Autacoids

🟢 Lite — Quick Review (1h–1d)

Autacoids — Key Facts for NEET PG

Histamine:

• H1 receptors: Smooth muscle contraction, vasodilation, pruritus
• H2 receptors: Gastric acid secretion (cimetidine = H2 blocker)
• Antihistamines: Cetirizine, Loratadine (H1); Diphenhydramine (sedating)

Prostaglandins (PGs):

• PGE1, PGE2: Vasodilation, protection, inflammation
• PGF2α: Bronchoconstriction, uterine contraction
• TXA2: Platelet aggregation (aspirin irreversibly inhibits COX → ↓TXA2)
• NSAIDs inhibit COX → ↓PGs (anti-inflammatory, anti-pyretic, analgesic)

Leukotrienes:

• LTC4, LTD4, LTE4 = “Cystinyl leukotrienes” — bronchoconstriction (1000× > histamine)
• Montelukast, Zafirlukast = Leukotriene receptor antagonists
• Zileuton = 5-LOX inhibitor

High-yield point: Aspirin + COX inhibition + TXA2 ↓ → antiplatelet effect (primary prevention vs bleeding risk)

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🟡 Standard — Regular Study (2d–2mo)

Autacoids — NEET PG Study Guide

Histamine:

Receptor	Location	Effect	Blocker
H1	Smooth muscle, vessels, skin	Pruritus, urticaria, bronchoconstriction	Cetirizine, Loratadine
H2	Gastric parietal cells	↑ Acid secretion	Cimetidine, Ranitidine, Famotidine
H3	Presynaptic CNS	Autoreceptor (↓ histamine release)
H4	Eosinophils, mast cells	Chemotaxis

Anaphylaxis treatment: Adrenaline (IM) → H1 + H2 blockers + steroids

Prostaglandins — Clinical Uses:

• PGE1 (Alprostadil): Patent ductus arteriosus (keeps it open)
• PGE2 (Dinoprostone): Cervical ripening, induction of labour
• Misoprostol (PGE1 analog): NSAID-induced ulcer prophylaxis, medical abortion
• Carboprost (PGF2α analog): Postpartum hemorrhage

Angiotensin:

• ATII: Potent vasoconstriction + aldosterone release
• ACE = Kininase II (breaks down bradykinin)
• ACE inhibitors → ↑ bradykinin → dry cough (common), angioedema (rare)

Platelet-Regulating Autacoids:

Autacoid	Effect on Platelet	Mechanism
TXA2	Aggregation	Via TP receptor
PGI2 (Prostacyclin)	Inhibition	Via IP receptor, ↑ cAMP
NO	Inhibition	↑ cGMP

Serotonin (5-HT):

• 5-HT1A: Anxiolysis, antidepressant (Buspirone)
• 5-HT2A: Vasoconstriction, platelet aggregation (Mirtazapine blocks it → ↑ appetite)
• 5-HT3: Vomiting (Ondansetron = 5-HT3 antagonist)
• 5-HT4: GI motility (Metoclopramide, Cisapride)

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🔴 Extended — Deep Study (3mo+)

Autacoids — Comprehensive NEET PG Notes

Detailed Pharmacology:

1. Histamine:

• Source: Mast cells, basophils (preformed + stored)
• Release triggers: IgE-mediated Type 1 hypersensitivity, morphine, codeine, contrast media, muscle relaxants
• Biosynthesis: Histidine → Histamine (histidine decarboxylase)

H1 Antihistamines — Classification:

Generation	Drug	Sedation	Key Points
1st	Diphenhydramine, Promethazine	High	Anticholinergic, antiemetic
2nd	Cetirizine, Loratadine	Low	Non-sedating, once daily
3rd	Fexofenadine, Desloratadine	Minimal	Active metabolites

2. Eicosanoids (Prostaglandins + Leukotrienes):

• Arachidonic acid → via COX → PGs, TXA2
• Arachidonic acid → via LOX → Leukotrienes (LTA4 → LTB4, LTC4-D4-E4)

Leukotriene Pathway:

AA → 5-HPETE → LTA4 → LTB4 (neutrophil chemotaxis)
                  → LTC4 → LTD4 → LTE4 (cys-LTs: bronchoconstriction)

• Cys-LT1 antagonists: Montelukast, Zafirlukast
• 5-LOX inhibitor: Zileuton

3. Renin-Angiotensin System:

• Renin cleaves Angiotensinogen → Angiotensin I
• ACE cleaves AT I → AT II
• AT II acts on AT1 receptors → vasoconstriction, aldosterone, ADH, thirst
• ACE inhibitors: Captopril, Enalapril, Ramipril
• ARBs: Losartan, Valsartan

4. Vasoactive Peptides:

Peptide	Effect	Blocker
Bradykinin	Vasodilation, pain, ↑ vascular permeability	Icatibant (B2 antagonist)
Natriuretic peptides (ANP, BNP)	Natriuresis, vasodilation	Nesiritide (recombinant)
Substance P	Pain transmission, NKA > NKB	Aprepitant (NK1 antagonist)

5. Adenosine:

• Antiplatelet + antiarrhythmic (slows AV conduction)
• Mechanism: A1 receptor → ↓ cAMP
• Clinical: Paroxysmal SVT (IV adenosine)
• Side effect: Bronchospasm (avoid in asthma)

6. Nitric Oxide (NO):

• EDRF from L-arginine (NOS)
• Vasodilator, ↓ platelet aggregation, neurotransmitter
• NO donors: GTN, Sodium nitroprusside
• PDE5 inhibitors (Sildenafil) → ↑ cGMP → enhanced NO effect

Drug Interactions with Autacoids:

• SSRIs + MAOIs + Tyramine → Hypertensive crisis (SSRI + MAOI: serotonin syndrome)
• NSAIDs + ACEi → ↓ renal PGs → ↓ renal blood flow → acute kidney injury
• Aspirin + Warfarin → ↑ bleeding (displacement from albumin)

Clinical Scenarios Frequently Asked in NEET PG:

1. Anaphylaxis → IM Adrenaline + IV fluids + H1/H2 blockers + steroids
2. Aspirin-exacerbated respiratory disease (AERD) → avoid NSAIDs → leukotriene antagonists
3. ACE inhibitor cough → switch to ARB
4. Carcinoid syndrome → Octreotide (somatostatin analog)
5. Migraine prophylaxis → Propranolol + Flunarizine + Amitriptyline

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