adrenergic receptor

Examples of adrenergic receptor in the following topics:

• Adrenergic Neurons and Receptors

  • Adrenergic receptors are molecules that bind catecholamines.
  • There are two main groups of adrenergic receptors, α and β, with several subtypes. α receptors have the subtypes α1 (a Gq coupled receptor) and α2 (a Gi coupled receptor).
  • Adrenaline or noradrenaline are receptor ligands to α1, α2, or β-adrenergic receptors (the pathway is shown in the following diagram).
  • α1-adrenergic receptors are members of the G protein-coupled receptor superfamily.
  • This schematic shows the mechanism of adrenergic receptors.

• Local Regulation of Blood Flow

  • Generally, norepinephrine and epinephrine (hormones secreted by sympathetic nerves and the adrenal gland medulla) are vasoconstrictive, acting on alpha-1-adrenergic receptors.
  • However, the arterioles of skeletal muscle, cardiac muscle, and the pulmonary circulation vasodilate in response to these hormones acting on beta-adrenergic receptors.

• Agonists, Antagonists, and Drugs

  • Acetylcholine receptor agonists and antagonists can have a direct effect on the receptors or exert their effects indirectly.
  • Muscarinic receptor antagonists bind to muscarinic receptors, thereby preventing ACh from binding to and activating the receptor.
  • Beta blockers (sometimes written as β-blockers) or beta-adrenergic blocking agents, beta-adrenergic antagonists, beta-adrenoreceptor antagonists, or beta antagonists, are a class of drugs used for various indications.
  • As beta-adrenergic receptor antagonists, they diminish the effects of epinephrine (adrenaline) and other stress hormones.
  • Beta blockers block the action of endogenous catecholamines—epinephrine (adrenaline) and norepinephrine (noradrenaline) in particular—on β-adrenergic receptors, part of the sympathetic nervous system that mediates the fight-or-flight response.

• Postganglionic Neurons

  • In the sympathetic division, most are adrenergic, meaning they use norepinephrine as their neurotransmitter.
  • In response to this stimulus, postganglionic neurons—with two important exceptions—release norepinephrine, which activates adrenergic receptors on the peripheral target tissues.
  • The activation of target tissue receptors causes the effects associated with the sympathetic system.
  • The ACh acts on two types of receptors, the muscarinic and nicotinic cholinergic receptors.
  • The postganglionic nerve then releases ACh to stimulate the muscarinic receptors of the target organ.

• Heart Circulation

  • This mechanism is due to beta-adrenergic receptors in the coronary arteries and helps enable the increased cardiac output associated with fight-or-flight responses.

• Classification of Receptors by Stimulus

  • Sensory receptors can be classified by the type of stimulus that generates a response in the receptor.
  • Sensory receptors perform countless functions in our bodies.
  • Cutaneous receptors are sensory receptors found in the dermis or epidermis.
  • Encapsulated receptors consist of the remaining types of cutaneous receptors.
  • A tonic receptor is a sensory receptor that adapts slowly to a stimulus, while a phasic receptor is a sensory receptor that adapts rapidly to a stimulus.

• Ionotropic and Metabotropic Receptors

  • Although both ionotropic and metabotropic receptors are activated by neurotransmitters, ionotropic receptors are channel-linked while metabotropic receptors initiate a cascade of molecules via G-proteins.
  • Two types of membrane-bound receptors are activated with the binding of neurotransmitters: ligand-gated ion channels (LGICs) inotropic receptors and metabotropic G- protein coupled receptors.
  • Examples of metabotropic receptors include glutamate receptors, muscarinic acetylcholine receptors, GABAB receptors, most serotonin receptors, and receptors for norepinephrine, epinephrine, histamine, dopamine, neuropeptides, and endocannabinoids.
  • Since opening channels by metabotropic receptors involves activating a number of molecules in the intracellular mechanism, these receptors take longer to open than the inotropic receptors.
  • While ionotropic channels have an effect only in the immediate region of the receptor, the effects of metabotropic receptors can be more widespread throughout the cell.

• Blocking of Hormone Receptors

  • A receptor antagonist does not provoke a biological response upon receptor binding, but limits or dampens agonist-mediated responses.
  • A receptor antagonist is a type of receptor ligand or drug that does not provoke a biological response itself upon binding to a receptor, but blocks or dampens agonist-mediated responses.
  • Binding to the active site on the receptor regulates receptor activation directly.
  • The current accepted definition of receptor antagonist is based on the receptor occupancy model.
  • Irreversible antagonists covalently bind to the receptor target and, in general, cannot be removed; inactivating the receptor for the duration of the antagonist effects is determined by the rate of receptor turnover, the rate of synthesis of new receptors.

• Classification of Receptors by Location

  • Some sensory receptors can be classified by the physical location of the receptor.
  • Sensory receptors code four aspects of a stimulus:
  • Receptors are sensitive to discrete stimuli and are often classified by both the systemic function and the location of the receptor.
  • Sensory receptors are found throughout our bodies, and sensory receptors that share a common location often share a common function.
  • For example, sensory receptors in the retina are almost entirely photoreceptors.

• Target Cell Specificity

  • Hormones target a limited number of cells (based on the presence of a specific receptor) as they circulate in the bloodstream.
  • This androgen insensitivity occurs when the receptors on the target cells are unable to accept the hormone due to an impairment in receptor shape.
  • Target cells are capable of responding to hormones because they display receptors to which the circulating hormone can bind.
  • Finally, hormone–receptor affinity can be altered by the expression of associated inhibitory or co-activating factors.
  • In some instances, alterations of receptor structure due to a genetic mutation can lead to a reduction in hormone–receptor affinity, as in the case of androgen insensitivity.