Prostaglandin I2 (prostacycin)

JOHN G. KELTON, MD, FRCP[C]; MORRIS A. BLAJCHMAN, MD, FRCP[C]

Prostaglandin 1 (PGIj), or prostacyclin, is a recently discovered prostaglandinthat affects many organ systems It Is both a potent inhibitor of plateletaggregation and a powerful vasodilator. The recent demonstration that it isthe main prostaglandin synthesized by the blood vessel wall suggests that itmay play an important role in limiting platelet-mediated thrombosis. However,despite considerable investigation, the exact physiological role of PGI,has yet to be elucidated.

La prostaglandine 12 (PGI), ou prostacycline, est une prostaglandine recemmentdecouverte qui affecte plusieurs systemes de l'organisme. C'est a la fois unpuissant agent inhibiteur de l'agregatlon plaquettaire et un agent vasodilatateurtres actif. La demonstration recente que c'est la principale prostaglandinesynthetisee par la paroi vasculaire indique qu'elle peut jouer un r6le importantdans la limitation des thromboses d'origing plaquettaire. Neanmoins, malgr6d'importantes recherches, le r6le physiologique precis de la PGI2 n'est pasencore 6lucid6.

The prostaglandins were discoveredover 40 years ago when it was notedthat seminal fluid had smooth mus-cle contracting activity. Since then,considerable progress has beenmade in understanding their bio-chemistry, metabolic pathways andphysiological roles in the varioustissues of the body.1 The prostaglan-dins are a family of 20-carbon un-saturated fatty acids that can be syn-thesized from a number of essentialfatty acids. In the body the pre-dominant precursor is arachidonicacid, an integral component of cellmembranes.2'3 While the function ofmany prostaglandins is only partlyunderstood, a number of pharma-cologic agents exert their action byinfluencing prostaglandin synthesis.

In 1976 a new prostaglandin, in-itially called prostaglandin X andnow known as prostaglandin L(PGL), or prostacyclin, was de-scribed.4 This prostaglandin, whichis synthesized by the blood vesselwall, is a potent inhibitor of plateletaggregation and a powerful vaso-dilator."6 Because of the uniqueproperties of PG12 it has been the

From the departments of pathologyand medicine, McMaster UniversityMedical Centre and the Canadian RedCross Blood Transfusion Service,Hamilton

Reprint requests to: Dr. Morris A.Blajchman, McMaster UniversityMedical Centre, Rm. 2N33, 1200 MainSt. W, Hamilton, Ont. L8S 4J9

subject of intense investigation. PGLhas a very short half-life in vivo andmay be thought of as a hormonewith a "short-loop function", withits site of action close to its site ofsynthesis.7 It thus differs from themore traditional hormones, whichare synthesized in one organ andaffect tissues remote from that site.As yet the exact physiological role

of PG12 has not been elucidated. Inthis review we will describe the dis-covery and characteristics of PGI2,and consider some current hypo-theses concerning its potential rolein health and disease.

Discovery and characterizationof PGI2

A number of mechanisms existin vivo to limit thrombus formation.Platelets do not stick to undamagedendothelial cells but adhere readilyto the vascular subendothelium.8'9This inhibitory property of the en-dothelial cell is due in part to theelectrostatic repulsion between thenegatively charged platelet and thenegatively charged endothelial cellsurface.10'11 In 1970 Ts'ao12 notedthat the addition of blood vesselsegments to platelet-rich plasma didnot induce aggregation, whereas theaddition of collagen from these ves-sels caused aggregation. Heyns andcolleagues11 suggested that endo-thelial cells could inhibit plateletaggregation as a result of an ade-nosine diphosphatase produiced bythese cells. In 1976 a group from

the Wellcome Research Labora-tories, Beckenham, England, headedby Vane, isolated a prostaglandinthat was shown to be a potent in-hibitor of platelet aggregation, andcalled it prostaglandin X4 It hasbeen given the trivial name prosta-cyclin because it has a second ring inits structure (Fig. 1). It has been de-signated PG12 because it was thenext prostaglandin in alphabeticalorder after prostaglandin H to becharacterized, and the subscript 2indicates that it is derived from fattyacids with two unsaturated bonds."4The substrates for its synthesis arethe endoperoxides, prostaglandin G2(PGG2) and prostaglandin H(PGH2).PGL is a potent inhibitor of plate-

let aggregation;""17 however, it isless potent as an inhibitor of plateletadhesion."8 It has a half-life at 37°Cand a pH of 7.4 of approximately 2minutes and its activity is gone in 20minutes at 22°C.4'16 It is more stableat an alkaline pH, and particularlyso at a pH greater than 8.4.19 It isa potent vasodilator, relaxing rabbitmesenteric and celiac arteries butnot rabbit aorta, pulmonary arteryor vena cava.4'6 It causes contrac-tion of rat stomach strip, chick rec-tum, guinea pig trachea and guineapig ileum.4"'6 Its synthesis from pros-taglandin precursors is catalyzed byPG12 synthetase, which is inhibitedby tranylcypromine and by 15-hy-droxyarachidonic acid.'6'" It is hy-drolyzed to a stable end product,6-ketoprostaglandin-Fi., and is pos-sibly oxidized to 1 5-keto-PG12.'sThe proposed synthetic pathway forPG12 is shown in Fig. 2.

Site of synthesis and sourceof PG12

Endothelial cells, smooth musclecells and fibroblasts have the poten-tial to synthesize PGI2.8 OriginallyVane and colleagues4 postulated thatthe substrates of PGI2 synthesis(PGG2 and PGH2) were providedby platelets. However, recent studieshave demonstrated that both endo-

CMA JOURNAL/JANUARY 26, 1980/VOL. 122 175

PROSTAGLANDIN 12

O /-/\ COOH

I \

HO OH

INHIBITION OFPLATELET AGGREGATION

VASODILATION

ARACHIDONICACID

THROMBOXANE A2

'0 /\ COOH

HO

PLATELET AGGREGATIONVASOCONSTRICTION

FIG. 1-Chemical structures of arachidonic acid, prostaglandin I (PGIL)and thromboxane A2.

Phospholipase (active)

Hydrocortisone |

Membrane Phospholipid - Arachidonic Acid

Arachidonic Acid

Indomethacin m| Cyco- oxygenaseAspirin

Cyclic Endoperoxides

Tranylcypromine- Prostaglandin 12

SynthetaseProstaglandin 12

6-ketoprostaglandin - Fia

FIG. 2-Proposed synthetic pathway for PGI2. Drugs known to inhibit variousenzymes in pathway are indicated on left.

thelial cells and subendothelial cells(smooth muscle cells and fibro-blasts) have the endoperoxide pre-

cursors required for de novo syn-

thesis of PG12.2' Furthermore, PGI2production is normal in thrombocy-topenic animals,30 and one investi-gator has demonstrated that plateletsdo not release prostaglandin endo-peroxides unless thromboxane syn-

thetase is inhibited.3"In vitro studies have demon-

strated that endothelial cell synthesisof PG12 is stimulated by physicalcontact, thrombin, trypsin and ara-chidonic acid but not by adenosinediphosphate or epinephrine.32 Thus,cells of the blood vessel wall areable to synthesize PGIL de novo,although it is possible that plateletsmay augment the synthesis by pro-viding additional substrate.

PGI and the platelet

PGI2 is the most powerful naturalinhibitor of platelet aggregationknown, and the inhibition is diffe-rent in various animals but indepen-dent of the aggregating agent.15 Theinhibition is mediated by cyclic ade-nosine monophosphate (AMP).PGIL stimulates the synthesis ofplatelet cyclic AMP by activatingplatelet adenyl cyclase.33 On a molarbasis PGI is 10 times more activethan PGD2, 30 times more activethan PGE1 and 1000 times more

active than 6-keto-PGF,a in stimul-ating the production of cyclicAMP.'3," The effect of PGI onplatelets is augmented by agents thatinhibit cyclic AMP degradation.'3-

Considerable controversy existsconcerning whether PGL2 is a cir-culating hormone. Recently Gry-glewski, Korbut and Ocetkiewicz3'and Moncada and associates40 havepresented evidence suggesting thatPGO2 is continually released by thelungs into the arterial circulation andthus continually affects plateletfunction. However, contrary evi-dence exists, in that if PGO2 were

circulating, then the platelet cyclicnucleotide concentration would beexpected to be raised, which is notthe case.41 The issue of whetherPGO2 is a circulating hormone thusremains to be resolved, and its re-

solution may clarify the physiolo-gical function of PGO2.

176 CMA JOURNAL/JANUARY 26, 1980/VOL. 122

KDKL$< COOH

PROSTAGLANDIN ENDOPEROXIDESPG:G2 PG:H2

Potential role of PGI2 in limitingthrombosis and in the pathogenesisof atherosclerosis

The synthesis of PGIL by theblood vessel wall may be importantin limiting platelet-augmentedthrombosis.42 We have demonstratedin rabbits that the inhibition of theproduction of PG12 by the vesselwall with large doses of acetylsalicy-lic acid, hydrocortisone or tranyl-cypromine augments thrombus for-mation.43" Other investigators havedemonstrated that the formation ofplatelet thrombi in arterioles andvenules is reduced by PGI2 infu-sion," whereas the inhibition ofPGIL by tranylcypromine results inenhanced formation of plateletthrombi in the cerebral microcircula-tion of the mouse."

These experimental observationssuggest the following physiologicalrole for PGL: Endothelial celldamage results in exposure of thesubendothelial layers, which leadsto platelet adhesion. Although PG12can limit platelet adhesion, theamount necessary is very high;therefore, it is less likely to be animportant factor in the control ofadhesion.'"47 However, once adhe-sion has occurred, subsequentplatelet aggregation to adherent pla-telets may be inhibited by physiol-ogic concentrations of PGI2. Thesubstrates for PGI2 synthesis maybe contributed to by prostaglandinintermediates released from the ad-herent platelets,' but it is likely thatthe synthesis of PGL2 proceeds with-out the need for substrate fromplatelets.A role for PGI2 in the develop-

ment of atherosclerosis has alsobeen suggested.48'" Lipid peroxidesthat are present in atheroscleroticplaques have been demonstrated tobe potent inhibitors of PGIL synthe-tase.49 Other risk factors for athero-sclerosis, such as hypercholesterole-mia and hypertension, produceendothelial cell damage, but becauseof the decreased amount of PGI2synthesized the vessel wall is unableto limit the resultant platelet aggre-gation. The platelet thrombi in turnrelease mitogenic factors that stimul-ate smooth muscle proliferation inthe vessel wall and potentiate thedeveloping atherosclerotic plaque.

This hypothesis is supported by thedemonstration that atheroscleroticvessels in animals synthesize re-duced amounts of PGI2." However,some investigators have questionedthis hypothesis," and therefore thepotential role of PGI2 in the patho-genesis of atherosclerosis remainsunresolved.

PGI and other organ systems

PGI may influence many organsystems in addition to hemostasisand thrombosis.

The kidney

Prostaglandins may play a rolein the regulation of renal vascularresistance, electrolyte excretion andplasma renin synthesis and release.PG12 is produced by the humanrenal cortex,51 and rabbit kidneyssynthesize PG12 from both endo-genous and exogenous arachidonicacid.52

Although the kidney can synthe-size PGL, it preferentially synthe-sizes thromboxane A2 (a potentvasoconstrictor and inducer ofplatelet aggregation) following ure-teric obstruction.52 Thus, PG12 mayact as the vasodilatory balance tothe vasoconstriction produced bythe kidney's thromboxane A2.

The cardiovascular system

PGL may regulate coronary bloodflow and may also limit systemichypertension. The isolated rabbitheart can synthesize PGI2 from en-dogenous or exogenous arachidonicacid;52 however, it is unlikely thatendogenous cardiac PG12 is impor-tant in limiting coronary platelet ag-gregation because of the low levelproduced plus the dilutional effectof the rapid flow.

Both PGE2 and PG12 may play arole as blood pressure regulators.Indeed, PGI2 may be the prostaglan-din of prime importance since it ismore potent than PGE and, unlikePGE2, is not completely inactivatedin the pulmonary circulation.5"5' Theinfusion of PG12 into hypertensiveanimals normalizes the blood pres-sure.' Thus, hypothetically, a rolefor PGI2 as a blood pressure reg-ulator may exist and act as follows:Hypertension may cause endothelialcell damage, which may lead to in-

creased synthesis of PGL. Thiswould result in a lowering of theblood pressure due to the vaso-dilatory action of PG12.The reproductive tract

PGI2 and its breakdown product,6-keto-PGF.a, are produced by theuterus, the placental vessels and theseminal vesicles. There is evidencethat PGI2 plays a role in uterinephysiology and may be a regulatorof fetal-placental blood flow.55 PGI2elicits a biphasic contractile responsefrom the human uterus - initialcontraction followed by relaxation.Thus, it may function as a balanceto the intrinsic myometrial stimulat-ing activity of other prostaglandins,particularly PGFa.."

Perfusion of the umbilical arteryof a human with PGIL induces avariable response that depends onthe concentration. Relaxation is ob-served at low concentrations, andcontraction is observed at higherconcentrations." The lack of sym-pathetic innervation- of the umbilicalvasculature suggests that humoralagents may be important in reg-ulating blood flow. The demonstra-tion that PG12 is the main fetal en-doperoxide metabolite suggests thatPGI2 may be an important regulatorof fetal blood flow.58

Inflammation

Prostaglandins of the E series(PGE1 and PGE) are released dur-ing inflammation and enhance thereaction by increasing vascular per-meability. PGI2 also potentiates theinflammatory response but, unlikePGE,, does not alter vascular per-meability but potentiates the vas-cular permeability induced by hista-mine, serotonin or adenosine di-phosphate."9Gastric acid output

PGL2 increases the gastric muco-sal blood flow through vasodilationand inhibits gastric secretion." Italso inhibits the formation of indo-methacin-induced gastric erosions.6"

Drugs and PGI2 synthesis

Drugs that inhibit the prostaglan-din pathway (Fig. 2) and block PGI2production have the potential toalter the physiological effects of

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PGLs. For example, hydrocortisoneinfused acutely into rabbits in verylarge doses or in lower doses forlonger periods, or applied topicallydirectly to the vessel wall, inhibitsPG12 synthesis and results in ashortening of the bleeding time inthrombocytopenic animals.61 Thiswas shown to be due to inhibitionof the normal vasodilatory effect ofPG12, which resulted in maintainedvasoconstriction following injury tothe vessel.6" It has also been demon-strated that massive doses of acetyl-salicylic acid inhibit PGL synthesisin the blood vessel in vivo and aug-ment the size of experimentally in-duced thrombi.6'The question then must be asked,

Why do patients given prostaglandininhibitors such as acetylsalicylic acidor corticosteroids not have throm-botic events? There are at least twoanswers. First, very large doses ofthe inhibiting agent are required tototally inhibit PG12 synthesis; how-ever, very low levels of PGI2 areadequate to limit thrombosis." Sec-ond, the inhibitory effect of acetyl-salicylic acid upon PGL synthesisis short-lived, ahnd the vessel wall israpidly able to produce more PG12.".In contrast, this drug irreversibly in-hibits prostaglandin formation in theplatelet for the life of that cell, andtherefore the effect on the platelet islonger lasting.

These differences between theability of the endothelial cell andthe platelet to regenerate prostaglan-dins may be due to the absence ofthe nucleus in the platelet and itsconsequent inability to synthesizenew cyclo-oxygenase enzyme to re-place that inhibited by the drug.Thus, the results of experiments inanimals are not necessarily in con-flict with the lack of thrombogenicityobserved in humans given drugs thatinhibit prostaglandin synthesis.

Recently the experimental drugBay g 6575 has been demonstratedto stimulate PGI2 release from theblood vessel wall.3 This action mayaccount for the antithrombotic ef-fects noted with this drug, which hastherefore been proposed for assess-ment in humans as an antithrombo-tic agent.3 In addition, the pharma-ceutical industry is extensivelysearching for PGL2 analogues thatretain the physiological function of

the molecule, yet are stable whenadministered in vivo."'"

Because of the potent vasodila-tory activity of PGI2 and its anal-ogues, PGI2 has been suggested forclinical use in a wide variety of dis-orders, including myocardial in-farction, angina, vascular occlusivediseases and hypertension, and dur-ing extracorporeal circulation."'PG12 has recently been used suc-cessfully in the treatment of patientswith advanced arteriosclerosis obli-terans."7

Conclusion

PGL, a recently discovered potentprostaglandin, may play a funda-mental physiological role in a num-ber of organ systems in addition tohemostasis and thrombosis. Its de-pletion by disease and drugs mayalso be important in a number ofdisorders. However, as yet the exactphysiological function of PGI2 isunclear.

We thank Drs. J. Hirsh and M.C.Brain for their helpful criticism ofthis manuscript, and Ms. J. Robertsonfor her expert secretarial assistance.Our research was supported by

grants from the Canadian Heart As-sociation and the Medical ResearchCouncil of Canada.

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