Adrenergic receptors

The adrenergic receptors (or adrenoceptors) are a class of G protein-coupled receptors that are targets of the catecholamines. Adrenergic receptors specifically bind their endogenous ligands, the catecholamines adrenaline and noradrenaline (also called epinephrine and norepinephrine in the USA), 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 fight-or-flight manner. For instance, the heart rate will increase and the pupils will dilate, energy will be mobilized, and blood flow diverted from other organs to skeletal muscle.

Subtypes of adrenergic receptors
There are several types of adrenergic receptors, but there are two main groups:

α-Adrenergic receptors
α Receptors bind norepinephrine and epinephrine, though norepinephrine has higher affinity. Phenylephrine is a selective agonist of the α receptor.

Type &alpha;1
Like all adrenergic receptors, the α1 subtype is a G protein-coupled receptor. Hormone binding activates the associated G q|undefined protein, which is linked to phospholipase C (PLC). PLC produces IP3, which causes a rise in intracellular calcium levels, and diacylglycerol. Elevated calcium and diacylglycerol activate protein kinase C, which mediates the downstream, intracellular effects of the hormone.

In blood vessels the principal effect is vasoconstriction. Blood vessels with α1 receptors are present in the skin and the gastrointestinal system, and during the fight-or-flight response vasoconstriction results in the decreased blood flow to these organs. This accounts for an individual's skin appearing pale when frightened.

There are three subtypes of α1 receptor:
 * A ("ADRA1A", )
 * B ("ADRA1B", )
 * D ("ADRA1D", )

At one point, there was a subtype known as C, but was found to be one of the previously discovered subtypes. To avoid confusion, it was decided that there would never be a C subtype again and so if any new subtypes were discovered, naming would start with E.

Type α2
The α2-adrenergic receptor family contains three subtypes:
 * A ("ADRA2A", )
 * B ("ADRA2B", )
 * C ("ADRA2C", )

α2 Receptors are found in pre- and postsynaptic nerve terminals, where they mediate synaptic transmission. Each subtype is linked to a Gi protein, which works in opposition to Gs proteins, suppressing adenylyl cyclase activity with a consequent decrease in intracellular cAMP levels.

β-Adrenergic receptors
Within this family there are three subtypes:
 * β1 ("ADRB1", ). The β1 subtype is mainly found in the heart (where agonists enhance myocardial contractility) and in the cerebral cortex
 * β2 ("ADRB2", ). The β2 subtype predominates in the lung (where agonists cause bronchiole dilation) and cerebellum.
 * β3 ("ADRB3", ). The β3 subtype is found in adipose tissue (where agonists enhance lipolysis).

All β receptors are linked to Gs proteins, which in turn are linked to adenylyl cyclase. Agonist binding thus causes a rise in the intracellular concentration of the second messenger cAMP. Downstream effectors of cAMP include cAMP-dependent protein kinase (PKA), which mediates some of the intracellular events following hormone binding.