Mechanism of Action of Noradrenaline (Norepinephrine)

1. Receptor pharmacology

Noradrenaline (norepinephrine, NA/NE) is an endogenous catecholamine that acts as a direct-acting sympathomimetic, stimulating adrenergic receptors:

α1-adrenergic receptors (main action): Gq-coupled → activates phospholipase C → increases IP3/DAG → raises intracellular Ca2+ in vascular smooth muscle → intense arterial and venous vasoconstriction. This is the dominant effect at clinically used doses.
β1-adrenergic receptors: Gs-coupled → activates adenylyl cyclase → increases cAMP → increases myocardial contractility (positive inotropy) and some chronotropic effect, and produces coronary artery vasodilation.
β2-adrenergic receptors: only weak/limited activation at clinical doses (much less than adrenaline), so bronchodilator/vasodilator effects are minimal.

Net hemodynamic effect: potent α1-mediated vasoconstriction raises systemic vascular resistance, and β1 stimulation increases contractility, together raising both systolic and diastolic blood pressure. Because afterload rises sharply, reflex (vagally-mediated) bradycardia often occurs, which can offset or blunt any rise in cardiac output (Morgan and Mikhail's Clinical Anesthesiology, 7e, p. 451-452; Tintinalli's Emergency Medicine, p. NOREPINEPHRINE section).

2. Downstream physiological consequences

Increased myocardial oxygen demand (from increased contractility and afterload)
Decreased renal and splanchnic blood flow due to intense vasoconstriction in those beds
Coronary vasodilation (a relatively favorable effect, partly flow-mediated/partly β1-linked)
Extravasation causes local tissue necrosis due to intense local vasoconstriction and ischemia

3. Clinical use

Noradrenaline is the first-line vasopressor of choice in septic shock and other forms of refractory/distributive shock, given as a continuous IV infusion (typical rate ~2-20 mcg/min or 30-300 ng/kg/min) because of its short plasma half-life. It is indicated for acute hypotension and shock refractory to adequate fluid resuscitation per sepsis guidelines - Tintinalli's Emergency Medicine.

Adverse effects: bradycardia, arrhythmias, peripheral/digital ischemia, hypertension, anxiety, and tissue necrosis on extravasation.

4. As an endogenous neurotransmitter (synaptic mechanism)

Beyond its use as an IV drug, endogenously noradrenaline is synthesized from dopamine by dopamine-beta-hydroxylase (DBH) in noradrenergic neurons (e.g., locus coeruleus, sympathetic postganglionic neurons), stored in vesicles, and released into the synapse on depolarization. It acts on pre- and post-synaptic α and β receptors, and its action is terminated mainly by reuptake (NET transporter, uptake-1) back into the presynaptic neuron, with a smaller contribution from extraneuronal uptake and enzymatic degradation by monoamine oxidase (MAO) and catechol-O-methyltransferase (COMT).

If by "Mg of NORAD" you meant something else - such as the magnesium dose used alongside noradrenaline, or a specific dosing/administration question - let me know and I can pull that specific detail from the pharmacology references.

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