Endothelial cell barriers: Transport of molecules between blood and tissues

An endothelial cell monolayer separates interstitia from blood and lymph, and determines the bidirectional transfer of solutes and macromolecules across these biological spaces. We review advances in transport modalities across these endothelial barriers. Glucose is a major fuel for the brain and peripheral tissues, and insulin acts on both central and peripheral tissues to promote whole-body metabolic signalling and anabolic activity. Blood-brain barrier endothelial cells display stringent tight junctions and lack pinocytic activity. Delivery of blood glucose and insulin to the brain occurs through their respective carrier (Glucose transporter 1) and receptor (insulin receptor), enacting bona fide transcytosis. At supraphysiological concentrations, insulin is also likely transferred by fluid phase cellular uptake and paracellular transport, especially in peripheral microvascular endothelia. The lymphatic microvasculature also transports insulin but in this case from tissues to lymph and therefrom to blood. This serves to end the hormone's action and to absorb highly concentrated subcutaneously injected insulin in diabetic individuals. The former function may involve receptor-mediated transcytosis into lymphatic endothelial cells, the latter fluid phase uptake and paracellular transport. Lymphatic capillaries also mediate carrier-dependent transport of other nutrients and macromolecules. These findings challenge the notion that lymphatic capillaries only transport macromolecules through intercellular flaps.