A beta agonist is a medication that is used to relax the muscles of the bronchial tubes to stimulate the heart muscle while dilating the blood vessels. A beta agonist interacts with the beta-2 receptors on the surface of the muscle cells causing relaxation in smooth muscles cells in the bronchial tubes or stimulation in the muscle cells of the heart. When used as a bronchodilator, the medication can be taken orally but is often administered as a mist with an inhaler or nebulizer. If the desired effect involves the muscles of the heart, it must be given as an intravenous injection.
After they branch off of the trachea, the bronchial tubes divide into increasingly smaller airways that lead into the alveoli, i.e., the little sacks in the lungs that conduct the exchange of oxygen and carbon dioxide. During asthma attacks or chronic obstructive pulmonary disease (COPD), the muscles of the bronchial tubes constrict and then cannot relax. The volume of air passing through them and into the lungs can be restricted enough that the body is no longer able to get the oxygen that it needs. Carbon dioxide level in the blood also rises to dangerous levels possibly causing loss of consciousness. A beta agonist works to relax the muscles in the airways and open the bronchial tubes to allow more airflow.
In heart muscle cells, a beta agonist stimulates increased calcium uptake, thereby producing stronger contractions in each muscle cell and increasing the heart rate. It also allows the muscles to relax more quickly and for signals from the pacemaker cells to travel through the heart more quickly. When a beta agonist is used to treat symptoms of heart disease, the effect that the medication has on the rest of the circulatory system is important to the outcome. Since the beta agonist causes the walls of the blood vessels to relax, it leads to dilation which helps to keep blood pressure from rising too much in response to the higher blood flow.
Oral beta agonists that act on the bronchial tubes are usually available in several forms such as syrups, tablets, nebulizer solution, and injection. Epinephrine, norepinephrine, and dopamine are examples of beta agonists that treat heart conditions and are injected into the blood stream. Other applications for beta agonists include stopping contractions in early labor, stimulating the liver to break down glucose, and causing the kidneys and pancreas to release renin and insulin respectively. Possible side effects include tremors, headaches, arrhythmias, and anxiety.