Adrenergic receptor
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, 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 three main groups:
Alpha α receptors
Agonist affinity: noradrenaline > adrenaline > phenylephrine > isoprenaline
Type α1
Acts by phospholipase C activation, which forms IP3 and DAG, one consequence of which is a rise in intracellular calcium.
In blood vessels these cause vasoconstriction. Blood vessels with alpha-1 receptors are present in the skin and the gastrointestinal system, and during the fight-or-flight response there is decreased blood flow to these organs. This is the reason people can appear pale when they've been frightened.
Within this family, there are three subtypes: A, B and D. 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.
Noradrenaline>Adrenaline>Phenylephrine order of potency for alpha-1
Type α2
Acts by inactivation of adenylate cyclase, cyclic AMP levels within the cell decrease.
These are found on both pre-synaptic and post-synaptic nerve terminals.
Within this family there are three subtypes, A, B and C.
Type β
Agonist affinity: isoprenaline > adrenaline > noradrenaline
All β receptors activate adenylate cyclase, raising the intracellular cAMP concentration.
Withing this family there are three subtypes β1, β2, and β3.