PT
● MODULE 06 / 20
Adrenergic Agonists & Antagonists
Predict cardiovascular, respiratory and central effects from alpha and beta receptor pharmacology.
Quick revision ↓ Medical Dictionary
FULL CHAPTER EXPLANATION
Deep Learning — Module 06
Read this section in sequence. It moves from drug targets to therapeutic effects, adverse reactions and safe use.
01
Adrenergic signaling
Noradrenaline released from sympathetic terminals is largely removed by neuronal reuptake. Alpha-1 activation contracts vascular smooth muscle, alpha-2 activation can reduce transmitter release, beta-1 activation increases cardiac activity and renin release, and beta-2 activation relaxes bronchial smooth muscle. Many agonists act at several receptor subtypes. Dose, tissue receptor distribution and baroreflex responses determine the final clinical effect, so a direct receptor action may differ from the observed change in pulse.
02
Sympathomimetic examples
Adrenaline combines alpha and beta actions and is a central medicine in supervised anaphylaxis treatment. Noradrenaline predominantly supports vascular tone with alpha activity and also stimulates beta-1 receptors. Dobutamine is used for inotropic support in selected monitored settings. Salbutamol illustrates beta-2 bronchodilation with possible tremor, tachycardia and hypokalemia. Phenylephrine is predominantly alpha-1 selective. These examples explain mechanisms rather than supply interchangeable indications or emergency dosing instructions.
03
Alpha antagonists and central agents
Prazosin blocks alpha-1 receptors and can cause postural hypotension, especially with initiation. Tamsulosin preferentially targets receptors relevant to lower urinary tract symptoms, although dizziness and other adverse effects still occur. Phenoxybenzamine irreversibly blocks alpha receptors. Clonidine stimulates central alpha-2 pathways, reducing sympathetic outflow; abrupt withdrawal can cause rebound hypertension. Reserpine depletes stored monoamines by inhibiting vesicular transport and illustrates adrenergic neuronal blockade.
04
Beta antagonists
Propranolol blocks beta-1 and beta-2 receptors. Metoprolol is relatively beta-1 selective, but selectivity is not absolute. Beta blockade can slow the heart, reduce renin release and mask some warning signs of hypoglycemia. Nonselective blockade can worsen bronchospasm in susceptible people. Some agents also block alpha receptors or have other properties, so the class is not clinically uniform. Abrupt withdrawal after chronic use may provoke rebound symptoms.
05
Integrated reasoning
An alpha-1 agonist may increase vascular resistance and blood pressure while reflex vagal activity slows the pulse. A beta blocker plus another atrioventricular nodal suppressant can cause excessive bradycardia. In catecholamine-secreting tumor preparation, specialist sequencing of alpha before beta blockade avoids unopposed vasoconstriction. Drug choice must account for blood pressure, rhythm, airway disease, volume status and the clinical setting rather than matching a receptor name to a symptom alone.
High-Yield Revision
- Baroreflexes can oppose a direct cardiac drug action.
- Beta-1 selectivity is relative and dose dependent.
- Review withdrawal risk for clonidine and chronic beta blockade.
Common Exam & Concept Pitfalls
- Do not treat all beta blockers as interchangeable.
- A sympathomimetic is not automatically suitable for every hypotensive state.
Sources & Further Reading
Curriculum scope checked 23 September 2026. These original study notes summarize established principles; current institutional guidance and product information govern clinical use. Faculty review is pending.
RAHBAR study structure: target → drug action → therapeutic use → adverse effects → monitoring. Use institutional curriculum and faculty guidance for local sequencing, practical requirements and assessment specifications.