Ion channels and their functional role in vascular endothelium

Endothelial cells (EC) form a unique signal-transducing surface in the vascular system. The abundance of ion channels in the plasma membrane of these nonexcitable cells has raised questions about their functional role. This review presents evidence for the involvement of ion channels in endothelial cell functions controlled by intracellular Ca2+ signals, such as the production and release of many vasoactive factors, e.g., nitric oxide and PGI(2). In addition, ion channels may be involved in the regulation of the traffic of macromolecules by endocytosis, transcytosis, the biosynthetic-secretory pathway, and exocytosis, e.g., tissue factor pathway inhibitor, von Willebrand factor, and tissue plasminogen activator. Ion channels are also involved in controlling intercellular permeability, EC proliferation, and angiogenesis. These functions are supported or triggered via ion channels, which either provide Ca2+-entry pathways or stabilize the driving force for Ca2+ influx through these pathways. These Ca2+-entry pathways comprise agonist-activated nonselective Ca2+-permeable cation channels, cyclic nucleotide-activated nonselective cation channels, and store-operated Ca2+ channels or capacitative Ca2+ entry. At least some of these channels appear to be expressed by genes of the trp family. The driving force for Ca2+ entry is mainly controlled by large-conductance Ca2+-dependent BKCa channels (slo), inwardly rectifying K+ channels (Kir2.1), and at least two types of Cl- channels, i.e., the Ca2+ +-activated Cl- channel and the housekeeping, volume-regulated anion channel (VRAC). In addition to their essential function in Ca2+ signaling, VRAC channels are multifunctional, operate as a transport pathway for amino acids and organic osmolytes, and are possibly involved in endothelial cell proliferation and angiogenesis. Finally, we have also highlighted the role of ion channels as mechanosensors in EC. Plasmalemmal ion channels may signal rapid changes in hemodynamic forces, such as shear stress and biaxial tensile stress, but also changes in cell shape and cell volume to the cytoskeleton and the intracellular machinery for metabolite traffic and gene expression.