Endothelin receptor A (ETA_rcpt)
Type: Family

G protein-coupled receptors (GPCRs) constitute a vast protein family that encompasses a wide range of functions, including various autocrine, paracrine and endocrine processes. They show considerable diversity at the sequence level, on the basis of which they can be separated into distinct groups . The term clan can be used to describe the GPCRs, as they embrace a group of families for which there are indications of evolutionary relationship, but between which there is no statistically significant similarity in sequence . The currently known clan members include rhodopsin-like GPCRs (Class A, GPCRA), secretin-like GPCRs (Class B, GPCRB), metabotropic glutamate receptor family (Class C, GPCRC), fungal mating pheromone receptors (Class D, GPCRD), cAMP receptors (Class E, GPCRE) and frizzled/smoothened (Class F, GPCRF) . GPCRs are major drug targets, and are consequently the subject of considerable research interest. It has been reported that the repertoire of GPCRs for endogenous ligands consists of approximately 400 receptors in humans and mice . Most GPCRs are identified on the basis of their DNA sequences, rather than the ligand they bind, those that are unmatched to known natural ligands are designated by as orphan GPCRs, or unclassified GPCRs .

The rhodopsin-like GPCRs (GPCRA) represent a widespread protein family that includes hormone, neurotransmitter and light receptors, all of which transduce extracellular signals through interaction with guanine nucleotide-binding (G) proteins. Although their activating ligands vary widely in structure and character, the amino acid sequences of the receptors are very similar and are believed to adopt a common structural framework comprising 7 transmembrane (TM) helices .

Endothelins are able to activate a number of signal transduction processes including phospholipase A2, phospholipase C and phospholipase D, as well as cytosolic protein kinase activation. The play an important role in the regulation of the cardiovascular system and are the most potent vasoconstrictors identified, stimulating cardiac contraction, regulating the release of vasoactive substances, and stimulating mitogenesis in blood vessels . As a result, endothelins are implicated in a number of vascular diseases, including the heart, general circulation and brain . Endothelins stimulate the contraction in almost all other smooth muscles (e.g., uterus, bronchus, vas deferens, stomach) and stimulate secretion in several tissues e.g., kidney, liver and adrenals . Endothelins have also been implicated in a variety of pathophysiological conditions associated with stress including hypertension, myocardial infarction, subarachnoid haemorrhage and renal failure .

Two endothelin receptor subtypes have been isolated and identified, endothelin A receptor(ETA) and endothelin B receptor (ETB) , and are members of the seven transmembrane rhodopsin-like G-protein coupled receptor family (GPCRA) which stimulate multiple effectors via several types of G protein . ETA and ETB receptors are both widely distributed, ETA receptors are mainly located on vascular smooth muscle cells, whereas ETB receptors are present on endothelial cells lining the vessel wall. Endothelin receptors have also been found in the brain, e.g. cerebral cortex, cerebellum and glial cells . ETA receptors are considered to be the primary vasoconstrictor and growth-promoting receptor, and the binding of endothelin to ETA increases vasoconstriction (contraction of the blood vessel walls) and the retention of sodium, leading to increased blood pressure . Endothelin B receptor on the other hand not only inhibits cell growth and vasoconstriction in the vascular system but also functions as a "clearance receptor". This receptor-mediated clearance mechanism is particularly important in the lung, which clears about 80% of circulating endothelin-1 .
Both receptors are localised to non-vascular structures such as epithelial cells as well as occurring in the central nervous system (CNS) on glial cells and neurones, where they are thought to mediate neurotransmission and vascular functions .

This entry represents the endothelin A receptor.