Eicosanoids in the Fetal and Transitional Pulmonary Circulation

SESSION

Elcosanolds In the Fetal and 'D'ansltlonal Pulmonary Circulation

PGD., PGI.) (prostacyclin), and thromboxane (fX) As via the endoperoxide intermediates, PGG1 and PGH1 • The lipoxygenase pathways proper yield a number of oxygenated products lacking the prostanoate structure, ie, hydroxy, hydroperoxy, hydroxyepoxy compounds (hydroxyepoxy acids from 12- and 15-carbon lipoxygenasesu), and the better known leukotrienes. Leukotrienes (I.:rs) are distinguished by their triene ~cture, and may or may not have a peptide moiety (peptidoleukotrienes, I..l'C4 , I.:rD. and I.:rE.). The third pathway, involving a cytochrome P-450-linked monooxygenase, is the most recent acquisition to the field and may generate several epoxides,• hydroxy acids, • and perhap~ other products as well (glutathione adduct?). 5 The possible occurrence of compounds requiring the concerted operation of monooxygenase and cyclooxygenase adds further complexity to this scheme. Two possibilities have been reported so far exemplifying the latter reaction sequence: 5,6-epoxyeicosatrienoic acid acting as a substrate for cyclooxygenase to form 5,6-epoxyprostaglandins• and PGI1 undergoing epoxidation to 5,6-oxido-PGI1• 7 Figure 1 also shows some of the drugs interfering with arachidonic acid metabolism. In this context, it must be stressed that any selective inhibitor, while inhibiting one pathwa~ may also promote arachidonic acid metabolism via unaffected pathways. An additional complication originates from the identification of thromboxane synthase with a cytochrome P-450 protein8 and the attendant possibility of imidazole and pyridine-based drugs inhibiting not only the formation of TXA.. but also the formation of monooxygenase products and possibly epoxyhydroxy acids as well. Most eicosanoids are vasoactive and may contribute to the control of vascular tone either directly or through the modulation of neural and hormonal influences. e Their relative importance, however, varies with the vascular bed, the

Flaolo Coceonl, M.D.;• and Peter M. Olley. M.D.t

prostaglandins and allied compounds (eicosanoids) occupy a central place in studies of the perinatal pulmoni!J')' circulation and have been implicated in several processes, both physiologic and pathologic. Our task was to review current concepts on this topic, directing particular attention to the outstanding or controversial issues and attempting at the same time to provide some perspective for future investigations. For sake of clarity, the subject is best introduced with a description of the compounds and their general organization in the control of vascular tone. FUNGnONAL ORGANIZATION OF THE EICOSANOID

SYSTEM

Initially limited to the sole prostaglandins (PGs), the eicosanoid system has grown in complexity and now includes a wide assortment of compounds differing in structure and function. Their common feature is the origin from arachidonic acid, hence the name eicosanoids. As indicated in Figure 1, the initial step in the metabolism of arachidonic acid involves incorporation of molecular oxygen at one or more than one site through a lipoxygenase-catalyzed reaction. Depending on the site, different compounds are formed.• The 11-carbon lipoxygenase pathway (cyclooxygenase pathway) generates the primary PGs (PGE1, PGF.,, From the •Research Institute, The Hospital lOr Sick Children, Toronto, Ontario; and the tDepartment of Pediatrics, University of Alberta, Edmonton, Alberta, Canada. Supported by grants from the Ontario Heart and Stroke Foundation and the Upjohn Comp~~ny of Canada. Hepnnt requem: Dr: Coceonl, Ho,uM fo! Sick Chfldrm, 1555 Unker~Uy Awnue,

3

lbronto, Ontario, Caniula M5G 1X8

Arachidonic Acid

!:5 }-UPOXL

~

12-Carbon 1S•Carbon

&W7MC

I

LEUKOTRIENES HYDROPEROXY/ HYDROXY PRODUCTS HYDROXYEPOXIDES

C'YCLOOX'YGENASE

IN~~+

l

PROSTAGLANDINS

PROSTAC'YCLIN THROMBOXANE

METYRAPONE

•

AOOH)

~A7 0

AAOH

5.6-EPOXY-PROSTAGLANDINS?

FIGURE 1. Pathways in the metabolism of arachidonic acid and their inhibition. Leukotrienes may be generated through 3 lipoxygenase pathways (5-, ~ and 15-carbon lipoxygenase); however, 5-carbon lipoxygenase is functionally most important BW755C: 3-amino-1-(m-(triHuoromethyl)phenyl)-2-pyrazoline.

1128

functional state, and intervening pathologic conditions. At any site, active compounds are formed within and without the vessel wall. Among the intramural eicosanoids, PGis is generally preeminent as a dilator influence, though exceptions have been noted. One such exception is the fetal ductus arteriosus whose patency is determined by PGE1 • In addition, vascular tissue has the potential for the formation of peptidoleukotrienes10 and perhaps 'fXA. as well, 11 both of which are constrictors and may override PGI1 action following insults of various nature. "Endothelium-derived relaxing factors" are a possible new addition to the eicosanoid system and may include both monooxygenase-derived epoxides11 and lipoxygenase-derived hydroperoxides. 13 Not included is the (classic) endothelium-derived relaxing factor (EDRF) induced by acetylcholine and bradykinin since it has recently been identified with nitric oxide. •• However, a product of arachidonate monooxygenase may also contribute to the latter relaxing activity15 and this remains to be verified. No information is available as yet on the function of vascular 12-carbon lipoxygenase;18 however, 12-hydroperoxy-arachidonic acid could act either by itself as a modulator of arachidonate cyclooxygenase/PGis synthase or through the conversion to an active derivative (epoxyhydroxy derivative?). Extramural eicosanoids originate from several sources, including parenchymal cells (cyclooxygenase products, leukotrienes, monooxygenase products?), invading leukocytes (cyclooxygenase products, leukotrienes) and platelets ('I'XA.). In the specific case of the lung, eicosanoid formation has been documented in interstitial cells (TXAJ, mast cells (PGDJ and, with some species (eg, sheepX in intravascular macrophages (cyclooxygenase products, leukotrienes). Blood-borne eicosanoids, reflecting the synthetic activity of a distant site, may also become important under certain conditions and could include not only relatively stable compounds such as PGE 2 , but also labile compounds ('fXAs, peptidoleukotrienes). In the latter case, plasma proteins are assigned a crucial role in protecting the eicosanoids from degradation. 17•18 Intra- and extramural sites may also operate in concert in generating vasoactive eicosanoids. For example, the possibility of platelets and polymorphonuclear leukocytes functioning as donors of, respectively, PGH1 1a and LTA.10 has been documented in vitro, and this occurrence may be important for the formation ofPGis and peptidoleukotrienes in endothelial cells. In sum, several cellular types generate vasoactive eicosanoids and their synthetic activity is conditioned by factors that may or may not be physiologic. This system of compounds is potentially very flexible and is well suited to the

regulation of vascular tone. The above data apply specifically to the adult. However, an eicosanoid-linked control mechanism is also operational in the fetus and the neonate, though it has some distinctive features reflecting the state of maturation of synthetic enzymes, the peculiar arrangement of the fetal circulation, and the normal adjustments of the transitional state at birth.•• Significantly, fetal blood vessels are endowed with enzyme(s) for the formation of peptidoleukotrienes and, in the case of the pulmonary artery, synthetic activity reportedly decreases with advancing gestation. 11 The potential for 1'XA. synthesis in lung tissue, on the other hand, increases towards term and attains a maximum around the time of birth. 13 While the former process may represent a priming condition fur postnatal dilation, the later process points to the potential impact of contractile influences on the perinatal pulmonary circulation. Despite the advances in the field, several issues remain outstanding. No information is available on the occurrence of eicosanoids in pulmonary resistance vessels, both prenatally and postnatally. Furthermore, fetal blood vessels have not been screened with regard to their ability to furm EDRFs. Conceptually, one would expect a lesser (or absent) synthesis oT EDRFs in the pulmonary vasculature compared to other vascular beds (eg. cerebral vasculature) and the systemic circulation as a whole. In fact, the distinctively low vascular resistance of the fetal compartment may signify a greater importance for EDRFs. ACI10N OF EICOSANOIDS

Eicosanoids have potent and varied effects on the perinatal pulmonary circulation (Table 1~ PGI1 is most important as a dilator, while 'fXA. and peptidoleukotrienes, specifically LTD., are preeminent among the constrictors. Leukotriene action is exerted directly on the vasculature and, unlike the adult, • it does not involve, or involves to a minor degree, a cyclooxygenase product (1'XA.) as an intermediary agent 31 PGD1 differs from other eicosanoids in producing an effect whose sign is species- and age-dependent When testing the lamb, PGD1 is a pulmonary relaxant in the fetus and young newborn, but it becomes a constrictor in the older newborn. Conversely, the pig pulmonary vasculature constricts to the compound from the immediate postnatal period onwards, both in vitro" and in vivo (unpublished data). This peculiar variation among species may have some bearing on the poor outcome of clinical trials with PGD1 •34 Quite possibly, failure of PGD1 therapy in 1- to 2-day-old infants with pulmonary hypertension-indeed, the demonstration that PGD1 may

Table 1-E./fecta q{EicosDnoida on the Perinatal Pulmonary Circulation Compound PGI1 PGE1 PGD1* PGF..

TXA.

Peptidoleukotrienest

Fetus Dilation Dilation Dilation Constriction Constriction Constriction

Reference

Newborn

Dilation Dilation Dilatio~Constriction

Constriction Constriction Constriction

Cassin;10 Lock et aJt5 Cassin;10 Lock et al.. Cassin et al;10 Lock et aJt5 Cassin;14 Lock et al.. Tyler et al17 Gause and Cassin;"" LeiBer et al;• Schreiber et al;30 Yokochi et a}3•

*Data apply to the lamb; no vasodilation is observed in the neonatal pig (see text~ tliD4 is considered most important among the leukotrienes. 30 CHEST I 93 I 3 I MARCH, 1988 I SUpplement

1138

increase rather than decrease pulmonary artery pressuremeans that the pig is a better model of the situation in humans than the lamb. Theoreticall~ the relative importance of individual eicosanoids in the prenatal vs postnatal control of pulmonary hemodynamics should become evident from tests of the precursor arachidonic acid. In fBct, however, the analysis of responses is fraught with difficulties. As shown by several investigators, 3&-37 arachidonic acid is a pulmonary constrictor in both the fetus and the newborn. Contrary to the adult, 38 no dilator response is ever observed, and the constrictor response occurs regardless of the intrinsic tone, dose, and mode of administration (bolus vs continuous infusion~ Pulmonary vasoconstriction abates upon treatment with indomethacin or the pyridine derivative, OKY-1581, indicating a role for intramural TXAs. but not the leukobienes, in the generation of tone. 31•38 However, indomethacin by itself is also a consbictor and its action conceivably results from interference with a PGI.-mediated relaxing mechanism. These inconsistencies would impl~ on one hand, that arachidonic acid undergoes a dUferent set ofcyclooxygenasecatalyzed transformations depending on whether it originates from an exogenous or an endogenous pool and, on the other hand, that the 5-carbon lipoxygenase pathway is not functional both prenatally and postnatally, notwithstanding its occurrence in blood vessels and the powerful constrictor effect of the leukobienes. The validity of either assumption will be considered in the following section while discussing current schemes of eicosanoid function. Tests with PGH1 have not been useful either in elucidating the functional organization of the pulmonary vascular cyclooyxgenase in vivo, even though this compound should be incorporated better than arachidonic acid into the endogenous pool. As shown by Cassin and associates, 40 somewhat paradoxically PGH1 is a constrictor in the ventilated fetus, when the potential for vasodilation should prevail, and a dilator in the unventilated fetus. Again, the constrictor response is reduced by pretreatment with a thromboxane synthesis inhibitor. In sum, the foregoing findings suggest that the neonatal pulmonary circulation, though relaxed by an endogenous dilator (conceivably PGIJ, may constrict in response to a cyclooxygenase product (conceivably TXAJ, and this latent constrictor influence can be easily expressed. Peptidoleukobienes are well suited for an effector role in the fetal circulation, but this possibility is not borne out by experiments with arachidonic acid. FuNcnONAL CoRRELATES OF EICOSANOID AcnoN

Knowledge of the eicosanoids has introduced new concepts on the mechanisms controlling the perinatal pulmonary circulation. In particular, eicosanoids have been assigned a key function in the maintenance of the elevated pulmonary vascular resistance in the fetus, the normal relaxation of pulmonary blood vessels at birth, and the pathogenesis of pulmonary hypertension. Evidence implicating peptidoleukotrienes in the prenatal control of pulmonary vascular tone is given in Thble 2. Collectively, these data form a good case in support of the hypothesis, even though results with the inhibitors might 1148

Table 2-Ecidence Supporting Leulwtriene InoolVftflmt in the PrenattJl Control cf Pulmonary VGICUlar 1bne Finding

Reference

Occurrence of 5-carbon lipoxygenase pathway in fetal vascular tissue Constrictor action of lJ'D4 on fetal pulmonary circulation Dilator effect of leukotriene synthesis inhibitor (compound U-60,257) and receptor antagonist (compounds FPL55712 and FPL57231) Lack of effect of thromboxane synthesis inhibitor (compound

Piper and Levene~! Gause and Cassin• Lebidois et al;•t Soifer et aJ4I

Clozel et al43

U-63,557A)

be questioned (see below~ Leaving aside the latter issue, it remains to be resolved why exogenous arachidonic acid is not metabolized via a pathway that is admittedly so important. Methodologic factors inherent to the fractional incorporation of the added precursor into the endogenous pool and the better accessibility of the (membrane-bound) cyclooxygenase compared to the (cytosolic) 5-carbon lipoxygenase could explain this apparent discrepancy. Whatever its sustaining mechanism, the contractile tension of the fetal vasculature may be overcome by the dilator influence of an eicosanoid under certain conditions. For example, the reactive pulmonary vasodilation resulting from occlusion of the ductus arteriosus is reversed by indomethacin" and, by inference, it is ascribed to PGI1 • While peptidoleukotrienes are considered important prenatall~ PGI. is assigned an effector role in the normal relaxation of the pulmonary vasculature at birth (Table 3). Significant!~ net production ofPGI1 by the lungs is detected only during the immediate postnatal periocf.a and, moreover, the magnitude of the pulmonary constrictor response to a cyclooxygenase inhibitor decreases with the age of the animal. 38 Both findings would imply the PGI1 is not essential once the pulmonary vasculature is dilated, and that other relaxing mechanism(s) become functional. Our results in the 2- to 5-week-old lamb- are consistent with this possibility. Briefly, we have found that long-term administration of indomethacin, unlike short-term administration, has no effect on baseline pulmonary tone, nor does it alter the pulmonary response to vasoactive agents and hypoxia. The Table 3-Ecidence Implicating PGI1 in the Normal Dilatation of the Pulmonary Circulation at Birth Finding Reduction in intramuralleukotriene synthesis towards term of gestation Production of PGis is stimulated by the onset of ventilation at birth Dilator effect of PGI1 on the fetal pulmonary circulation 'Iieatment with non-steroidal antiinflammatory drugs interferes with the postnatal pulmonary

vasodilatation

Reference Piper and

Levene~!

LefHer et aJe

Levin et al;411

LefHer et aJ41

identity of this compensatory mechanism is not known. However, an appealing possibility would be that, in the absence ~f a viable cyclooxygenase pathway, arachidonic acid is converted to alternative vasodilator agents via monooxygenase or lipoxygenase pathways. Such agents would complement PGI1 action and, when necessary, would also reestablish normal homeostasis. While this sequence of events requires verification, the actual role assigned to PGI. needs to be reconciled with the observed constrictor response to arachidonic acid and PGH1 in the newborn. In either case, responses imply preferential conversion of the exogenous substrate to 'fXAt rather than PGI1 • Perhaps the answer to this apparent paradox lies in the peculiar arrangement of the neonatal circulation in which latent influences from contractile eicosanoids seem prominent (see above) and may be expressed following the slightest insult In fact, it should be pointed out that arachidonic acid, like any fatty acid, acts as a detergent and the ensuing perturbation inflicted on cells could either condition subsequent transfOrmations of the exogenous compound or could promote the ex novo formation of endogenous eicosanoids. A final point to be discussed concerns the postulated role of eicosanoids in the pathogenesis of neonatal pulmonary hypertension. For sake of simplification, we may distinguish 3 forms of hypertension depending on the nature of the insult and the eicosanoid involved. Hypertension from maternal ingestion of nonsteroidal anti-inflammatory drugs has a better characterized pathogenetic sequence comprising both hemodynamic events (constriction of the ductus arteriosus and the pulmonary circulation) and structural changes in the pulmonary vasculature. 41 Limiting our analysis to the eicosanoid system, the degree of pulmonary vasoconstriction is likely to be conditioned by at least 3 factors, that is, suppression of PGI. synthesis, promotion ofleukotriene synthesis, and effectiveness of compensatory mechanism(s) that may or may not be eicosanoid-mediated. The complex interplay between these factors may weD explain the variable incidence of the disease. 48 Table 4-Evidence Supporting a Leulwtriene Role in the

Mediation of Hyporic Pulmonary Vaaoconstriction Finding

Reference

Occurrence of 5-carbon lipoxygenase pathway in vascular tissue Constrictor action of l.l'D• on neonatal pulmonary circulation

Piper and Levenell

Reversal of hypoxic response by leukotriene synthesis inhibitor (compound U-60,257) and receptor antagonist (compounds FPL55712 and FPL57231) Potentiation of hypoxic response fOllowing indomethacin treatment Lack of effect of thromboxane synthesis inhibitor (compound OKY-1581) Presence of peptidoleukotrienes in endotracheal washout of infants with persistent fetal circulation

LeiBer et al;• Schreiber et al;30 Yokochi et a}31 Soifer et al;51 Goldberg et al;SI Schreiber et aJI4

Cassin14 Tod and Cassin311 Stenmark et al•

There is also agreement on the importance ofTXAs in the genesis of pulmonary hypertension following streptococcal sepsis and endotoxemia.II0.5 1 An active debate, however, centers on the role of eicosanoids in the pulmonary constrictor response to hypoxia. According to some investigators, $1, 54 peptidoleukotrienes have an intermediary function in this process and supporting data are listed in Thble 4. Nevertheless, this concept is questioned by others111•1111 and their main criticism is directed against findings with the leukotriene antagonists, FPL 55712 and FPL 57231. FPL 55712, and possibly FPL 57231, also inhibit cyclic nucleotide phosphodiesterase57 and this action, rather than an interference with leukotriene action, may explain the reported blunting of the hypoxic response. $1,54 Indeed, the pulmonary response is often associated with generalized vasodilation and, at times, with an increase in cardiac OUtput as we0113•1111 (unpublished data~ The likelihood of an unspecific effect is further supported by our finding that a more specific and potent leukotriene antagonist, ON0-411, 58 leaves the constrictor response to hypoxia unaltered (unpublished data~ Probative results"" obtained recently with the leukotriene synthesis inhibitor, U-60,257, are also questionable because the compound is effective at exceedingly high doses and, being a PGI1 analog, may retain some of the dilator activity of the parent compound. Undoubtedly, the foregoing arguments also introduce an element of doubt in the scheme implicating leukotrienes in the sustained pulmonary tone of the fetus. CONCLUSION

Eicosanoids are formed at multiple sites in the lung and exert a wide range of effects on the perinatal pulmonary circulation. Several processes, both physiologic and pathologic, may be conditioned or modulated by an eicosanoid. The specific role played by the peptidoleukotrienes remains uncertain and needs to be re-evaluated using newly developed inhibitors of synthesis and action. Advances in this field have important implications for the prevention and management of pulmonary hypertensive disorders of the newborn. REFERENCES

1 \\blfe LS. Eicosanoids: prostaglandins, thromboxanes, leukotrienes, and other derivatives of carbon-20 unsaturated &tty acids. J Neurochem 1982; 38:1-14 2 Pace-Asciak CR. Arachidonic acid epoxides. J Bioi Chern 1984; 259:8332-37 3 ~iss RH, Arnold JL, Estabrook Rw. Thmsfurmation of an arachidonic acid hydroperoxide into epoxyhydro%y and trihydroxy fatty acids by liver microsomal cytochrome P-450. Arch Biochem Biophys 1987; 252:334-38 4 Capdevi1a J, Saeki ~ Falck JR. The mechanistic plurality of cytochrome P-450 and its biological ramifications. Xenobiotica 1984; 14:101>18 5 Spearman ME, Prough RA, Estabrook Rw, Falck JR, Manna S, Leibman KC, et al. Novel glutathione conjugates fOrmed from epoxytrienoic acids (EETs~ Arch Biochem Biophys 1985; 242: 22.>30 6 Oliw EH, Benthin G. On the metabolism of epoxyeicosatrienoic acids by ram seminal vesicles: isolation of 5(6)epoxy-prostaglandin F ~a· Biochem Biophys Res Comm 1985; 126:1090-96 7 \\bng PY-K, Malik KU. Epoxidation ofprostacyclin in the rabbit kidne)t J Bioi Chern 1985; 260:9150-S3 CHEST I 93 I 3 I MARCH, 1888 I Supplement

1158

8 Haurand M, Ullrich V. Isolation and characterization of thrombo~&ne synthase from human platelets as a cytochrome P-450 enzyme. J Bioi Chern 1985; 260:15059-67 9 W!ksler 88. Prostaglandins and vascular function. Circulation 1984; 70 Suppl3:63-71 10 Galton SA, Piper PJ. The role of blood vessels in the bioconwrsion ofluekotrienes in the pig. Br J Pharmac 1987; 90:617-22 11 Neri Serneri GG, Abbate R, Gensini GF, Panetta A, Casolo GC, Carini M. TXA. production by human arteries and wins. Prostaglandins 1983; 25:753-66 12 Pinto A, Abraham NG, Mullane KM. Cytochrome P-450dependent monooxygenase activity and endothelial-dependent relaxation induced by arachidonic acid. J Pharm Exp Ther 1986; 236:445-51 13 Uotila I! Vargas R, Wroblenska B, d'Alarcao M, Metsuda Sl! Corey EJ, et al. Relaxing effects of 15-lipoxygenase products of arachidonic acid on rat aorta. J Pharm Exp Ther 1987; 242:94549 14 Palmer RMJ, Ferrige AG, Moncada S. Nitric oxide release accounts ror the biological activity of endothelium-derived relaxing factor. Nature 1987; 327:524-26 15 \anhoutte PM. The end of the quest? Nature 1987; 327:459-60 16 Powell WS, FUnk CD. Metabolism of arachidonic acid and other polyunsaturated fatty acids by blood vessels. Progr Lipid Res 1987; 26:183-10 17 Folco G, Granstrom E , Kindahl H. Albumin stabilizes thrombou.ne At. FEBS Lett 1977; 82:321-24 18 Fitzpatrick FA, Morton DR, Wynalda MA. Albumin stabilizes leukotriene A.. J Bioi Chern 1982; 257:4680-83 19 Marcus AJ, Wlksler 88, Jaffe EA. Enzymatic conversion of prostaglandin endoperoxide H 1 and arachidonic acid to prostacyclin by cultured human endothelial cells. J Bioi Chern 1978; 253:7138-41 20 Feinmark SJ, Cannon PJ. Endothelial cellleukotriene C, synthesis results from intercellular transfer of leukotriene A. synthesized by polymorphonuclear leukocytes. J Bioi Chern 1986; 261: 16466-72 21 Coceani F, Olley PM. In: Herman AG, \anhoutte PM, Denolin H, Goossens A, eds. Cardiovascular pharmacology of the prostaglandins. New York: Raven Press, 1982;303-14 22 Piper PJ, Lewne S. Generation of leukotrienes from fetal ~d neonatal porcine blood vessels. Bioi Neonate 1986; 49:109-12 23 Printz Ml! Skidgel RA, Freedman WF. Studies of pulmonilry prostaglandin biosynthetic and catabolic enzymes as factors in ductus arteriosus patency and closure. Evidence for a shift in products with gestational age. Pediatr Res 1984; 18:19-24 24 Cassin S. Role of prostaglandins and thrombou.nes in the control of the pulmonary circulation in the fetus and newborn. Semin Perinatol1980; 4:101-07 25 Lock JE, Olley PM, Coceani F. Direct pulmonary vascular responses to prostaglandins in the conscious newborn lamb. Am J Physioll980; 238:H631-38 26 Cassin S, Tod M, Philips J, Frisinger J, Jordan J, Gibbs C. Effects of prostaglandin D 1 on perinatal circulation. Am J Physiol1981; 240:H755-60 27 '!Yler TL, LeiBer CW. Cassin S. Circulatory responses of perinatal goats to prostaglandin precursors. Prostaglandins Med 1978; 1:213-29 28 Gause G, Cassin S. Leukotrienes and the fetal pulmonary circulation. In: Southeastern symposium on pulmonary and cardiovascular effects ofeicosanoids. Southeastern Pharmacology Society Meeting. Orlando, March 13-15, 1986. 29 LeiBer CW, Mitchell JA, Green RS. Cardiovascular effects of leukotrienes in neonatal piglets. Role in hypoxic pulmonary vasoconstriction? Circ Res 1984; 55:780-87 30 Schreiber MD, Heymann MA, Soifer SJ. The differential effects of leukotriene C, and D, on the pulmonary and systemic circulations in newborn lambs. Pediatr Res 1987; 21:176-82

1188

31 Yokochi K, Olley PM, Sideris E, Hamilton F, Huhtanen D, Coceani F. In: Samuelsson B, Paoletti R, eds. Leukotrienes and other lipoxygenase products. Advances in prostaglandins, thrombou.ne and leukotriene research, vol. 9. New York: Raven Press, 1982; 211-14 32 Kadowitz PJ, Hyman AL. Analysis of responses to leukotriene 0 4 in the pulmonary vascular bed. Circ Res 1984; 55:707-17 33 Perrault 'I: Olley PM, Coceani F. The vascular effect of PGD1 and its metabolite, 9Cl, 11P-PGF1, in the neonatal pig lung. Clin Invest Med 1987; 10:C71 34 Soifer ~J, Clyman Rl, Heymann MA. Prostaglandin 0 1 does not lower pulmonary arterial pressure or improve oxygenation in infants with persistent pulmonary hypertension syndrome <(PPHN~ Pediatr Res 1985; 19:365A 35 LeiBer CW. Green RS, Jerkins RK, Horton JL, Desiderio OM, Cassin S. Arachidonic acid metabolism by neonatal lungs perfused with Krebs bicarbonate buffer. Prostaglandins Leukcr trienes Med 1984; 15:115-28 36 Redding'.GJ, McMurtry I, Reeves JT. Effects of meclofenamate on pulmonary vascular resistance correlate with postnatal age in young piglets. Pediatr Res 1984; 18:579-83 37 Tod ML, Cassin S. Perinatal pulmonary responses to arachidonic acid during normoxia and hypoXia. J Appl Physiol 1984; 57:

977-83 38 Hyman AL, Spannhake EW. Kadowitz PJ. Divergent responses to arachidonic acid in the feline pulmonary vascullri- bed. Am J Physioll980; 239:H40-6 · 39 Tod ML, Cassin S. Thrombou.ne synthase inhibition and perinatal pulmonary response to arachidonic acid. J Appl Physiol 1985; 58:710-16 40 Tod ML, Cassin S, McNamara DB, Kadovitz PJ. Effects of prostaglandin H 1 on perinatal pulmonary circulation. Pediatr Res 1986; 20:565-69 41 Lebidois J, Soifer SJ, Clyman RI, Heymann MA. Piriprost: a putative leukotriene synthesis inhibitor increases pulmonary blood flow in fetal lambs. Pediatr Res 1987; 22:350-54 42 Soifer SJ, Loitz RD, Roman C, Heymann MA. Leukotriene end organ antagonists increase pulmonary blood flow in fetal lambs. Am J Physiol1985; 249:H570-76 43 Clozel M, Clyman RI, Soifer SJ, Heymann MA. Thrombou.ne is not responsible for the high pulmonary vascular resistance in fetal lambs. Pediatr Res 1985; 19:1254-57 44 Hammerman C, Scarpelli EM. Indomethacin and the cardiopulmonary adaptations of transition. Pediatr Res 1984; 18:

842-45 45 LeiBer CW, Hessler JR, Green RS. The onset of breathing at birth stimulates pulmonary vascular prostacyclin synthesis. Pediatr Res 1984; 18:938-42 46 Levin DL, Mills LJ, Wlinberg AG. Hemodynamic, pulmonary , vascular, and myocardial abnormalities secondary to pharmaro, ) logic constriction of the fetal ductus arteriofus. Circulation 1979; 60:360-64 47 LeiBer CW, Tyler TL, Cassin S. Effect of indomethacin on pulmonary vascular response to ventilation of fetal goats. Am J Physiol1978; 234:H346-51 48 Lock JE, Olley PM, Soldin S, Coceani F. Indomethacin-induced pulmonary vasoconstriction in the conscious newborn lamb. Am J Physiol1980; 238:H639-44 49 Coceani F, Olley PM. In: Barnett HJM, Hirsch J, Mustard JF, eds. Acetylsalicylic acid: new uses for an old drug. New York: Raven Press, 1982; 109-22 50 RunJde 8, Goldberg RN, Streitfeld MM, Clark MR, Buron E, Setzer ES, et al. Cardiovascular changes in group B streptococcal sepsis in the piglet: response to indomethacin and relationship to prostacyclin and thrombou.ne At. Pediatr Res 1984; 18:874r> 78 ' 51 Schumacher WA, Adams HD, Ogletree ML. Effect of the thrombou.ne At-receptor antagonists, SQ29,548 and SQ28,668,

52

53

54

55

56

57

58

on the pulmonary hypertensive response to endotoxemia in swine. Pharmacology 1987; 34:301-08 Soifer SJ, Schreiber MD, Frantz EG, Heymann MA. Inhibition of leukotriene synthesis attenuates hypoxia-induced pulmonary vasoconstriction in newborn lambs. Pediatr Res 1986; 20:441A Goldberg RN, Sugihara C, Ahmed T, De Cudermus BD, Banios P, Setzer ES, et al. InHuence of an antagonist of slow-reacting substance of anaphylaxis on the cardiovascular manifestations of hypoxia in piglets. Pediatr Res 1985; 19:1201-05 Schreiber MD, Heymann MA, Soifer SJ. Leukotriene inhibition prevents and reverses hypoxic pulmonary vasoconstriction in newborn lambs. Pediatr Res 1985; 19:437-41 Stenmark KR, James SL, Voelkel NF, Toews WH, Reeves JT, Murphy RC. Leukotrienes c. and D 4 in neonates with hypoxemia and pulmonary hypertension. N Engl J Med 1983;. 309: 77-80 Kulik TJ, Schutjer RK, Howland DF, Lock JE. Pulmonary and systemic vascular effects of SRS-A blockade in conscious lambs. Am J Physiol1985; 249:H968-73 Chasin M, Scott C. Inhibition of cyclic nucleotide phosphodiesterase by FPL55712, an SRS-A antagonist. Biochem Pharmacal 1978; 27:2065-67 Obata T, Katsube N, Miyamoto T, Toda M, Okegawa T, Nakai H, et al. In: Hayaishi 0, Yamamoto S, eds. Advances in prostaglandin, thromboxane, and leukotriene research, vol 15. New York: Raven Press, 1985; 229-31

Leukotriene Synthesis Inhibition Increases Pulmonary Blood Flow in Fetal Lambs* M. A Heymarm, M.D.;]. LeBidms, M.D.; S.]. Soifer, M.D.; and R. I. Clyman, M.D.

W

e considered that leukotrienes may control fetal pulmonary vascular tone because infusion of a putative leukotriene receptor antagonist (FPL57231) markedly increased pulmonary blood flow and decreased pulmonary vascular resistance in near-term fetal lambs. 1 This hypothesis would be considerably strengthened if inhibition of leukotriene synthesis also produced similar hemodynamic changes. MATERIAL AND METHODS

Pregnant ewes were operated on at 133-136 days' gestation using epidural and local anesthesia. Polyvinyl catheters were inserted into peripheral fetal vessels as well as into the fetal pulmonary artery and left atrium fOllowing left lateral thoracotomy. About 4 days' recovery were allowed befOre study. The effects of ± 30 mglkg Piriprost (U 60257), a lipoxygenase inhibitor, given directly into the pulmonary artery over 5 minutes were evaluated in 9 fetal lambs, all with normal baseline blood gas values and pH. Pulmonary blood How was measured before and after Piriprost with injection of microspheres and a pulmonary arterial reference sample, • and pulmonary vascular resistance was calculated from the pulmonary arterial/left arterial mean pressure difference.

REsuus Pulmonary blood flow increased maximally by 502% be*From the Cardiovascular Research Institute and Departments of Pediatrics, Physiology, Obstetrics, Gynecology, and Reproductive Sciences, University of CalifOrnia, San Francisco.

tween 15 and 60 minutes after Piriprost injection (p<0.05, Friedman nonparametric AN OVA), and pulmonary vascular resistance fell by 87% (p<0.05). DISCUSSION

This study further investigated the role of leukotrienes in the control of pulmonary blood flow and pulmonary vascular resistance in fetal lambs. Intrapulmonary injection of U 60257, an inhibitor of leukotriene synthesis, significantly increased pulmonary blood flow and decreased pulmonary vascular resistance in late-gestation fetal lambs-changes similar in magnitude to those seen with leukotriene receptor antagonism as well as to those that occur with the normal initiation of ventilation at birth. 3 That structurally dissimilar compounds, one a leukotriene synthesis inhibitor and the other a leukotriene receptor antagonist, similarly increased pulmonary blood flow and decreased pulmonary vascular resistance, strongly suggests that leukotnenes play a role in controlling pulmonary vascular tone in the fetal lamb. REFERENCES

1 Soifer SJ, Loitz RD, Roman C, Heymann MA. Leukotriene end organ antagonists increase pulmonary blood How in fetal lambs. Am J Physiol 1985; 249:H570-76 2 Heymann MA, Payne BD, Hoffman JIE, Rudolph AM. Blood How measurements with radionuclide-labelled particles. Prog Cardiovasc Dis 1977; 20:55-79 3 Heymann MA. Control of the pulmonary circulation in the perinatal period. J Dev Physiol1984; 6:281-90

Acute and Chronic Feta' Pulmonary Hypertension Alter Pulmonary Vasoreactlvlty* Steven H. Abman, M.D.; and Frank]. Accurso, M.D.

P

ersistent pulmonary hypertension of the newbprn (PPHN) is the failure of the pulmonary circulation to adapt successfully to postnatal conditions. Associated with a wide variety of neonatal cardiopulmonary disprders, PPHN represents the inability to achieve or sustain a drop in pulmonary vascular resistance during the immediate newborn period. Previous clinical and experimental studies suggested that intrauterine events may play important roles in its pathogenesis. Supportive evidence includes: (1) the timing of onset of disease within hours of birth; (2) the severity ofhistologic findings at autopsy of infants with severe PPHN dying within the first few days, including marked smooth muscle and perivascular adventitial thickening;' and (3) animal studies suggesting that fetal stresses, such as hypoxia or hypertension, can cause smooth muscle thickening of small pulmonary arteries. a.3 Recent studies, however, failed to confirm these observations. • The actual mechanisms *From the DepartmentofPediatrics, University of Colorado J{ealth · Sciences Center, Denver. This work was supported in part by grants from the American Heart Association (SHA, AHA-Squibb Clinician-Scientist Award), the National Institutes of Health (HD-01866 and HD-00781), and the Colorado Heart Association.

Reprint requests: Dr. Abman, University of Colorado Health Sciences Center, 4200 East Ninth Avenue, Denver 80262 CHEST I 93 I 3 I MARCH, 1988 I Supplement

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