Plasmalemmal vesicles function as transcytotic carriers for small proteins in the continuous endothelium

We investigated the location and the structural identity of the small pore system, postulated by the pore theory of capillary permeability, using a murine heart perfusion system and small protein molecules as preferential probes for the small pores. Dinitrophenylated proteins were perfused in situ in the absence and in the presence of N-ethylmaleimide (NEM), a reagent known to interfere with membrane fusion of vesicular carriers with their target membranes. The exit pathways of the tracers from vascular lumina to the interstitia were followed by immunoelectron microscopy and by tissue fractionation biochemistry to quantitate their transport and to estimate the extent of transport inhibition by NEM. After 5 min of perfusion, all tracers used were found essentially restricted to plasmalemmal vesicles (PVs) within the endothelium and NEM inhibited their transport by 80-85%. The transport of [C-14]inulin and [C-14]sucrose, assumed to follow the paracellular pathway was marginally affected by NEM. These findings indicate that PVs function as structural equivalents of the small pore system for molecules >2 nm in diameter.