Norepinephrine

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Definition

Small-molecule transmitter substance grouped under “biogenic amine transmitters,” which includes the catecholamines (dopamine, epinephrine, norepinerphrine) as well as serotonin and histamine.

All catecholamines are derived from the amino acid tyrosine, have a catechol nucleus (3,4-dihydroxylated benzene ring) and share a common synthetic pathway that uses the following enzymes (in order):

  • tyrosine hydroxylase
  • aromatic amino acid decarboxylase
  • dopamine β-hydroxylase
  • pteridine reductase
  • phenylethanolamine-N-methyl transferase (converts norepinephrine to epinephrine, only found in epinephrine-producing cells)

Distribution

In the CNS: norepinephrine is released by cells whose cell bodies are in the locus ceruleus in the brainstem. These neurons have diffuse projections in the cortex, cerebellum and spinal cord.

In the PNS: norepinephrine is released by the postganglionic neurons of the sympathetic nervous system.

In the adrenal medulla, norephinephrine is mostly converted to epinephrine which is released into the systemic circulation.

Receptors

  • α-adrenergic
  • β-adrenergic

As a pharmacological agent

Classification

Adrenergic agonist, inotrope, vasopressor

Description

Norepinephrine is a potent vasoconstrictor and inotrope.

Mechanism of action

Norepinephrine acts predominantly on alpha-adrenergic receptors to produce constriction of resistance and capacitance vessels, thereby increasing systemic blood pressure and coronary artery blood flow.

Pharmacokinetics

Norepinephrine is ineffective orally, and subcutaneous absorption is poor. It is recommended that norepinephrine be administered only via the intravenous route. The onset of activity is rapid after administration, with a short duration of action after discontinuation. The drug does not cross the blood-brain barrier.

Contraindications

Generally contraindicated with MAOI therapy due to risk of severe, prolonged hypertension.

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