An adrenergic agonist, sometimes referred to as a sympathomimetic agent, is a drug which acts on what is known as the sympathetic nervous system, preparing the body to act in an emergency. The heart rate speeds up, the airways widen, blood is diverted from the skin to the muscles, and the bladder and bowel become less active. Medically, an adrenergic agonist may be used in the treatment of conditions such as asthma, allergy, cardiac arrest or shock.
Adrenergic agonists may cause their effects by attaching directly to what are called adrenergic receptors on nerve endings or cells, or they might act indirectly by increasing the amounts of norepinephrine. Norepinephrine, or noradrenaline, is the sympathetic nervous system's neurotransmitter, the messenger substance that normally binds to adrenergic receptors. In some cases, adrenergic receptor agonists can work both directly and indirectly.
An adrenergic agonist can be a substance that is naturally present inside the body, such as epinephrine or adrenaline. Dopamine is another adrenergic agonist which occurs in the body and which is converted into noradrenaline. Cocaine and amphetamines occur outside the body, and are both examples of adrenergic agonists that act indirectly.
When the body's natural noradrenaline is released from the ending of a sympathetic nerve, in response to a nerve signal or an adrenergic drug, it may attach itself to either alpha or beta adrenergic receptors. These receptors could be sited on another nerve ending, or on the cells of a body tissue or organ. An adrenergic agonist binding to alpha receptors may cause effects such as narrowing of the blood vessels supplying the gut and skin or widening of the pupils. Activation of beta receptors increases the force and speed at which the heart beats, widens the blood vessels that supply muscles and opens up the airways. The metabolism is affected, making more glucose and fatty acids available for use, and the body's allergic response is damped down.
An adrenergic agonist may be used in the treatment of asthma, given in an inhalable form. A severe asthma attack causes muscles in the airway walls to contract, leading to life-threatening narrowing. An adrenergic agonist can reverse this narrowing through its relaxing effect on muscle. Typically, the drug used will be one such as albuterol or salbutamol, which acts on specific types of beta receptors which are mainly located inside the lungs. This helps to avoid side effects such as a rapid heartbeat or shaking muscles, which can be experienced with drugs which act on beta receptors throughout the body.