Definition

        •      Group of compounds that are produced by the action of the enzyme prostaglandin synthetase on the precursor substance,                   arachidonic acid

        •      Prostaglandins & thromboxanes are formed by the cyclooxygenase pathway from arachidonic acid

        •      The cyclooxygenase reaction is inhibited by NSAIDs including aspirin

        •      The vascular endothelium synthesises PGE2 & PGI2

        •      Platelets synthesise thromboxane

 

Action

        •      Vascular smooth muscle:

                                   Relax: PGE2; PGI2 (prostacyclin)

                                   Constrict:      TXA2; PGF2

 

        •      Respiratory smooth muscle

                                   Relax: PGE2; PGI2 (prostacyclin)

                                   Constrict:      TXA2; PGF2

 

        •      Platelets

                                   Inhibit aggregation:  PGE1; PGI2 (prostacyclin)

                                   Promote aggregation:          TXA2

 

Clinical application:

        •      PGI2 (prostacyclin) lowers peripheral & coronary resistance; used in pulmonary hypertension [however its pulmonary vasodilator                         effects are not specific enough- the patients BP must be supported by pressor drugs]

        •      Aspirin inhibits platelet cyclooxygenase to produce a mild clotting defect

        •      Patency of foetal ductus arteriosus is dependent on PGI2 (prostacyclin) & PGE2 synthesis: to maintain its patency before            surgery in transposition of great vessels, pulmonary artery stenosis/atresia PGE1 (alprostadil) is given iv

        •      To close patent ductus , cyclooxygenase inhibitors to inhibit synthesis of vasodilators PGI2 (prostacyclin) & PGE2 eg                  indomethanin