Pharmacology — Autonomic Pharmacology, NMC MBBS licence examination syllabus (Nepal Medical Council).
Why a drug for angina also treats tremor, glaucoma, thyrotoxicosis and stage fright.
Start with a question worth sitting with: why does the same drug class lower blood pressure, slow a racing heart, prevent migraine, reduce portal pressure in cirrhosis, and stop a pianist's hands shaking? Most drug classes do one thing. Beta-blockers appear to do everything. The answer is that they do exactly one thing — occupy the beta-adrenergic receptor — and it is the receptor that is scattered across the body doing all those different jobs.
Get the receptor map right and every indication, every adverse effect, and every contraindication in this chapter follows from it. Memorise the indications as a list and you will fail the clinical-reasoning questions. This chapter builds the map first.
Beta receptors are G-protein coupled receptors for the sympathetic transmitters noradrenaline and adrenaline. All three subtypes couple to Gs, so activation raises intracellular cyclic AMP. What that does depends entirely on which tissue the receptor sits in — which is the single fact this whole chapter rests on.
Beta-3 receptors matter far less here, but are worth a line: they sit on adipose tissue (lipolysis) and on the detrusor muscle of the bladder (relaxation), and they are the target of mirabegron in overactive bladder. No beta-blocker in common use is aimed at them.
Beta-blockers are competitive antagonists. That word is doing real work: because the block is competitive, a high enough concentration of agonist can displace it. This explains something that otherwise looks arbitrary — why adrenaline is an unreliable rescue in beta-blocker overdose, and why the correct antidote works by a completely different route.
Because the antagonism is competitive rather than permanent, beta-blockers have little effect on a resting, unstimulated system. This is why they matter most where sympathetic drive is high — exercise, stress, heart failure, thyrotoxicosis, phaeochromocytoma — and why a healthy person taking one notices remarkably little at rest but cannot raise their heart rate on exertion.
Note the second arm carefully. The antihypertensive action is not mainly the fall in cardiac output — that is offset early on by reflex vasoconstriction, which is exactly why blood pressure takes days to weeks to settle rather than dropping on the first dose. The sustained effect comes largely from renin suppression and reduced central sympathetic outflow.
The diastole point is worth dwelling on, because it is counter-intuitive and it is examinable. The coronary arteries fill during diastole, not systole — the myocardium squeezes them shut when it contracts. Slowing the heart lengthens diastole disproportionately, so a beta-blocker improves coronary supply at the same time as it cuts demand. It attacks the angina mismatch from both sides at once.Three properties separate the drugs, and each one maps onto a real bedside decision.
SelectivityCardioselective (beta-1 preferring): atenolol, bisoprolol, metoprolol, esmolol, nebivolol. Non-selective (beta-1 + beta-2): propranolol, timolol, nadolol, sotalol. Selectivity is relative, not absolute — it is lost at high dose.
Intrinsic sympathomimetic activity (ISA)Partial agonists: pindolol, acebutolol. They block a high-sympathetic state but give weak stimulation at rest, so they cause less bradycardia. Generally avoided post-MI, where the aim is maximal sympathetic suppression.
Additional vasodilator actionCarvedilol and labetalol also block alpha-1 (vasodilation). Nebivolol releases nitric oxide. These are the ones used where afterload reduction is wanted.
This table is the one to revise from. Read it as a set of reasons, not a list to memorise — the "why it matters" column is what scenario questions actually test.
| Drug | Selectivity | Solubility | Distinguishing feature — and why it matters |
|---|---|---|---|
| Propranolol | Non-selective | Lipophilic | The all-rounder for non-cardiac uses: tremor, migraine prophylaxis, portal hypertension, thyrotoxicosis. Crosses into the CNS, so it causes vivid dreams. |
| Atenolol | Cardioselective | Hydrophilic | Renally cleared, so it accumulates in renal impairment. Few CNS effects. |
| Metoprolol | Cardioselective | Lipophilic | Hepatically cleared. Only the succinate (extended-release) has heart-failure mortality evidence. |
| Bisoprolol | Cardioselective | Moderate | Highly selective; a first-line heart-failure agent. |
| Carvedilol | Non-selective + alpha-1 | Lipophilic | Vasodilating. Heart-failure agent; favourable metabolic profile. |
| Labetalol | Non-selective + alpha-1 | Moderate | IV-capable. Agent of choice in hypertension in pregnancy and in hypertensive emergency. |
| Esmolol | Cardioselective | — | IV only, half-life ≈9 minutes, esterase-metabolised. Use when you may need to stop the effect fast. |
| Sotalol | Non-selective | Hydrophilic | Also class III: prolongs QT and can cause torsades. The exception to nearly every generalisation. |
| Nebivolol | Cardioselective | Lipophilic | Releases nitric oxide → vasodilation. Exception to the alphabet mnemonic. |
| Timolol | Non-selective | — | Topical for glaucoma — but systemically absorbed, so it can still cause bronchospasm. |
Two properties alter which patient gets which drug, and both appear in scenario questions disguised as a comorbidity.
Angina↓ rate and ↓ contractility cut myocardial oxygen demand; slower rate lengthens diastole, when coronary perfusion actually happens. Both sides of the supply–demand mismatch.
Post-MIReduce mortality — through reduced oxygen demand, reduced arrhythmia risk, and blunted ventricular remodelling.
Heart failure (HFrEF)Counteract chronic sympathetic overdrive, which is toxic to myocardium long term. Only bisoprolol, carvedilol, metoprolol succinate and nebivolol have good mortality evidence. Start low, go slow, never in decompensated failure.
HypertensionRenin suppression plus reduced sympathetic outflow. No longer routine first-line in uncomplicated hypertension in most major guidelines, but preferred when there is a compelling co-indication such as angina, post-MI or heart failure.
ArrhythmiasSlowed AV conduction controls ventricular rate in atrial fibrillation and flutter. Class II antiarrhythmics; sotalol additionally has class III action.
ThyrotoxicosisRapid symptom control — tremor, tachycardia, anxiety. Propranolol also inhibits peripheral T4→T3 conversion at high dose, a genuine added benefit rather than just symptom masking.
Migraine prophylaxisPropranolol — lipophilic, and it is prophylaxis, never treatment of an acute attack.
Essential tremorPropranolol, via beta-2 blockade on skeletal muscle. This is the common indication that needs a non-selective agent.
Portal hypertensionNon-selective agents (propranolol, nadolol) reduce portal pressure and prevent variceal bleeding — splanchnic beta-2 blockade leaves alpha-mediated vasoconstriction to reduce inflow, so selectivity would defeat the purpose.
GlaucomaTopical timolol reduces aqueous humour production. Systemically absorbed — it can still cause bronchospasm.
Performance anxietyPropranolol blunts the peripheral adrenergic symptoms (tremor, palpitations) without sedation.
A negative inotrope for a failing heart sounds like a contradiction, and students reasonably stumble on it. The resolution is that acute and chronic heart failure are pharmacologically different situations.
In decompensated failure, cardiac output is being propped up by sympathetic drive; remove it and the patient deteriorates. In chronic stable failure, that same sympathetic drive is progressively destroying myocardium — driving remodelling, apoptosis and arrhythmia. Blocking it slows the disease. Same drug, same receptor, opposite outcome, and the variable is time course rather than pharmacology.
The hypoglycaemia interaction deserves its own figure, because it is both dangerous and reliably misunderstood.
AbsoluteSevere bradycardia · second- or third-degree AV block without a pacemaker · cardiogenic shock · acute decompensated heart failure · severe asthma with active bronchospasm.
Relative / cautionCOPD (cardioselective agents are generally tolerated, and the post-MI mortality benefit usually outweighs the risk) · peripheral vascular disease · brittle diabetes with hypoglycaemia unawareness · Prinzmetal (vasospastic) angina.
This is the highest-yield reasoning thread in the whole topic, because a single mechanism generates at least three separate exam scenarios that look unrelated on the surface.
Beta-blocker overdose presents with bradycardia and hypotension, and in severe cases seizures and hypoglycaemia (particularly in children). Standard first measures are IV fluids and atropine, but the specific answer the examiner wants is different — and the reason is the point.
Refer back to the cascade figure above: glucagon enters the pathway downstream of the blockade. That single spatial fact is the entire explanation, and a question can test it from either direction — "why glucagon?" or "why might adrenaline fail?"
PresentationA 58-year-old man with stable angina and well-controlled asthma needs an anti-anginal. He is already on aspirin and a statin.
Key clueAsthma — so beta-2 blockade is the hazard.
ReasoningThe therapeutic target is purely beta-1 (cardiac oxygen demand). Nothing about angina requires beta-2 blockade, so a cardioselective agent gives the benefit while minimising bronchospasm risk.
AnswerBisoprolol or metoprolol, started low, with the caution that selectivity is relative and lost at high dose. Propranolol would be the wrong choice.
PresentationA 34-year-old woman reports palpitations, heat intolerance, weight loss and a fine tremor. Pulse 118 and irregular. TSH suppressed; free T4 raised.
Key cluesThyrotoxicosis with tremor and tachycardia — two different receptor populations, beta-2 and beta-1.
ReasoningA cardioselective agent controls the rate but leaves the tremor. Propranolol covers both, and at higher doses additionally reduces peripheral T4→T3 conversion. Definitive treatment is still antithyroid therapy — the beta-blocker buys symptomatic control while that takes effect.
AnswerPropranolol, alongside definitive antithyroid management.
PresentationA 26-year-old man presents with chest pain, agitation, BP 210/120, pulse 140. Pupils dilated. He admits to cocaine use two hours ago.
TrapThe tachycardia and hypertension make a beta-blocker look like the obvious answer. It is the harmful answer.
ReasoningCocaine drives both alpha and beta stimulation. Blocking beta alone removes beta-2 vasodilation and leaves alpha-mediated vasoconstriction unopposed — coronary vasoconstriction and blood pressure both worsen.
AnswerBenzodiazepines first — they reduce the central sympathetic drive and so lower both pressure and rate — with nitrates or an alpha-blocker such as phentolamine for ongoing ischaemia.
PresentationA 72-year-old on long-term atenolol 50 mg for hypertension is admitted with diarrhoea and dehydration. Creatinine has risen; eGFR has fallen from 62 to 28. On day two her pulse is 38 and she is hypotensive. The dose has not changed.
Key clueNothing was prescribed differently — so what changed is clearance, not dose.
ReasoningAtenolol is hydrophilic and renally excreted. Acute kidney injury reduces clearance, the drug accumulates, and an unchanged dose becomes an effective overdose. A lipophilic, hepatically cleared agent such as metoprolol would not behave this way.
AnswerWithhold the atenolol, support with fluids, and reassess. If ongoing blockade is needed, switch to a hepatically cleared agent. This is the clinical cash value of the lipophilic/hydrophilic split.
| Confusion | The distinction | Why it matters clinically |
|---|---|---|
| Cardioselective vs non-selective | Selectivity is relative and dose-dependent | Never tell an asthmatic the drug is "safe" because it is cardioselective — at high dose it is not. |
| Metoprolol tartrate vs succinate | Only the succinate (extended-release) has heart-failure mortality evidence | Prescribing the tartrate for HFrEF is not equivalent therapy. |
| Sotalol vs the rest | Sotalol is class II and class III | It prolongs QT and can cause torsades — unique in the class. |
| Labetalol / carvedilol vs pure beta-blockers | These also block alpha-1 | Labetalol is the agent of choice in hypertension in pregnancy. |
| Beta-blocker vs CCB in vasospasm | Vasospastic angina needs a CCB | A beta-blocker can worsen the spasm — unopposed alpha again. |
| Verapamil/diltiazem vs amlodipine | Only the non-dihydropyridines slow the AV node | Amlodipine combines safely; verapamil can cause complete heart block. |
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