Gap junctions (GJ) are formed by a family of integral membrane proteins termed connexins (Cx). Vascular endothelial cells express three connexin family members Cx37, Cx40, and Cx43. In each gap junction, six connexins oligomerize to form a connexon hemichannel at one cell membrane; this connexon docks with a connexon from another cell's membrane to establish an intercellular channel. At the arteriolar level, the presence of gap junctions between endothelial cells, between smooth muscle cells, and between endothelial and smooth muscle cells has been established structurally (i.e., by electron microscopy, immunohistochemistry), and functionally (i.e., by dye coupling and electrical coupling). In mammalian capillaries, gap junctions have been detected by electron microscopy and immunohistochemistry (Cx37 and Cx40 appear to dominate here; ).
Direct intercellular communication via gap junctions can be regulated by numerous mechanisms (e.g., gating, con-nexin protein expression, rates of connexin insertion and removal from the cell membrane) and can be affected by a number of intracellular signaling pathways. At the "macroscopic" level, communication has been shown to be modulated by changes in cytosolic pH and calcium, and by changes in trans-GJ and transmembrane voltages. Thus, capillary-arteriolar communication and, consequently, the local blood flow control, could be modulated acutely (e.g., via altered GJ gating) and chronically (via altered connexin expression) during disease processes whose targets include gap junctions.
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