This chapter follows the supplied KMU learning outcomes and builds prostaglandins from their basic concept to mechanism, organ-system actions and clinical applications. First understand how prostaglandins act through their receptors, then use the high-yield review for revision.

Topic 9 — Prostaglandins: Biological Actions and Clinical Applications

Module/Theme: Infection and Inflammation

A focused undergraduate guide to the main prostaglandins, their receptor-mediated mechanism of action, important organ-system effects and major therapeutic applications.

Topic Introduction

Prostaglandins are locally acting lipid mediators derived from arachidonic acid. They belong to the larger group of eicosanoids and are produced in many tissues in response to physiological and inflammatory stimuli. Unlike classical endocrine hormones, they are usually formed when needed, act close to their site of synthesis and are rapidly inactivated. Different prostaglandins can produce quite different effects because they act on different receptors in different tissues. In this topic, you will learn the major prostaglandins, how they signal through specific receptors, how they influence blood vessels, platelets, the gastrointestinal tract, uterus, airways, kidneys and inflammation, and how these actions are used clinically. :contentReference[oaicite:0]{index=0}

A. Major Prostaglandins and Their Basic Organization

Prostaglandins are members of the eicosanoid family. They are formed from arachidonic acid through the cyclooxygenase pathway. The term “prostaglandin” includes several related molecules, but each has its own receptor profile and biological effects. For undergraduate learning, the most important members are PGE₂, PGF_2α, PGD₂, prostacyclin or PGI₂, and thromboxane A₂ or TXA₂, which is closely related to the prostaglandin pathway.

Important prostanoids

• PGE₂: important in vasodilation, pain sensitization, fever, gastric protection, uterine activity and maintenance of renal blood flow.
• PGF_2α: produces strong contraction of uterine smooth muscle and can also influence bronchial and ocular smooth muscle.
• PGD₂: is particularly associated with mast cells and allergic inflammatory responses.
• PGI₂ (prostacyclin): is produced mainly by vascular endothelium and causes vasodilation while inhibiting platelet aggregation.
• TXA₂: is produced mainly by activated platelets and promotes platelet aggregation and vasoconstriction.

A useful way to remember their physiological balance is that vascular endothelium tends to produce PGI₂, which keeps vessels relatively dilated and platelets less active, whereas activated platelets produce TXA₂, which promotes platelet aggregation and vasoconstriction.

AIM VISUAL 01 — Major Prostanoids at a Glance

B. Formation and Mechanism of Action of Prostaglandins

Prostaglandins are not stored in large intracellular reservoirs. Instead, they are synthesized when cells are stimulated. The starting material is arachidonic acid, which is released from membrane phospholipids. Cyclooxygenase enzymes then convert arachidonic acid into unstable prostaglandin intermediates, from which individual tissues form specific prostanoids.

Once produced, prostaglandins bind to specific cell-surface receptors. These receptors are mainly G-protein-coupled receptors. Their effects therefore depend not simply on which prostaglandin is present, but also on which receptor subtype is expressed in the target tissue.

Important receptor families

• EP receptors: activated mainly by PGE₂.
• FP receptors: activated mainly by PGF_2α.
• DP receptors: activated mainly by PGD₂.
• IP receptors: activated by PGI₂.
• TP receptors: activated mainly by TXA₂.

Different receptor subtypes couple to different intracellular signalling pathways. Some increase cyclic AMP, some decrease cyclic AMP, and others increase intracellular calcium. This explains why prostaglandins may relax smooth muscle in one organ but contract it in another.

AIM VISUAL 02 — Prostaglandin Synthesis and Receptor Signalling

C. Cardiovascular and Platelet Effects

Prostaglandins have important effects on vascular smooth muscle and platelets. These effects help regulate local blood flow and hemostasis. The most important physiological contrast is between prostacyclin and thromboxane A₂.

Prostacyclin — PGI₂

PGI₂ is formed mainly by vascular endothelial cells. It activates IP receptors, increases cyclic AMP in relevant target cells and produces vasodilation. In platelets, increased cyclic AMP reduces platelet activation and therefore inhibits platelet aggregation.

Thromboxane A₂

TXA₂ is formed mainly by activated platelets. It acts through TP receptors and promotes platelet aggregation and vasoconstriction. This supports formation of the platelet plug at sites of vascular injury.

PGE₂

PGE₂ commonly causes vasodilation in several vascular beds. The exact effect varies with receptor subtype and tissue, but its vasodilator action contributes to increased blood flow during inflammation.

AIM VISUAL 03 — PGI₂–TXA₂ Balance

D. Inflammation, Pain, Fever and Gastrointestinal Effects

PGE₂ is especially important in inflammation. It does not simply “cause inflammation”; rather, it amplifies several characteristic inflammatory responses. Its effects help explain why drugs that inhibit cyclooxygenase reduce pain, fever and inflammation.

Pain

PGE₂ sensitizes peripheral nociceptive nerve endings to inflammatory mediators. The nerves therefore respond more strongly to stimuli that would otherwise be less painful. This produces hyperalgesia, meaning increased sensitivity to pain.

Fever

During infection or inflammation, endogenous pyrogens promote prostaglandin formation in the hypothalamic region. PGE₂ contributes to elevation of the thermoregulatory set point, producing fever.

Gastric mucosal protection

PGE₂ and PGI₂ support normal gastric mucosal defense. They decrease gastric acid secretion and promote protective mechanisms such as mucus and bicarbonate secretion and adequate mucosal blood flow. Therefore, marked inhibition of protective prostaglandin synthesis can make the gastric mucosa more vulnerable to injury.

Vascular component of inflammation

PGE₂ and PGI₂ can produce vasodilation. Increased local blood flow contributes to the redness and warmth of inflamed tissue.

AIM VISUAL 04 — PGE₂ in Inflammation

E. Effects on Uterus, Airways, Kidneys and Other Organs

Because prostaglandin receptors are widely distributed, these mediators influence several organ systems. The direction and strength of the response depend on the prostaglandin, receptor subtype, tissue and physiological state.

Uterus and cervix

Prostaglandins are important regulators of uterine smooth-muscle activity. PGF_2α strongly contracts the uterus, while PGE compounds can also increase uterine activity. Certain PGE compounds additionally promote cervical ripening, making the cervix softer and more favorable for dilation. These actions form the basis of several obstetric and gynecological uses.

Airways

Different prostanoids can have different effects on bronchial smooth muscle. PGF_2α and TXA₂ can favor bronchoconstriction, while PGI₂ has relaxant effects in some vascular and smooth-muscle tissues. PGD₂, released particularly from mast cells, contributes to allergic airway responses and can promote bronchoconstriction.

Kidneys

Renal prostaglandins, particularly PGE₂ and PGI₂, help maintain renal perfusion by producing local vasodilation. This role becomes especially important when renal blood flow is threatened by vasoconstrictor influences.

Eye

PGF_2α-related receptor activation in the eye can increase aqueous humor outflow through the uveoscleral pathway. This lowers intraocular pressure and has an important therapeutic application.

Fetal circulation

PGE compounds help maintain patency of the ductus arteriosus during fetal life. This action can also be used therapeutically when temporary maintenance of ductal patency is required in selected congenital cardiac conditions.

AIM VISUAL 05 — Organ-System Actions of Prostaglandins

F. Clinical Applications of Prostaglandins and Their Analogues

The clinical value of prostaglandins comes from reproducing their normal physiological actions in a controlled manner. Different prostaglandin analogues are therefore selected according to the receptor and organ effect required. The most important undergraduate applications are summarized below.

1. Obstetric and gynecological applications

Prostaglandin preparations are used when uterine contraction or cervical ripening is therapeutically desirable.

• Dinoprostone is a PGE₂ preparation used for cervical ripening and induction of labor.
• Misoprostol is a PGE₁ analogue that promotes cervical ripening and uterine contraction and is used in obstetric and gynecological practice.
• Carboprost is a PGF_2α analogue that produces strong uterine contraction and may be used when a marked uterotonic effect is required.

2. Gastric mucosal protection

Misoprostol can reduce gastric acid secretion and enhance mucosal protective mechanisms. Its therapeutic action reflects the normal gastric-protective role of prostaglandins.

3. Glaucoma

PGF_2α analogues such as latanoprost reduce intraocular pressure mainly by increasing uveoscleral outflow of aqueous humor. This is an important example of converting a prostaglandin smooth-muscle and outflow effect into a therapeutic benefit.

4. Maintaining ductus arteriosus patency

Alprostadil, a PGE₁ preparation, can maintain ductus arteriosus patency when continued ductal blood flow is temporarily required in certain congenital cardiac lesions.

5. Pulmonary vascular disease

Prostacyclin and related analogues produce strong pulmonary vasodilation and inhibit platelet activation. These properties make PGI₂-based drugs useful in selected patients with pulmonary arterial hypertension.