Overview
The adrenal medulla releases the catecholamines epinephrine and norepinephrine, and the gastrointestinal tract releases peptide hormones that control secretion and motility. This topic covers their synthesis, secretion, signaling and effects.
Adrenal medulla and catecholamines
The adrenal medulla is the inner part of the adrenal gland. It makes up approximately 20% of the gland and is derived from neural ectoderm. It is a highly modified sympathetic ganglion. The cells of the adrenal medulla are called chromaffin cells, which are essentially modified postganglionic sympathetic neurons. The adrenal medulla secretes epinephrine and norepinephrine. These hormones are catecholamines.
- Epinephrine (80% of secretion).
- Norepinephrine (20% of secretion).
Catecholamines are made from the amino acid tyrosine in chromaffin cells. Tyrosine is hydroxylated to L-DOPA by tyrosine hydroxylase (the rate-limiting enzyme, inhibited by its end products), DOPA decarboxylase makes dopamine, dopamine is taken into the storage granules and converted to norepinephrine by dopamine β-hydroxylase (which needs vitamin C), and in the adrenal medulla phenylethanolamine N-methyltransferase (PNMT), induced by cortisol from the cortex, methylates norepinephrine to epinephrine (S-adenosylmethionine is the methyl donor). The hormones are stored in granules with ATP and chromogranins and are released by exocytosis.
Catecholamines act on adrenergic receptors (α1 and β receptors raise Ca²⁺ or cAMP respectively; α2 lowers cAMP). They have a plasma half-life of only minutes and are inactivated by catechol-O-methyltransferase (COMT) and monoamine oxidase (MAO); the end product vanillylmandelic acid (VMA) and metanephrines are excreted in the urine and are raised in pheochromocytoma.
The effects of epinephrine and norepinephrine are similar to the effects of sympathetic nervous system activation, but the hormonal effects last longer. The effects of epinephrine and norepinephrine take seconds to manifest.
- "Fight or flight" response: The body responds to fear, extreme stress, and intense exercise with massive activation of the sympathetic nervous system, including the adrenal medulla. This is the "fight or flight" response.
- Increase plasma glucose levels.
- Increase cardiovascular function:
- Increase heart rate.
- Increase blood pressure via vasoconstriction.
- Increase vasodilation in heart vessels.
- Increase lipid breakdown.
- Increase bronchial dilation.
- Decrease gastrointestinal activity.
- Muscle glycogen is converted into glucose for rapid energy.
- Epinephrine: binds to α- and β-adrenergic receptors (α1, α2, β1, β2). It has a high affinity for β receptors.
- α-adrenergic receptor binding causes vasoconstriction.
- β2-adrenergic receptor binding causes vasodilation (especially in skeletal muscle).
- Norepinephrine: binds to α- and β1-adrenergic receptors. It is primarily a vasoconstrictor.