Adrenergic is a term used to describe proteins and drugs that interact with adrenaline or noradrenaline, also known as epinephrine and norepinephrine, respectively. For example, adrenergic receptors are membrane proteins that are the target for epinephrine and norepinephrine, while adrenergic transporters are proteins that carry norepinephrine across the cell membrane of nerve cells. An adrenergic agonist is a drug that typically produces the same effect as epinephrine or norepinephrine, where as an adrenergic antagonist is a drug that generally blocks the effects of epinephrine and norepinephrine. Epinephrine and norepinephrine act as hormones and neurotransmitters in numerous biological processes.
When epinephrine is released, it contracts and relaxes smooth muscle in the air passageways and arteries, typically resulting in deep breathing and increased blood pressure. Epinephrine can also increase the level of glucose and fatty acids in the blood, generally leading to increased energy production within cells. In addition, epinephrine and norepinephrine initiate the flight-or-fight response.
Norepinephrine typically increases the heart rate and blood flow to muscles during stressful situations and affects the part of the brain that is responsible for attention and response. It also increases blood glucose levels, thus providing needed energy for cells. Norepinephrine also acts as an anti-inflammatory agent when it is released as a neurotransmitter between nerve cells of the brain.
Epinephrine and norepinephrine bind to either an alpha or a beta adrenergic receptor to carry out its function. Alpha receptors are responsible for smooth muscle contractions and neurotransmitter inhibition, while beta receptors relax smooth muscles and contract the heart muscle. There are two subtypes of alpha receptors and three subtypes of beta receptors, each with their own agonist and antagonist.
Many alpha 1 receptor agonists, including methoxamine and oxymetazoline, typically target the phospholipase component of the receptor to generate the same effect as epinephrine and norepinephrine. Clonidine and guanabenz are alpha 2 receptor agonists that produce the same effect as epinephrine and norepinephrine by inhibiting the adenylyl cyclase component of the receptor. Drugs that block the effects of epinephrine and norepinephrine on alpha 1 and alpha 2 receptors include alfuzosin and atipamezole.
Beta 1 adrenergic receptor agonists, such as dobutamine and isoproterenol, stimulate adenylyl cyclase activity to open calcium ion channels in the heart muscle, while beta 2 agonists stimulate adenylyl cyclase to close calcium ion channels in smooth muscles. Beta 3 agonists, such as amibegron and solabegron, stimulate adenylyl cyclase to increase the production of fatty acids. Beta adrenergic receptor antagonists, also called beta blockers, include metoprolol and butoxamine.