Endocytosis and exocytosis of transferrin by isolated capillary endothelium

Vesicular transport of solutes across capillary walls may be regulated by specific solute-endothelial interactions. Little data is available on the vesicular transport of serum proteins which may transit the capillary wall in situ. Capillaries were isolated from epididymal fat and incubated in fluorescent-labeled transferrin and radiolabeled sucrose. Endocytosis and exocytosis of these tracers were quantitated on a picomolar basis over timed intervals and standardized against the amount of endothelial DNA present in the isolate. The rate of vesicular endocytosis of transferrin was 6-7 times greater than that of sucrose indicating a mechanism of selection for transferrin. Endocytosis as a function of external concentration exhibited complex kinetics for transferrin that was consistent with an adsorptive component and a fluid component. Sucrose uptake appeared to be simple fluid endocytosis but with a rate-limiting concentration at 500-600 microM. Vesicular exocytosis of both solutes from preloaded capillaries appeared to occur more rapidly than their endocytosis. This was probably not due to different rates of filling and emptying of attached vesicles nor to an intrinsic difference in rates of vesicle interiorization and refusing with the plasma membrane. Different rates of endocytosis and exocytosis may only be apparent since exocytosis of marker before the capillaries reach ingestion equilibrium would reduce the measured uptake rate.