Transport of protein hormones through the vascular endothelium

Introduction In most organs, the vascular endothelium forms a barrier through which the passage of macromolecules is slow and relatively inefficient (Williams 1983). Protein hormones are secreted at very low concentration and act rapidly at their target organs. There is thus a need for specific transendothelial transport mechanisms for these hormones. We will review here what is known about the structures of endothelial cells in relation to protein transport. We will then describe the specific mechanism we have recently observed for human chorionic gonadotropin/luteinizing hormone (hCG/LH) and discuss its potential generalization to the transendothelial transport of other hormones. The vascular endothelium constitutes a major barrier for the blood–tissue exchange of macromolecules The exchange of metabolites and secretory products between the blood and tissues takes place at the level of the microvasculature (i.e. in capillaries and venules). The vascular endothelium is the main barrier although the basal lamina, pericytes