This class of hormones are structural derivatives of the amino acid tyrosine and include the catecholamines adrenaline and noradrenaline.
Within the cytoplasm of catecholamine secreting cells, tyrosine is enzymatically converted to dihydroxyphenylalanine (L-DOPA) and then dopamine (DA). The dopamine is transported into into membrane-bound secretory vesicles where the enzymes required for its conversion to noradrenaline are located. The noradrenaline is then stored in these vesicles until released by exocytosis. 
In the adrenal medulla however the noradrenaline is transported back into the cytoplasm where it is acted upon by an enzyme only found in the cytoplasm of these cells. The result of this enzymatic reaction is adrenaline which is transported back into the secretory vesicles for storage and subsequent secretion.
Following their release amine hormones circulate in a free state, enter into the extracellular space and bind to adrenergic receptors present on their target tissues.
Like peptide hormones, adrenergic receptors are all membrane-spanning proteins with the hormone binding site of the extracellular side. Activation of these receptors produces a conformation change in the protein which triggers intracellular effects by G-protein-mediated up- or down-regulation of a number of second messengers.
There are five different types of adrenergic receptor and with quite marked variation in expression of these in different tissues. These are summarised in the table below:.
| Receptor | Location | Effect of Activation |
|---|---|---|
| α1 | Smooth muscle of blood vessels in skin Smooth muscle in sphincters of urogenital tract. Smooth muscle in sphincters of digestive tract. |
Contraction (vasoconstriction). Contraction (urine retention). Contraction (inhibition of defecation). |
| α2 | Smooth muscle of digestive tract. Secretory glands of digestive system. Pancreas. |
Decrease motility. Decrease in secretion. Decrease in insulin secretion. |
| β1 | Cardiac muscle. | Increase in rate & force of contraction. |
| β2 | Smooth muscle of blood vessels in heart. Smooth muscle of blood vessels in skeletal muscle. Smooth muscle of airways. Smooth muscle of digestive tract. |
Relaxation (vasodilation). Relaxation (vasodilation). Dilation (increased flow). Decrease motility. |
| β3 | Adipose tissue. | Lipolysis (fatty acid release). |
Both adrenaline and noradrenaline can bind to these different classes of adrenergic receptors so in many instances the same receptors that are activated by noradrenaline released from sympathetic postganglionic neurones can also be activated by the hormone adrenaline reaching them through the circulatory system. In many respects then adrenaline secreted by the endocrine system complements the effects of the sympathetic division of the autonomic nervous system.