Alpha vs Beta Receptors
Catelocholamines are the sympathetic neurohumodal transmitters including noradrenaline and dopamine. These chemicals interact directly with receptors on the cell membrane in order to complete their action. This interaction is a vital step in the mechanisms of action of catecholamines, which ultimately directs to either increase or decrease tissue activity in the body. The increase of activity is called excitation, whereas the decrease is called inhibition. In 1948, Ahlquist proposed two receptor types; alpha and beta receptors, which define these two differences in responses (excitation and inhibition). Body tissues may consist of either alpha or beta receptors or both types of receptors.
What are Alpha Receptors?
Stimulation of alpha receptors is mainly accountable for excitatory effects of catecholamines. However, in certain places, alpha receptors can possibly inhibit the activities. For example; alpha receptors of GI react are inhibitory in action. There are two types of alpha receptors; alpha1 and alpha2. Each of this type has three subtypes.
Alpha1receptors are mainly found in the vascular smooth muscles, which are excitatory in action. They results in constriction of muscles in blood vessels located in the skin and cerebral, and contraction of pilomotor muscles of skin and radial muscles of the iris. The mechanism of alpha1 is the alteration of cellular calcium ion fluxes. Alpha2 receptors are mainly found in effector tissues and on neuronal endings. The mechanism of action of alpha2 is the inhibition of adenylyl cyclase.
What are Beta Receptors?
Beta receptors are usually responsible for inhibitory actions of tissues. But there are some exceptions. For example, the beta receptors located in the heart are excitatory; thus responsible to increase the heart rate. Moreover, beta receptors can results in relaxation of bronchial muscles, change the contractibility of skeletal muscles and dilatation of skeletal muscle blood vessels. The three beta receptor subtypes are (1) beta1 receptors; responsible for myocardial stimulation and renin release, (2) beta2 receptors; responsible for bronchial muscle relation, vasodilation of skeletal muscles and uterine relaxation, and (3) beta3 receptors; responsible for lipolysis of adipocytes.
What is the difference between Alpha and Beta Receptors?
• Stimulation of alpha receptors is often responsible for excitatory effects, whereas that of beta receptors is responsible for inhibitory effects.
• Alpha receptors are further divided into alpha1 and alpha2 receptors while beta receptors are further divided into beta1, beta2, and beta3 receptors.
• Alpha receptors are mainly found in vascular smooth muscles, effector tissues, and on neuronal endings, whereas beta receptors are mainly found in bronchial muscles, heart muscles and uterine muscles.