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-1 receptors
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.
Alpha-2 receptors
When an agonist binds to an α2-receptor, cyclic AMP levels within the cell decrease. These are found on pre-synaptic
Beta-1 receptors
These are present in heart tissue, and cause an increased heart rate by acting on the cardiac pacemaker cells. Many beta-blockers for treatment of angina will only affect these receptors and not the beta-2 receptors.
Beta-2 receptors
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. Anti-asthma drugs, such as salbutamol (Ventolin) work by binding to 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.
Beta-3 receptors
Beta-3 receptors are present in adipose tissue and are thought to have a role in the regulation of lipid metabolism.
