Norepinephrine (noradrenaline)
Also known as: noradrenaline, NE, NA
A catecholamine that is both the transmitter of most postganglionic sympathetic neurons and a minor (≈20 %) adrenal medullary hormone; acting mainly on α1, α2 and β1 receptors, it raises blood pressure by vasoconstriction.
Overview
Norepinephrine is formed from dopamine by dopamine β-hydroxylase inside secretory vesicles. Most plasma norepinephrine is spill-over from sympathetic nerve terminals rather than adrenal secretion; it therefore reflects overall sympathetic activity. Compared with epinephrine it has little β2 activity, so its dominant systemic effect is α1-mediated vasoconstriction of arterioles and veins, raising both systolic and diastolic pressure; the resulting baroreceptor reflex often slows the heart despite direct β1 stimulation.
Its action at synapses is terminated chiefly by reuptake into the nerve terminal by the norepinephrine transporter (NET, uptake-1), then by MAO; circulating norepinephrine is cleared by COMT and MAO to normetanephrine and VMA.
- Location
Sources: postganglionic sympathetic nerve terminals (main) and adrenal medullary chromaffin cells (≈20 % of medullary output); also brainstem neurons (locus coeruleus). Targets: vascular smooth muscle, heart, adipose tissue and many sympathetically innervated organs.
Function
- Maintains vascular tone and blood pressure (α1 vasoconstriction).
- Increases myocardial contractility and (directly) heart rate (β1).
- Stimulates lipolysis and renin release (β1).
- Inhibits further transmitter release (presynaptic α2).
- Physiology
Class: catecholamine. Receptors: α1 (Gq), α2 (Gi), β1 (Gs); weak at β2. Half-life ≈2 min. Metabolites: normetanephrine, VMA.
- Regulation
Sympathetic nerve activity (baroreflex, posture, exercise, cold, stress); presynaptic α2 autoreceptors provide negative feedback on release.
Clinical significance
Vasopressor therapy
Norepinephrine infusion is the first-line vasopressor in septic and vasodilatory shock, raising mean arterial pressure by α1 vasoconstriction; extravasation can cause tissue necrosis.
Paraganglioma
Extra-adrenal chromaffin tumours (no cortisol-induced PNMT) secrete predominantly norepinephrine, causing sustained hypertension; raised plasma normetanephrine.
Compiled from standard anatomy and physiology references — see the Endocrine System sources. For education only; not medical advice.