Effect of plasma proteins on endothelial binding and vesicle loading of anionized ferritin in rabbit aorta

Endothelial binding and vesicle loading of anionized ferritin (AF, isoelectric point, 3.8 to 4.2) in plasma and in Tyrode's solution were investigated in the rabbit aorta. After 2 minutes of in situ perfusion, binding to vesicle necks was significantly higher than elsewhere on the membrane and was independent of AF concentration in the range of 0.05 g/ml to 0.2 g/ml for both perfusates. After 30 minutes, no particles were seen in abluminal vesicles and few had reached the basement membrane. Compared to AF in plasma, AF in Tyrode's solution showed lower endothelial binding and greater vesicle loading (p less than 0.005). The average number of particles per loaded vesicle was equal to the average particle density at vesicle necks in Tyrode's solution, but not in plasma, for 0.1 g/ml AF. Gel chromatography and electrophoresis demonstrated no detectable difference between AF particles incubated in plasma or Tyrode's solution. Our experiments indicate that adherence to, or entrapment in, the glycocalyx is an important step for molecules that are transported by vesicles. We hypothesize that plasma proteins adsorb to the endothelial surface and partially shield its net negative charge so that more AF may bind. Plasma proteins also inhibit vesicle loading either sterically or by modifying the electrical potential at the vesicle neck compared to that within the vesicle cavity.