Beta receptors, or beta adrenergic receptors, are part of the sympathetic nervous system. The sympathetic nervous system acts continuously but it is also responsible for the bodily changes that occur in an emergency, creating the well-known fight or flight response. These effects in the body are caused by release of hormones called adrenaline and noradrenaline, which bind to alpha and beta adrenergic receptors on target organs and activate them to cause specific changes. The heart beats more quickly, blood pressure rises, the airways widen and digestion is inhibited. Beta receptors are found in muscle in the heart, blood vessels, airways and uterus and in fatty tissue.
Three main types of beta receptor are found in the body, known as beta1, beta2 and beta3. A beta receptor should not be confused with what is called a beta cell receptor, which is found on beta cells in the pancreas. In many cases, beta receptors are located on smooth muscle, which is not under voluntary control, and their activation leads to muscle relaxation. Beta1 receptors, which are found on heart muscle, are different. When hormones attach to them and activate them, the heart is stimulated to beat faster and more forcefully.
Beta2 receptors are found on smooth muscle in the walls of blood vessels, respiratory passages and in the uterus. Their activation inhibits muscle contraction and this causes blood vessels to dilate, airways to widen and the uterus to relax. Beta3 receptors are located in fatty tissue. When they are activated they stimulate the breakdown of fat, a process known as lipolysis, although the significance of this is not fully understood.
Medications which stimulate beta receptors may be used to treat conditions such as asthma, in which airway muscle spasms prevent normal breathing. Drugs are used which selectively act on beta2 receptors, which are found on smooth muscle in the walls of respiratory passages. They cause the muscle to relax, widening the airways and enabling the patient to breathe.
Drugs known as beta-blockers have been developed to block beta receptors, preventing hormones from binding to them and activating them. Some of these drugs block both beta1 and beta2 receptors. One example is propranolol, which is used to treat high blood pressure, or hypertension. It lowers the speed and force with which the heart beats, but can also affect breathing by narrowing the airways. Other drugs which selectively block beta1 receptors have less of an effect on respiration, although that may still be too much for some people with asthma.