Flow-dependent concentration polarization of plasma proteins at the luminal surface of a cultured endothelial cell monolayer

Flow-dependent concentration or depletion of atherogenic low density lipoproteins which has been theoretically predicted to occur at a blood/endothelium boundary may play an important role in the genesis, progression, and regression of atherosclerosis in man and intimal hyperplasia in vascular grafts implanted in the arterial system in man and experimental animals. Hence to explore such a possibility, we have studied the effect of a steady shear flow on concentration polarization of plasma proteins and lipoproteins at the luminal surface of a cultured bovine aortic endothelial cell (BAEC) monolayer which served as a model of the vessel wall of an artery or an implanted vascular graft. The study was carried out by circulating a cell culture medium containing fetal calf serum or bovine plasma lipoproteins in steady flow through a parallel-plate flow cell in which a cultured BAEC monolayer was installed, over the physiologic ranges of wall shear rate and water filtration velocity at the BAEC monolayer. The water (cell culture medium) filtration velocity at the BAEC monolayer was determined to provide a measure of the change in concentration of plasma protein particles at the luminal surface of the BAEC monolayer. It was found that for perfusates containing plasma proteins and/or lipoproteins, water filtration velocity varied as a function of flow rate, being lowest in the absence of flow. Water filtration velocity increased or decreased as flow rate increased or decreased from an arbitrarily set non-zero value, indicating that surface concentration of protein particles varied as a direct function of flow rate, and the process was reversible. It was also found that at particle concentrations equivalent to those found in a culture medium containing serum at 20% by volume, plasma lipoproteins which were much smaller in number and lower in concentration but larger in size than albumin, showed almost the same effect as observed with serum which contained both lipoproteins and albumin, indicating that the substance responsible for this phenomenon is not albumin but lipoprotein whose diffusivity is much smaller than that of albumin. The results strongly support our hypothesis that flow-dependent concentration polarization of lipoproteins occurs at a blood endothelium boundary, and this in turn promote the localization of various vascular diseases which develop in our arterial system.