Prostaglandins, Leukotrienes (Eicosanoids) and Platelet Activating Factor

Chapter 13

Prostaglandins, Leukotrienes (Eicosanoids) and Platelet Activating Factor 

PROSTAGLANDINS AND LEUKOTRIENES

• Prostaglandins (PGs) and Leukotrienes (LTs) are biologically active derivatives of 20 carbon atom polyunsaturated essential fatty acids that are released from cell membrane phospholipids. They are the major lipid derived autacoids.

• In the 1930s human semen was found to contract isolated uterine and other smooth muscle strips and to cause fall in BP in animals. The active principle was termed ‘prostaglandin’, thinking that it was derived from prostate. Only in the 1960s it was shown to be a mixture of closely related compounds, the chemical structures were elucidated, and widespread distribution was revealed. In 1970s it became clear that aspirin like drugs act by inhibiting PG synthesis, and that in addition to the classical PGs (Es and Fs), thromboxane (TX), prostacyclin (PGI) and leukotrienes (LTs) were of great biological importance. Bergstrom, Samuelsson and Vane got the Nobel prize in 1982 for their work on PGs and LTs. Over the past 40 years they have been among the most intensely investigated substances.

CHEMISTRY, BIOSYNTHESIS AND DEGRADATION

• Chemically, PGs may be considered to be derivatives of prostanoic acid, though prostanoic acid does not naturally occur in the body. It has a five membered ring and two side chains projecting in opposite directions at right angle to the plane of the ring. There are many series of PGs and thromboxanes (TXs) designated A, B, C....I, depending on the ring structure and the substituents on it. Each series has members with subscript 1, 2, 3 indicating the number of double bonds in the side chains. Leukotrienes are so named because they were first obtained from leukocytes (leuko) and have 3 conjugated double bonds (triene). They have also been similarly designated A, B, C.....F and given subscripts 1, 2, 3, 4.

• In the body PGs, TXs and LTs are all derived from eicosa (referring to 20 C atoms) tri/tetra/ penta enoic acids. Therefore, they can be collectively called eicosanoids. In human tissues, the fatty acid released from membrane lipids in largest quantity is 5,8,11,14 eicosa tetraenoic acid (arachidonic acid). During PG, TX and prostacyclin synthesis, 2 of the 4 double bonds of arachidonic acid get saturated in the process of cyclization, leaving 2 double bonds in the side chain. Thus, subscript 2 PGs are most important in man, e.g.

• PGE2, PGF2α, PGI2, TXA2. No cyclization or reduction of double bonds occurs during LT synthesis— the LTs of biological importance are LTB4, LTC4, LTD4.

• Eicosanoids are the most universally distributed autacoids in the body. Practically every cell and tissue is capable of synthesizing one or more types of PGs or LTs. The pathways of biosynthesis of eicosanoids are summarized.

• There are no preformed stores of PGs and LTs. They are synthesized locally at rates governed by the release of arachidonic acid from membrane lipids in response to appropriate stimuli. These stimuli activate hydrolases, including phospholipase A, probably through increased intracellular Ca2+.

• The cyclooxygenase (COX) pathway generates eicosanoids with a ring structure (PGs, TXs, prostacyclin) while lipoxygenase (LOX) produces open chain compounds (LTs). All tissues have COX—can form cyclic endoperoxides PGG2 and PGH2 which are unstable compounds. Further course in a particular tissue depends on the type of isomerases or other enzymes present in it. PGE2 and PGF2α are the primary prostaglandins (name based on the separation procedure: PGE partitioned into Ether while PGF into phosphate [Fosfat in Swedish] buffer; α in PGF2α refers to orientation of OH group on the ring). PGs A, B and C are not found in the body: they are artifacts formed during extraction procedures. Lung and spleen can synthesize the whole range of COX products. Platelets primarily synthesize TXA2 which is —chemically unstable, spontaneously changes to TXB2. Endothelium mainly generates prostacyclin (PGI2); also chemically unstable and rapidly converts to 6-keto PGF1α.

• Cyclooxygenase is now known to exist in two isoforms COX-1 and COX-2. While both isoforms catalyse the same reactions, COX-1 is a constitutive enzyme in most cells—its activity is not changed once the cell is fully grown. On the other hand, COX-2 normally present in insignificant amounts, is inducible by cytokines, growth factorsand other stimuli during the inflammatory response. It is believed that eicosanoids produced by COX-1 participate in physiological (housekeeping) functions such as secretion of mucus for protection of gastric mucosa, haemostasis and maintenance of renal function, while those produced by COX-2 lead to inflammatory and other pathological changes. However, certain sites in kidney and brain constitutively express COX-2 which may play physiological role.

• A splice variant of COX-1 (designated COX-3) has been found in the dog brain. This isoenzyme is inhibited by paracetamol, but its role in humans is not known.

• Lipoxygenase pathway appears to operate mainly in the lung, WBC and platelets. Its most important products are the LTs, (generated by 5- LOX) particularly LTB4 (potent chemotactic) and LTC4, LTD4 which together constitute the ‘slow reacting substance of anaphylaxis’ (SRS-A) described in 1938 to be released during anaphylaxis. A membrane associated transfer protein called FLAP (five lipoxygenase activating protein) carrys arachidonic acid to 5-LOX, and is essential for the synthesis of LTs. Platelets have only 12-LOX.

• HPETEs produced by LOX can also be converted to hepoxilins, trioxilins and lipoxins. A third enzymatic pathway involving cytochrome P450 can metabolize arachidonic acid into 19- and 20-HETEs and epoxyeicosatrienoic acids. Free radicals can attack arachidonic acid to produce isoprostanes nonenzymatically. Brain cells couple arachidonic acid with ethanolamine to produce anandamide which has cannabinoid like action. The above-named metabolites of arachidonic acid have a variety of vascular, inflammatory and other actions, but their pathophysiological role is not clear.
• Inhibition of synthesis Synthesis of COX products can be inhibited by nonsteroidal anti-inflammatory drugs (NSAIDs). Aspirin acetylates COX at a serine residue and causes irreversible inhibition while other NSAIDs are competitive and reversible inhibitors. Most NSAIDs are nonselective COX-1 and COX-2 inhibitors, but some newer ones like celecoxib, rofecoxib are selective for COX-2at lower doses. NSAIDs do not inhibit the production of LTs: this may even be increased since all the arachidonic acid becomes available to the LOX pathway.

• Zileuton inhibits LOX and decreases the production of LTs. It was used briefly in asthma but has been withdrawn. Glucocorticosteroids inhibit the release of arachidonic acid from membrane lipids (by stimulating production of proteins called annexins or lipocortins which inhibit phospholipase A2) — indirectly reduce production of all eicosanoids— PGs, TXs and LTs. Moreover, they inhibit the induction of COX-2 by cytokines at the site of inflammation.

• Degradation of arachidonates occurs rapidly in most tissues, but fastest in the lungs. Most PGs, TXA2 and prostacyclin have plasma t½ of a few seconds to a few minutes. First a specific carrier mediated uptake into cells occurs, the side chains are then oxidized, and double bonds are reduced in a stepwise manner to yield inactive metabolites. Metabolites are excreted in urine. PGI2 is catabolized mainly in the kidney.

ACTIONS AND PATHOPHYSIOLOGICAL ROLES

Prostaglandins, thromboxanes and prostacyclin

• The cyclic eicosanoids produce a wide variety of actions depending upon the particular PG (or TX or PGI), species on which tested, tissue, hormonal status and other factors. PGs differ in their potency to produce a given action and different PGs sometimes have opposite effects. Even the same PG may have opposite effects under different circumstances. The actions of PGs and TXA2 are summarized. Since virtually all cells and tissues are capable of forming PGs, they have been implicated as mediators or modulators of a number of physiological processes and pathological states.

CVS 

• PGE2 and PGF2α cause vasodilatation in most, but not all, vascular beds. In isolated preparations, they are more potent vasodilators than ACh or histamine. PGF2α constricts many larger veins including pulmonary vein and artery. Fall in BP occurs when PGE2 is injected i.e., but PGF2α has little effect on BP.

1. PGI2 is uniformly vasodilatory and is more potent hypotensive than PGE2. 
2. TXA2 consistently produces vasoconstriction.
3. PG endoperoxides (G2 and H2) are inherently vasoconstrictor, but often produce vasodilatation or a biphasic response due to rapid conversion to other PGs, especially PGI2 in the blood vessels themselves.
4. PGE2 and F2α stimulate heart by weak direct but more prominent reflex action due to fall in BP. The cardiac output increases.

Role

1. PGI2 is probably involved in the regulation of local vascular tone as a dilator. 
2. PGE2 and PGI2 are believed to be continuously produced locally in the ductus arteriosus during foetal life—keep it patent; at birth their synthesis is inhibited, and closure occurs. Aspirin and indomethacin induce closure when it fails to occur spontaneously. These PGs may also be important in maintaining placental blood flow. 
3. PGs, along with LTs and other autacoids may mediate vasodilatation and exudation at the site of inflammation.

Platelets

• TXA2, which can be produced locally by platelets, is a potent inducer of aggregation and release reaction. The endoperoxides PGG2 and PGH2 are also proaggregatory. On the other hand, PGI2 (generated by vascular endothelium) is a potent inhibitor of platelet aggregation. PGD2 has antiaggregatory action, but much less potent than PGI2. PGE2 has inconsistent effects.

Role 

• TXA2 produced by platelets and PGI2 produced by vascular endothelium probably constitute a mutually antagonistic system: preventing aggregation of platelets while in circulation and inducing aggregation on injury, when plugging and thrombosis are needed. Aspirin interferes with haemostasis by inhibiting platelet aggregation which is due toTXA2 production. Before it is deacetylated in liver, aspirin acetylates COX in platelets while they are in portal circulation. Further, platelets are unable to regenerate fresh COX (lack nucleus: do not synthesize protein), while vessel wall is able to do so (fresh enzyme is synthesized within hours). Thus, in low doses, aspirin selectively inhibits TXA2 production and has antithrombotic effect lasting > 3 days.

Uterus 

• PGE2 and PGF2α uniformly contract human uterus, pregnant as well as nonpregnant in vivo. The sensitivity is higher during pregnancy and there is a further modest increase with progress of pregnancy. However, even during early stages uterus is quite sensitive to PGs though not to oxytocin. PGs increase tone as well as amplitude of uterine contractions. When tested in vitro, PGF2α consistently produces contraction while PGE2 relaxes nonpregnant but contracts pregnant human uterine strips. At term, PGs at low doses soften the cervix and make it more compliant.

Role

• Foetal tissues produce PGs and at term PGF2α has been detected in maternal blood. It has been postulated that PGs mediate initiation and progression of labour. Aspirin has been found to delay the initiation of labour and also prolongs its duration. 

• Because PGs are present in high concentration in semen and can be rapidly absorbed when lodged in the vagina at coitus, it is believed that they so coordinate movements of the female genital tract that transport of sperms and fertilization is facilitated.

• Dysmenorrhoea in many women is associated with increased PG synthesis by the endometrium. This apparently induces uncoordinated uterine contractions which compress blood vessels → uterine ischaemia → pain. Aspirin group of drugs are highly effective in relieving dysmenorrhoea in most women.

Bronchial muscle 

• PGF2α, PGD2 and TXA2 are potent bronchoconstrictors (more potent than histamine) while PGE2 is a powerful bronchodilator. PGI2 produces mild dilatation. Asthmatics are more sensitive to constrictor as well as dilator effects of PGs. PGE2 and PGI2 also inhibit histamine release and are effective by aerosol—but produce irritation of the respiratory tract and have a brief action.

Role 

• Asthma may be due to an imbalance between constrictor PGs (F2α, PGD2, TXA2) and LTs on one hand and dilator ones (PGE2, PGI2) on the other. In few individuals' aspirin-like drugs consistently induce asthma, possibly by diverting arachidonic acid to produce excess LTC4 and D4. This sensitivity is not shared by selective COX-2 inhibitors, indicating that suppression of COX-1 at the pulmonary site is responsible for the reaction. In allergic human asthma, LTs are more important and COX inhibitors are without any effect in most patients.

GIT 

• (i)In isolated preparations, the longitudinal muscle of gut is contracted by PGE2 and PGF2α while the circular muscle is either contracted (usually by PGF2α) or relaxed (usually by PGE2). Propulsive activity is enhanced in man, especially by PGE2 → colic and watery diarrhoea are important side effects. PGE2 acts directly on the intestinal mucosa and increases water, electrolyte and mucus secretion. PGI2 does not produce diarrhoea and infact opposes PGE2 and toxin induced fluid movement.

Role

• PGs may be involved in mediating toxin induced increased fluid movement in secretory diarrhoeas. In certain diarrheas', aspirin can reduce stool volume, but is not uniformly effective. PGs appear to play a role in the growth of colonic polyps and cancer. Association of low incidence of colon cancer with regular intake of aspirin is now established. NSAIDs afford relief in familial colonic polyposis by reducing polyp formation.

• (ii)PGE2 markedly reduces acid secretion in the stomach. Volume of juice and pepsin content are also decreased. It inhibits fasting as well as stimulated secretion (by feeding, histamine, gastrin). The gastric pH may rise upto 7.0. PGI2 also inhibits gastric secretion but is less potent. Secretion of mucus in stomach and mucosal blood flow are increased; PGs are antiulcerogenic

Role 

• PGs (especially PGI2) appear to be involved in the regulation of gastric mucosal blood flow. They may be functioning as natural ulcer protectives by enhancing gastric mucus production. The ulcerogenic action of NSAIDs may be due to loss of this protective influence. Normally, gastric mucosal PGs are produced by COX-1. Selective COX-2 inhibitors are less ulcerogenic. However, COX-2 gets induced during ulcer healing, and COX-2 inhibitors have the potential to delay healing.

Kidney 

• PGE2 and PGI2 increase water, Na+ and K+ excretion and have a diuretic effect. PGE2 has been shown to have a furosemide-like inhibitory effect on Cl¯ reabsorption as well. They cause renal vasodilatation and inhibit tubular reabsorption. PGE2 antagonizes ADH action, and this adds to the diuretic effect. In contrast, TXA2 causes renal vasoconstriction. PGI2, PGE2 and PGD2 evoke release of renin.

Role 

• PGs appear to function as intrarenal regulators of blood flow as well as tubular reabsorption in kidney. The NSAIDs tend to retain salt and water. The diuretic action of furosemide is blunted by indomethacin— indicating a facilitatory role of PGs by increasing renal blood flow and/or augmenting inhibition of tubular reabsorption. 

• Renin release in response to sympathetic stimulation and other influences may be facilitated by PGs. 

• Bartter’s syndrome, characterized by decreased sensitivity to angiotensin II is associated with increased PG production; many of the manifestations are improved by prolonged use of NSAIDs. 

 CNS 

• PGs injected i.e. penetrate brain poorly and central effects are not prominent. However, injected intracerebroventricularly PGE2 produces a variety of effects—sedation, rigidity, behavioral changes and marked rise in body temperature. PGI2 also induces fever, but TXA2 is not pyrogenic.

Role 

• PGE2 may mediate pyrogen induced fever and malaise. Aspirin and other inhibitors of PG synthesis are antipyretic. Pyrogens, including cytokines released during bacterial infection, trigger synthesis of PGE2 in the hypothalamus, which resets the thermostat to cause fever. COX-2 is the major isoenzyme involved; selective COX-2 inhibitors are equally efficacious antipyretics. A role of COX-3 has also been proposed. (ii) PGs may be functioning as neuromodulators in the brain by regulating neuronal excitability. A role in pain perception, sleep and some other functions has been suggested.

ANS 

• Depending on the PG, species and tissue, both inhibition as well as augmentation of NA release from adrenergic nerve endings has been observed.

Role 

• PGs may modulate sympathetic neurotransmission in the periphery.

Peripheral nerves 

• PGs (especially E2 and I2) sensitize afferent nerve endings to pain inducing chemical and mechanical stimuli. They irritate mucous membranes and produce long lasting dull pain on intradermal injection.

Role 

• PGs appear to serve as algesic agents during inflammation. They cause tenderness and amplify the action of other algesics. Inhibition of PG synthesis is a major anti-inflammatory mechanism. Aspirin injected locally decreases pain produced by injection of bradykinin at the same site.

Eye

• PGF2α induces ocular inflammation and lowers i.o.t by enhancing uveoscleral outflow.Non irritating congeners like latanoprost are now first line drugs in wide angle glaucoma.

Role 

• Locally produced PGs appear to facilitate aqueous humor drainage, since COX-2 expression in the ciliary body has been found to be deficient in wide angle glaucoma patients.

Endocrine system 

• PGE2 facilitates the release of anterior pituitary hormones—growth hormone, prolactin, ACTH, FSH and LH as well as that of insulin and adrenal steroids. It has a TSH like effect on thyroid. PGF2α causes luteolysis and terminates early pregnancy in many mammals, but this effect is not significant in humans. Though PGs can terminate early pregnancy in women, this is not associated with fall in progesterone levels.
• Metabolism PGEs are antilipolytic, exert an insulin like effect on carbohydrate metabolism and mobilize Ca2+ from bone: may mediate hypercalcaemia due to bony metastasis.

Leukotrienes 

• The straight chain lipoxygenase products of arachidonic acid are produced by a more limited number of tissues (LTB4 mainly by neutrophils; LTC4 and LTD4—the cysteinyl LTs—mainly by macrophages), but probably they are pathophysiologically as important as PG.

CVS and blood 

• LTC4 and LTD4 injected i.v. evoke a brief rise in BP followed by a more prolonged fall. The fall in BP is not due to vasodilatation because no relaxant action has been seen on blood vessels. It is probably a result of coronary constriction induced decrease in cardiac output and reduction in circulating volume due to increased capillary permeability. These LTs markedly increase capillary permeability and are more potent than histamine in causing local edema formation. LTB4 is highly chemotactic for neutrophils and monocytes; this property is shared by HETE but not by other LTs. Migration of neutrophils through capillaries and their clumping at sites of inflammation in tissues is also promoted by LTB4.

Role 

• LTs are important mediators of inflammation. They are produced (along with PGs) locally at the site of injury. While LTC4 and D4 cause exudation of plasma, LTB4 attracts the inflammatory cells which reinforce the reaction. 5-HPETE and 5-HETE may facilitate local release of histamine from mast cells.

Smooth muscle 

• LTC4 and D4 contract most smooth muscles. They are potent bronchoconstrictors and induce spastic contraction of g.i.t. at low concentrations. They also increase mucus secretion in the airways.

Role 

• The cysteinyl LTs (C4 and D4) are the most important mediators of human allergic asthma. They are released along with PGs and other autacoids during AG: AB reaction in the lungs. In comparison to other mediators, they are more potent and are metabolized slowly in the lungs, exert a long lasting action. LTs may also be responsible for abdominal colics during systemic anaphylaxis.

Afferent nerves 

• Like PGE2 and I2, the LTB4 also sensitizes afferents carrying pain impulses— contributes to pain and tenderness of inflammation.

PROSTANOID RECEPTORS

PGs, TX and prostacyclin act on their own specific receptors located on cell membrane. Five major types of prostanoid receptors have been designated, each after the natural PG for which it has the greatest affinity. This has been supported by receptor cloning. All prostanoid receptors are G-protein coupled receptors which utilize the IP3/ DAG or cAMP transducer mechanisms. Some selective antagonists of prostanoid receptors have been produced. The prostanoid receptors are:

• DP Has greatest affinity for PGD2, but PGE2 also acts on it; activation increases cAMP which inhibits platelet aggregation

• EP Has greatest affinity for PGE2; enprostil is a selective agonist. It has been subdivided into EP1 which causes smooth muscle contraction through IP3/DAG pathway and EP2 which mediates smooth muscle relaxation by increasing cAMP. Cloning studies have identified two more subtypes EP3 and EP4. PGE2 enhances Cl¯ and water secretion in intestinal mucosa also by increasing cAMP. However, in some tissues (adipocytes) PGE2 inhibits cAMP formation—responsible for its antilipolytic action. EP1 receptors are activated by PGF2α also.

• FP Has greatest affinity for PGF2α; fluprostenol is a selective agonist. The most prominent effect of activation of this receptor is smooth muscle contraction mediated through IP3/DAG formation.

• IP Has greatest affinity for PGI2; PGE also acts on it and cicaprost is a selective agonist. It functions by activating adenylyl cyclase in platelets (inhibiting aggregation) and smooth muscles (relaxation).

• TP Has greatest affinity for TXA2; PGH2 also acts on it. It utilizes IP3/DAG as second messengers which mediate platelet aggregation and smooth muscle contraction.

LEUKOTRIENE RECEPTORS

Separate receptors for LTB4 (BLT) and for the cysteinyl LTs (LTC4, LTD4) have been defined. Two subtypes, cys LT1 and cysLT2 of the cysteinyl LT receptor have been cloned. All LT receptors function through the IP3/DAG transducer mechanism. The cysLT1 receptor antagonists, viz. Montelukast, Zafirlukast, etc are now valuable drugs for bronchial asthma.

USES

• Clinical use of PGs and their analogues is rather restricted because of limited availability, short lasting action, cost, side effects and other practical considerations. Their approved indications are:

• Abortion During first trimester, termination of pregnancy by transcervical suction is the pro-cedure of choice. Intravaginal PGE2 pessary inserted 3 hours before attempting dilatation can minimise trauma to the cervix by reducing resistance to dilatation.

• Medical termination of pregnancy of upto 7 weeks has been achieved with high success rate by administering mifepristone (antiprogestin) 600 mg orally 2 days before a single oral dose of misoprostol 400 μg. Uterine contractions are provoked and the conceptus is expelled within the next few hours. Ectopic pregnancy should be ruled out beforehand and complete expulsion should be confirmed afterwards. Uterine cramps, vaginal bleeding, nausea, vomiting and diarrhoea are the possible complications. Methotrexate administered along with misoprostol is also highly successful in inducing abortion in the first few weeks of pregnancy.

• PGs have a place in midterm abortion, missed abortion and molar gestation, though delayed and erratic action and incomplete abortion are a problem. The initial enthusiasm has given way to more considered use. PGs convert the oxytocin resistant midterm uterus to oxytocin responsive one: a single extraamniotic injection (PGE2) followed by i.v. infusion of oxytocin or intraamniotic (PGF2α) with hypertonic solution produces 2nd trimester abortion in a high percentage without undue side effects. Pretreatment with mifepristone improves the efficacy of PGE as abortifacient 

• Induction/augmentation of labour PGs do not offer any advantage over oxytocin for induction of labour at term. They are less reliable and show wider individual variation in action. PGE2 and PGF2α (rarely) have been used in place of oxytocin in toxaemic and renal failure patients, because they do not cause fluid retention. PGE2 may also be used to augment labour, if it is slow, in primipara. Intravaginal route is preferred now: side effects are milder; extra/intra amniotic route is infrequently used.

• Cervical priming Applied intravaginally or in the cervical canal, low doses of PGE2 which do not affect uterine motility make the cervix soft and compliant. This procedure has yielded good results in cases with unfavourable cervix. If needed labour may be induced 12 hours later with oxytocin: chances of failure are reduced.

• Postpartum haemorrhage (PPH) Carboprost (15-methyl PGF2α) injected i.m. is an alternative for control of PPH due to uterine atony, especially in patients unresponsive to ergometrine and oxytocin.

• Peptic ulcer Stable analogue of PGE1 (misoprostol) is occasionally used for healing peptic ulcer, especially in patients who need continued NSAID therapy or who continue to smoke.

• Glaucoma Topical PGF2α analogues like latanoprost and isopropyl unoprostone are one of the first-choice drugs in wide angle glaucoma.

• To maintain patency of ductus arteriosus in neonates with congenital heart defects, till surgery is undertaken. PGE1 (Alprostadil) is used; apnoea occurs in few cases.

• To avoid platelet damage PGI2 (Epoprostenol) can be used to prevent platelet aggregation and damage during haemodialysis or cardiopulmonary bypass. It also improves harvest of platelets for transfusion. Few cases of primary pulmonary hypertension have been successfully maintained on epoprostenol infusion.

 FLOLAN 0.5 mg vial for reconstitution.

The other suggested uses of PGs are:

1. Peripheral vascular diseases PGI2 (or PGE1) infused i.v. can relieve rest pain and promote ulcer healing in severe cases of intermittent claudication and in Raynaud’s disease. 
2. Impotence Alprostadil (PGE1) injected into the penis causes erection lasting 1–2 hours. However, oral sildenafil/ tadalafil is now preferred for erectile dysfunction.

SIDE EFFECTS

• Side effects are common in the use of PGs, but their intensity varies with the PG, the dose and the route. These are: nausea, vomiting, watery diarrhoea, uterine cramps, unduly forceful uterine contractions, vaginal bleeding, flushing, shivering, fever, malaise, fall in BP, tachycardia, chest pain.

PLATELET ACTIVATING FACTOR (PAF)

• Like eicosanoids, platelet activating factor (PAF) is a cell membrane derived polar lipid with intense biological activity; discovered in 1970s and now recognized to be an important signal molecule. PAF is acetyl-glyceryl ether-phosphoryl choline.

• Synthesis and degradation PAF is synthesized from precursor phospholipids present in cell membrane by the following reactions. The second step is rate limiting. Antigen-antibody reaction and a variety of mediators stimulate PAF synthesis in a Ca2+ dependent manner on demand: there are no preformed stores of PAF. In contrast to eicosanoids, the types of cells which synthesize PAF is quite limited—mainly WBC, platelets, vascular endothelium and kidney cells.

• Actions PAF has potent actions on many tissues/organs.

• Platelets Aggregation and release reaction; also releases TXA2; i.v. injection results in intravascular thrombosis.

• WBC PAF is chemotactic to neutrophils, eosinophils and monocytes. It stimulates neutrophils to aggregate, to stick to vascular endothelium and migrate across it to the site of infection. It also prompts release of lysosomal enzymes and LTs and generation of superoxide radical by the polymorphs. The chemotactic action may be mediated through release of LTB4. It induces degranulation of eosinophils.

• Blood vessels Vasodilatation mediated by release of EDRF occurs → fall in BP on i.v. injection. Decreased coronary blood flow has been observed on intracoronary injection, probably due to formation of platelet aggregates and release of TXA2. PAF is the most potent agent known to increase vascular permeability. Wheal and flare occur at the site of intradermal injection.Injected into the renal artery PAF reduces renal blood flow and Na+ excretion by direct vasoconstrictor action, but this is partly counteracted by local PG release.

• Visceral smooth muscle Contraction occurs by direct action as well as through release of LTC4, TXA2 and PGs. Aerosolized PAF is a potent bronchoconstrictor. In addition, it produces mucosal edema, secretion and a delayed and long-lasting bronchial hyper-responsiveness. It also stimulates intestinal and uterine smooth muscle.

• Stomach PAF is ulcerogenic: erosions and mucosal bleeding occur shortly after i.v. injection of PAF. The gastric smooth muscle contracts.

• Mechanism of action Membrane bound specific PAF receptors have been identified. The PAF receptor is a G-protein coupled receptor which exerts most of the actions through intracellular messengers IP3/DAG → Ca2+ release. As mentioned above, many actions of PAF are mediated/augmented by PGs, TXA2 and LTs which may be considered its extracellular messengers. PAF also acts intracellularly, especially in the endothelial cells; rise in PAF concentration within the endothelial cells is associated with exposure of neutrophil binding sites on their surface. Similarly, its proaggregatory action involves unmasking of fibrinogen binding sites on the surface of platelets.

• PAF antagonists A number of natural and synthetic PAF receptor antagonists have been investigated. Important among these are ginkgolide B (from a Chinese plant), and some structural analogues of PAF. The PAF antagonists have manyfold therapeutic potentials like treatment of stroke, intermittent claudication, sepsis, myocardial infarction, shock, g.i. ulceration, asthma and as contraceptive. Some of them have been tried clinically but none has been found worth marketing. Alprazolam and triazolam antagonize some actions of PAF.

• Pathophysiological roles PAF has been implicated in many physiological processes and pathological states, especially those involving cell-to-cell interaction. These are:

1. Inflammation: Generated by leukocytes at the site of inflammation PAF appears to participate in the causation of vasodilatation, exudation, cellular infiltration and hyperalgesia. 
2. Bronchial asthma: Along with LTC4 and LTD4, PAF appears to play a major role by causing bronchoconstriction, mucosal edema and secretions. It is unique in producing prolonged airway hyper-reactivity, so typical of bronchial asthma patient. 
3. Anaphylactic (and other) shock conditions: are associated with high circulating PAF levels. 
4. Haemostasis and thrombosis: PAF may participate by promoting platelet aggregation. 5. Rupture of mature graafian follicle and implantation: Early embryos which produce PAF have greater chance of implanting. However, PAF is not essential for reproduction.
5. Ischaemic states of brain, heart and g.i.t., including g.i. ulceration