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.

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 nerve terminals.

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 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.

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. 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.

Beta-3 receptors

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