Pharmacodynamics and Receptor Profile
Norepinephrine exerts a direct stimulating effect on adrenergic receptors, which determines its primary pharmacological effects.
- Vascular effects: Stimulates $\alpha_1$- and $\alpha_2$-adrenergic receptors, leading to powerful vasoconstriction and a rapid increase in blood pressure (BP). Unlike epinephrine, norepinephrine lacks a secondary (subsequent) phase of blood pressure drop. This is because it has virtually no effect on $\beta_2$-adrenergic receptors, which mediate vasodilation.
- Cardiac effects: The drug stimulates myocardial $\beta_1$-adrenergic receptors, directly increasing stroke volume and myocardial contractility. However, the sudden spike in blood pressure activates baroreceptors in the aortic arch. The resulting impulse stimulates the vagus nerve (n. vagus) center, enhancing its inhibitory effect on the heart—leading to reflex bradycardia (decreased heart rate). Consequently, despite the increased stroke volume, cardiac output remains unchanged.
Clinical pearl: Reflex bradycardia can be prevented pharmacologically by pre-administration of an M-cholinergic receptor blocker (e.g., atropine).
Pharmacokinetics and Administration Rules
The only appropriate route for systemic administration is intravenous (most commonly as a continuous infusion to maintain a stable therapeutic effect).
- Oral administration: Ineffective, as the substance is completely destroyed in the gastrointestinal tract.
- Subcutaneous and intramuscular administration: Strictly contraindicated. Its potent vasoconstrictive action causes local arteriolar spasm, leading to ischemia and severe tissue necrosis at the injection site. (In general, absorption from subcutaneous tissue depends on blood flow; it can be accelerated by massage or hyaluronidase, or slowed by vasoconstrictors. Due to excessive vasoconstriction, norepinephrine itself is never given subcutaneously).
Biotransformation of norepinephrine occurs via non-microsomal oxidation (oxidative deamination) involving mitochondrial and cytosolic enzymes—monoamine oxidase (MAO) and catechol-$O$-methyltransferase (COMT). Metabolites, along with a small fraction of unchanged drug, are eliminated via the kidneys.
Clinical Application and Safety Profile
The primary indication for norepinephrine is acute hypotension (conditions accompanied by a critical drop in blood pressure).
Adverse Effects:
- Headache
- Respiratory disturbances
- Cardiac rhythm disorders (arrhythmias)
Contraindications:
- Heart failure and atrioventricular block
- Severe atherosclerosis
- Halothane anesthesia. This inhalation anesthetic dramatically increases myocardial sensitivity (sensitization) to catecholamines, which, in the presence of norepinephrine, creates an extremely high risk of fatal arrhythmias.
Physiological Regulation and Reuptake
The release of endogenous norepinephrine into the synaptic cleft is tightly regulated by presynaptic receptors:
- Stimulation of presynaptic $\alpha_2$-adrenergic receptors inhibits neurotransmitter release (negative feedback mechanism).
- Stimulation of presynaptic $\beta$-adrenergic receptors, conversely, enhances its release (positive feedback mechanism).
After fulfilling its function, the neurotransmitter undergoes neuronal reuptake. Pharmacology utilizes drugs that block this transport mechanism. These include selective inhibitors such as reboxetine (selective blockade) and maprotiline (primarily blocks norepinephrine reuptake over serotonin, similar to tricyclic antidepressants).
There are also mixed-action drugs—serotonin-norepinephrine reuptake inhibitors (SNRIs), such as venlafaxine. Both the parent drug and its metabolites are active. SNRI therapy may be accompanied by central nervous system side effects (somnolence, dizziness) and autonomic disturbances (dry mouth, ejaculation difficulties).