Mechanism of action and cardiac effects
Propranolol directly affects myocardial electrophysiology by blocking $\beta_1$-adrenoceptors. It suppresses the automatism of the sinoatrial node, thereby reducing the heart rate. Additionally, the drug suppresses conduction and automatism in the atrioventricular (AV) node, and also reduces the activity of Purkinje fibers and ventricular cardiomyocytes.
In cardiological practice, these properties are used for:
- Treatment of supraventricular tachyarrhythmias: slowing AV conduction helps control ventricular rate (e.g., in tachyarrhythmic atrial fibrillation).
- Management of extrasystoles: suppresses ectopic foci of excitation in both the atria and the ventricles.
- Secondary prevention after myocardial infarction: the drug is prescribed after the acute period. It protects the heart from hyperactivation of the sympathetic nervous system, reduces the zone of necrosis, and prevents fatal arrhythmias (ventricular fibrillation). As a result, patient mortality is reliably reduced by 20–50%.
Non-cardiac indications
Due to the blockade of various types of adrenoceptors, the drug is actively used outside of cardiology.
- Endocrine system: In thyrotoxicosis, propranolol performs a dual function. Symptomatically, it eliminates tachycardia and palpitations. Pathogenetically, it inhibits the peripheral conversion of the hormone thyroxine ($T_4$) into the more active triiodothyronine ($T_3$).
- Neurology and psychiatry: The drug tones cerebral vessels, preventing their painful pulsation, which is used for the prevention of migraine attacks. Blockade of skeletal muscle $\beta_2$-receptors helps successfully combat essential tremor. In addition, the depressing effect on the CNS helps relieve panic attacks, fear, and anxiety.
Fun fact: Although $\beta$-blockers reduce intraocular pressure by decreasing aqueous humor secretion, propranolol itself is not used in glaucoma. It has a strong local anesthetic effect, which creates an unacceptable risk of damaging the desensitized cornea.
Side effects and safety profile
The non-selectivity of the drug implies a high probability of adverse reactions from various body systems.
- Cardiac risks ($\beta_1$ blockade): Excessive reduction of pumping function and drop in cardiac output (risk of heart failure), marked bradycardia, arterial hypotension, and depression of AV conduction up to complete block.
- Respiration and vessels ($\beta_2$ blockade): Increased bronchial tone creates a risk of bronchospasm (the drug is contraindicated in bronchial asthma). Increased vascular tone leads to a feeling of coldness in the extremities and a risk of tissue ischemia.
- Systemic effects: Lipid profile disturbances (increase in triglyceride and VLDL levels against the background of a decrease in anti-atherogenic HDL due to the inhibition of endothelial lipoprotein lipase), stimulation of uterine contractions (threat of miscarriage), diarrhea, and risk of erectile dysfunction.
- CNS effects: Crossing the blood-brain barrier, the drug can provoke drowsiness, lethargy, sleep disturbances, depressive states, and centrally mediated nausea.
Withdrawal considerations and use in diabetes
Prolonged administration (more than 3 months) requires extremely careful termination of the course. Due to the up-regulation phenomenon (compensatory increase in receptor synthesis in response to blockade), abrupt discontinuation of the drug leads to catastrophic consequences. Endogenous catecholamines sharply increase renin secretion, vascular tone, and heart rate. In the first 7 days after withdrawal, this can provoke exacerbation of arterial hypertension, severe angina attacks, and myocardial infarction. To avoid this, the dose is reduced gradually over 2 weeks.
Special restrictions apply to patients with diabetes mellitus. Propranolol inhibits glycogenolysis in the liver, which causes drug-induced hypoglycemia to last longer and be more severe. The most dangerous is the "masking" phenomenon. Receptor blockade conceals sympathoadrenal danger signals (tachycardia and tremor), leaving only sweating noticeable. This prevents timely recognition of a drop in blood sugar levels, which is especially critical in type 1 diabetes.
Briefly: answers to topical questions
What are the absolute contraindications to the use of propranolol?
- Bronchial asthma is a contraindication due to the risk of bronchospasm upon $\beta_2$-receptor blockade.
- Second- or third-degree atrioventricular block in patients without a functioning pacemaker.
- Pronounced bradycardia.
- Acute and severe chronic heart failure.
- Arterial hypotension.
- Vasospastic angina (Prinzmetal's angina).
- Obliterating peripheral vascular disease.
- Cardiogenic shock.
- Severe chronic obstructive pulmonary disease in the acute stage.
- Allergy or intolerance to drug components.
Class and exam questions
Why is propranolol not used to treat glaucoma, unlike other beta-blockers?
Propranolol has pronounced local anesthetic activity. This deprives the cornea of the eye of sensitivity and significantly increases the risk of its injury.
What is the mechanism behind propranolol withdrawal syndrome?
With prolonged blockade, the body compensatorily synthesizes new adrenoceptors. If the blocker is abruptly removed, endogenous catecholamines massively affect these receptors, causing vasospasm, a rise in blood pressure, and the risk of a heart attack.
How does propranolol affect its own pharmacokinetics during a long-term course?
The drug is capable of reducing hepatic blood flow. Because of this, it slows down the rate of its own metabolism in the liver, which leads to a prolongation of its elimination half-life.
Why are non-selective beta-blockers dangerous in hypoglycemia?
They block glycogen breakdown in the liver, exacerbating hypoglycemia, and mask its main symptoms—tachycardia and tremor, due to which the patient may not notice the dangerous condition.
What else to study on this topic
- Mechanisms of receptor up-regulation with long-term antagonist use
- The effect of non-selective beta-blockers on the lipid profile (mechanism of atherogenic action)
- Biotransformation of propranolol: first-pass effect and formation of active metabolites
- Rationale for choosing between cardioselective and non-selective beta-blockers in diabetes mellitus
- Pathogenesis of the effect of propranolol on thyroid hormone metabolism
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