM and at their interfaces with noncharged regions of the BBB. On the basis of permeability data for the positively charged ribonuclease $(+4,radius=2.01nm)$ and negatively charged $α$-lactalbumin $(−10,radius=2.08nm)$ measured in intact rat mesenteric and pial microvessels, our model predicted that the charge density in both SGL and BM would be $∼30mEq/L$⁠, which is comparable to that in the SGL of mesenteric microvessels. Interestingly, our model also revealed that due to the largest concentration drop in the BM, there is a region with a higher concentration of negatively charged $α$-lactalbumin in the uncharged inter-endothelial cleft, although the concentration of $α$-lactalbumin is always lower than that of positively charged ribonuclease and that of a neutral solute in the charged SGL and BM.