Table of Contents

Autocoids

Autacoids are locally acting endogenous substances that function as short-lived biological mediators. The term “autacoid” means self-remedy, highlighting their ability to regulate physiological and pathological processes at the site of synthesis.

Unlike hormones that circulate systemically, autacoids:

  • Act locally
  • Have short duration of action
  • Are rapidly metabolized
  • Play major roles in inflammation, allergy, gastric secretion, vascular tone, and pain

They are critically important in understanding inflammation, hypersensitivity reactions, asthma, shock, and gastrointestinal disorders.

Classification of Autacoids

Autacoids are broadly divided into:

  1. Amines (Histamine, Serotonin)
  2. Peptide autacoids (Bradykinin)
  3. Lipid-derived autacoids (Prostaglandins, Leukotrienes, Thromboxanes)
  4. Gaseous mediators (Nitric Oxide)

1. Histamine

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Source

  • Stored in mast cells and basophils

  • Released during allergic reactions and tissue injury

Receptor Types

  • H1 – Allergy, bronchoconstriction, vasodilation

  • H2 – Gastric acid secretion

  • H3 – CNS regulation

  • H4 – Immune modulation

Mechanism of Action

Histamine binds to G-protein coupled receptors leading to:

  • Increased vascular permeability

  • Smooth muscle contraction

  • Increased gastric acid secretion

Clinical Importance

  • H1 blockers → Allergy treatment

  • H2 blockers → Peptic ulcer disease

  • Major role in anaphylaxis

2. Serotonin (5-Hydroxytryptamine)

 

Source

  • Enterochromaffin cells (GIT)
  • Platelets
  • Central nervous system

Receptors

Multiple subtypes (5-HT1 to 5-HT7), mostly GPCRs.

Physiological Roles

  • Mood regulation
  • Gastrointestinal motility
  • Platelet aggregation
  • Vasoconstriction

Clinical Relevance

  • Triptans → Migraine
  • SSRIs → Depression
  • 5-HT3 antagonists → Anti-emetic drugs

3. Prostaglandins & Thromboxanes

https://www.ncbi.nlm.nih.gov/books/NBK470143/bin/chapter7.f1.jpg

Origin

Derived from arachidonic acid via cyclooxygenase (COX) pathway.

Functions

  • Fever
  • Pain
  • Inflammation
  • Gastric protection
  • Uterine contraction
  • Platelet aggregation (Thromboxane A2)

Clinical Significance

  • NSAIDs inhibit COX enzymes
  • COX-2 inhibitors reduce inflammation
  • Prostaglandin analogues used in glaucoma and obstetrics

4. Leukotrienes

 
 
https://ars.els-cdn.com/content/image/1-s2.0-S1094553913001338-gr1.jpgOrigin

Produced via lipoxygenase pathway from arachidonic acid.

Functions

  • Bronchoconstriction
  • Increased vascular permeability
  • Mucus secretion

Clinical Importance

  • Major mediators in asthma
  • Leukotriene receptor antagonists used in asthma management

5. Bradykinin

 
 
https://www.researchgate.net/publication/353640291/figure/fig4/AS%3A11431281386771331%401745099044659/Receptor-stimulation-of-B1-and-B2-receptors-Kallidin-KD-and-Bradykinin-BK-act.tifSource

Formed from kininogen via kallikrein.

Effects

  • Vasodilation
  • Pain sensation
  • Increased permeability
  • Hypotension

Clinical Correlation

ACE inhibitors increase bradykinin levels → cough & angioedema.

6. Nitric Oxide (NO)

 
 
https://cvphysiology.com/s9v2f-3r4p7/share/vascular-no-cgmp-pathway.png

Synthesis

Produced from L-arginine by nitric oxide synthase (NOS).

Mechanism

Activates guanylyl cyclase → increases cGMP → smooth muscle relaxation.

Physiological Role

  • Vasodilation

  • Neurotransmission

  • Platelet inhibition

Clinical Applications

  • Nitrates in angina

  • Sildenafil enhances NO pathway

Role of Autacoids in Inflammation

Autacoids are central mediators in inflammation:

  • Histamine → Early phase response
  • Prostaglandins → Pain & fever
  • Leukotrienes → Bronchospasm
  • Bradykinin → Pain & edema

Their coordinated release produces redness, swelling, heat, and pain.

Drug Interaction and Autacoids

Autacoids interact pharmacodynamically with many drug classes:

  • NSAIDs reduce prostaglandin synthesis
  • Antihistamines block H1 receptors
  • Leukotriene antagonists reduce asthma severity
  • ACE inhibitors increase bradykinin
  • Nitrates enhance nitric oxide signaling

Understanding these interactions prevents adverse effects and improves therapeutic outcomes.

Clinical Importance for Medical & OT Students

Knowledge of autacoids is crucial in:

  • Managing allergic reactions

  • Treating asthma

  • Handling anaphylaxis in operation theatre

  • Controlling inflammation and pain

  • Understanding shock mechanisms

In anesthesia and emergency medicine, autacoid imbalance can be life-threatening.

Summary

Autacoids are powerful local mediators involved in:

  • Allergy
  • Inflammation
  • Pain
  • Vascular regulation
  • Gastric secretion
  • CNS function

Their pharmacology forms the foundation for understanding antihistamines, NSAIDs, anti-asthmatics, antidepressants, and cardiovascular drugs.

Mastery of autacoids enables safe prescribing, rational drug selection, and improved patient care.