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Adrenal Glands: Catecholamines

• Anatomy of the Adrenal Glands: Gross appearance, vascular supply, innvervation.
• Anatomy of the Adrenal Glands: Corticosteroids.
• Anatomy of the Adrenal Glands: Catecholamines.

Biosynthesis of Catecholamines

The amino acid phenylalanine is the precursor of all catecholamines, they are synthesized via the following steps: tyrosine, dopa, dopamine, norepinephrine and epinephrine. The rate-limiting enzyme is tyrosine hydroxylase, which catalyzes the conversion of phenylalanine into tyrosine and dopa.

Effects of catecholamines

Adrenaline acts on alpha-1, beta-1 and beta-2 receptors. Noradrenaline acts mainly on alpha-1 receptors, rather than the beta-1 receptors. Dopamine acts in small doses on the dopamine receptor DA 1 and 2, in higher doses on alpha-1 receptors.

• Alpha-1 receptors: vasoconstriction, smooth muscle contraction of the bladder neck, glycogenesis.
• Alpha-2 receptors: vein constriction, central attenuation of the sympathetic nervous system, inhibition of insulin release, relaxation of the intestine, preventing lipolysis.
• Beta-1 receptors: positive inotropic and positive chronotropic to the heart, lipolysis, and renin release.
• Beta-2 receptors: bronchodilation, vasodilation, gluconeogenesis, relaxation of the uterus, relaxation of the intestine.
• DA1 receptors: vasodilation.
• DA2 receptors: Prevent noradrenaline release (presynaptic).

Metabolism of Catecholamines

The plasma half-life of epinephrine is around 20 s. The inactivation of adrenaline depends mainly on monoamine oxidase (MAO) and catecholamine-O-methyltransferase (COMT). The metabolic products are vanillylmandelic acid, metadrenaline (metanephrine), and normetadrenaline (normetanephrine); they are excreted via the urine. Furthermore, catecholamines are inactivated via re-uptake and re-used as neurotransmitters.

References

  Deutsche Version: Katecholamine der Nebennieren