Adrenergic receptor
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The adrenergic receptors (or adrenoceptors) are a class of G-protein coupled receptors that is the target of catecholamines. Adrenergic receptors specifically bind their endogenous ligands, the catecholamines adrenaline and noradrenaline (also called epinephrine and norepinephrine) and are activated by these.
Many cells possess these receptors, and the binding of an agonist will generally cause the cell to respond in a flight-fight manner. For instance, the heart will start beating quicker and the pupils will dilate.
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Sub-types of adrenergic receptors
There are several types of adrenergic receptors, but there are five main groups:
Alpha α receptors
Agonist affinity: noradrenaline > adrenaline > isoprenaline
Type α1
Acts by phospholipase C activation, which forms IP3 and DAG.
In blood vessels these cause vasoconstriction. Blood vessels with alpha-1 receptors are present in the skin and the gastrointestinal system, and during the flight-or-fight response there is decreased blood flow to these organs. This is the reason people can appear pale when they've been frightened.
Type α2
Acts by inactivation of adenylate cyclase, cyclic AMP levels within the cell decrease.
These are found on pre-synaptic nerve terminals.
Beta ß receptors
Agonist affinity: isoprenaline > adrenaline > noradrenaline
All ß receptors by activating adenylate cyclase, raising the intracellular cAMP concentration.
Type ß1
These are present in heart tissue, and cause an increased heart rate by acting on the cardiac pacemaker cells. Many beta-blockers used for treatment of angina will mainly affect these receptors and the beta-2 receptors to a lesser extent. These are referred to as 'cardio-selective' beta-blockers.
Type ß2
These are in the vessels of skeletal muscle, and cause vasodilation, which allows more blood to flow to the muscles, and reduces total peripheral resistance. These tend to work with adrenaline (epinephrine), but not noradrenaline (norepinephrine).
Beta-2 receptors are also in bronchial smooth muscle, and cause bronchodilation when activated. Some Anti-asthma drugs, such as the bronchodilator salbutamol (Ventolin) work by binding to and stimulating the β2 receptors.
Non-selective beta-blocking drugs, such as propranolol (Inderal), can represent a risk to people with asthma by blocking the beta-2 receptors, causing bronchoconstriction.
Type ß3
Beta-3 receptors are present in adipose tissue and are thought to have a role in the regulation of lipid metabolism.
External links
Brief overview of functions of the beta-3 receptor (http://www.ncbi.nlm.nih.gov/entrez/dispomim.cgi?id=109691)de:Adrenozeptor fr:Récepteur adrénergique