Beta receptors are part of the sympathetic nervous system and respond to adrenaline and norepinephrine. There are three types of beta receptors, found in the heart, blood vessels, airways, uterus, and fatty tissue. Drugs that stimulate or block beta receptors are used to treat conditions like asthma and high blood pressure.
Beta receptors, or beta adrenergic receptors, are part of the sympathetic nervous system. The sympathetic nervous system acts continuously but 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 the release of hormones called adrenaline and norepinephrine, which bind to alpha and beta adrenergic receptors on target organs and activate them to cause specific changes. The heart beats faster, blood pressure rises, the airways dilate, and digestion is inhibited. Beta receptors are found in the heart muscles, blood vessels, airways and uterus, and in fatty tissue.
Three main types of beta receptors 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 the beta cells of the pancreas. In many cases, beta receptors are found on smooth muscle, which is not under voluntary control, and their activation leads to muscle relaxation. The beta1 receptors, found on heart muscle, are different. When hormones attach to them and activate them, the heart is stimulated to beat faster and harder.
Beta2 receptors are found on smooth muscle in blood vessel walls, the airways, and the uterus. Their activation inhibits muscle contraction and this causes blood vessels to dilate, widen the airways and relax the uterus. Beta3 receptors are found in adipose tissue. When activated they stimulate the breakdown of fat, a process known as lipolysis, although the significance of this is not fully understood.
Drugs that stimulate beta receptors can be used to treat conditions such as asthma, in which muscle spasms in the airways prevent normal breathing. Drugs are used that act selectively on beta2 receptors, which are found on the smooth muscles of the walls of the airways. They cause the muscle to relax, widening the airways and allowing the patient to breathe.
Drugs known as beta blockers were developed to block beta receptors, preventing hormones from binding to them and activating them. Some of these drugs block both beta1 and beta2 receptors. An example is propranolol, which is used to treat high blood pressure or hypertension. It reduces the speed and force with which the heart beats, but it can also affect breathing by narrowing the airways. Other drugs that selectively block beta1 receptors have a lesser effect on breathing, although this may still be too much for some people with asthma.
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