- Email: [email protected]
Some investigations into the role of prostacyclin in thromboregulation
(Received
6.9.1977; Accepted
by
in revised form 19.i2.1977. Editor J.J. Sixma)
ISTRODCCTIOX The interesting finding that blood vessel microsomes are able to convert prostaglandin endoperoxides into an extremely potent anti-aggregating substance, prostacyclin (PGI2, Ref. l-6), may provide an explanation of the unique anti-thrombotic properties of intact vascular endothelium, especially since it was observed that endothelial cells in culture also produce As one of the physiological implications prostacyclin from endoperoxides (7). of this finding, it has immediately been assumed that a mural platelet thrombus only forms at places of functionally damaged or disrupted endothelium, because at that particular site there will be no PCI2-generation to prevent thronbogenesis (3,4,8). However, this theory can be accepted only after it has been shown that vascular PGI2-synthesis is mainly located in viable endothelium and occurs in sub-endothelial tissue to only a negligible extent. We wish to present some evidence that this may not be true. Moreover, we investigated the physiological relevance of PC12 in thromboregulation using rats with different arterial thrombosis tendencies.
XETHODS The production of PGL2-like material was determined as described by Buntin:: et al. (4) with slight modifications. A small piece of tissue, diameter 3 mn, dry weight approximately 100 pg, was punched out of the aorta of rats or rabbits and incubated in 200 ~1 0.02 X TRIS buffer at pH 7 .? and rooz temperature. After various incubation times 50 ~1 of the incubation fluid was transferred into 1 ml titrated platelet rich plasma of normal rats, diluted with saline to a platelet concentration of about 180,00O/al. Aggregation was triggered 20 s later with ADP (final concentration 0.3 x 20 s after administrati>n 10-6 mol/l). Control measurements were performed
367
of (2)
3 of
“1 05 frcsl? .ADP-induced
315 ‘3,ffer. aggregation.
T’ne effect of endothelial iatS, in a rabbit aorta acids (EFA) . EPA-deficient Incubations tendency (9). vascular endothelium by washing.
PGI?-production -
xas
e:xpressed
as
ichi’hiti-n
disruption was investigated in aortas of normal deficient in essential fatty and in aortas of rats, rats have a decreased arterial thrombosis were carried out before and after disruption of carefully rubbing with sand paper and subsequent
In a separate experiment PGI?-formation was measured in rats diets containing 35 calX of sither sunflowerseed oil (SO) or These diets cause a low and coconut oil (HCO) for 2-3 weeks. In this experiment the thrombosis tendency respectively (9). PGI2 generated cas expressed as equivalents of prostaglandin
fed adequate hydrogenated high arterial activity of El-activity.
the
Upon incubation of arterial tissue in TRIS buffer, a substance if formed which inhibits ADP-induced aggregation. The amounts produced depend on the incubation time and the substance is rather labile when kept at room temperature (Fig. 1). Heating at 80°C for a few minutes destroys the aggregation inhibiting potency (not shown). Incubation of aortic tissue with indomethacin (1 ;ig/ml) prevents the whereas subsequent addition formation OE this substance almost completely, of prostaglandin H7 restores the formation (Fig. 2). These findings confirm the results of Bunting et al. (4) and Honcada et al. (5) and indicate that rat aortic tissue can release a prostacyclin-like substance. Disruption of endothelium resulted in an increased and rabbit aorta.
by rubbing release of
with sand paper prostacyclin-like
(Table 1 and Fig. 3) material from rat
TABLE 1 Effect of Disruption of Aortic paper on Vascular prostacyclin as Inhibition (X -+ s.e.m.) of
I
Type
of
Normal
rats
Endothelium production, ADP-induced
(n = 3)
with Sand measured Aggregation
I EFA-deficient
(n = 3)
Endothelium
98 -+ 1.2 11 min
90
+ 4.1
I
15 +
3.7
44 -+ 4.4
Tisatasy
CPGI?)
formatian
j;;
Diets containing 35 caiK SurliLovar3eed :il (.SO, or Hydrogenated Coconut oil (HCil) for 2-3 x~eks. Activity oi PG12 is expressed in equi~;a?cnts 2: 11~ PCEi activity generated per min (n = 8, mea:. A s.e.z.!. For all experiments P3 .> 0.10 fed
PROSTACYCLIS:
I W’
vo1.12,xo.2
THROYB~REGU.ATIO~
?F
alla
4
3
’
tims hn,
FIG.?
.\: 50 :!I
incubation
incubation
fluid
(TRIS
buffer);
after
9 nin
with
I. no dOrCd 2. 1.
nondl aurtd
aorta pretreated
4.
pretreated
5.
no
aorta,
with
aorta only
indomethasin
+ PCH, PCHL
.3 Effect
of
released TRIS A:
endochelial upon
incubation
TRIS
buffer
disruption of
on
aggregation
inhibition
(left)
or
(rigbc)
rabbit
buffer
50
~1
I : no aorta 2 : nonnina arJrt.a 3
8:
rat
ADP.
fins1
concentration
:
aorta 0.3
vith x
disrupted IO -6
mol/l
endothelium
caused aorta
for
by
prostacyclin
4 min
in
light transmission
IA I 0
1 1
f 2
I 3
I 4
I 5
1 6
I 7
, f 8 9 time (min)
FCC.1
I. reference aorta, reference platelets. 2. EFA drficisnt aorta, EFA-deficient platel?cr.
DISCUSSIOX
Itat aorta, when incubated in TRIS buffer, produces a substance which inhibits platelet aggregation very effectively. This substance is rather labile and since its production is inhibited by indomethacin i;hi?e the endoperoxide PGH2 acts as a precursor, it is likely to be prostacyclin (PGI2). However, confirmation by other means is required. :tZter endothelial disruption the aggregation inhibiting vascular incubate had increased considerably. This is an increased prostacyclin content since its occurrence upon pre-Incubation with indomethacin of the vascular of the endothelium. Ftoreover the inactivation at room creasing storage time parallels that of PGIz released
potency of ihe most likely due tg> is completely urevented tissue before d’isruptiJn temperature with infrom non-damaged aor:a.
It is well known that mechanical activation of tissues may enhance their prostaglandin production and we convincingly shoT;ed this to apply also to vascular PGI2-formation (unpublished results). Since scanning electron the rubbing procedure did not completely remove the microscopy showed that endothelial layer, the increased PGI3-formation observed after this treatment
may partly be due to activ;acion of remaining endothelia: :ells. HoTever, this finding alss strongly suggests the ability of snbend3thelium to generate PC17 from endogenous precursors. This should, therefore, be investigated further,
using
Recently it was to convert PGH2 culture do (7), capacity may be
less
trajumstic
procedures.
shown that medial smooth muscle cells in culture are into prostacyclin as effectively as endotheiial cells Therefore the suggested subendotbelial ?GI2-producing activity . due to medial smooth muscle cell
The increased PGI2-production on endothelial damage makes it hard understand why endotbelial removal by ballooning (also a traumatic ment) leads to the immediate formation of mural thrombi (I!).
able in
to treat-
It is thought that the platelet release reaction, which is essential for is mediated by formation of endothe formation of a platelet thrombus, the platelets (12). In peroxides (EP) and thronbosane A2 (TxA.2) within EFA-deficient rats the formation of platelet EP and TxA2 is very low. However, after adequate platelet activation the release reaction - and consequently platelet aggregation - is almost normal, which indicates the existence of a thrombosis mediating system different from the AA-EP-TxA 2 In EFA-deficiency the balance between prostacyclin pathway (10,13,14). formation and the platelet release reaction will, therefore, be very much in favour of the latter. If this balance - as has been suggested (1,2,3) indeed determines thrombosis tendency, this situation will result in a prothrombotic state. This is in line with our finding that a piece of EFA-deficient aorta, brought into contact with a suspension of EFA-deficient of control aorta in platelets induces much more aggregation than 3 piece a suspension of control platelets (Fig. 4). ?Ioreover, similar differences between EFA-deficient and control aortas were observed after “crossing over” the platelet suspensions whereas no differences occurred after pre-treatment of the aortas with indomethacin. Although all these in vitro findings point to an increased arterial thrombosis tendency in EFA-deficiency, in vivo experiments invariably show this tendency to be considerably depressed (9): This is not in line with a major role of vascular PGI2 in chromboregulation. The same conclusion can be drawn from the experiment with rats fed either SO of HCO. Although these animals have different thrombosis tendendies (9) their aortic PGI2-generation is essentially similar. The present findings indicate that more experimental evidence is required before an ultimate thromboregulatory role of the EP-TxAz-PGIZ-tri3d can be accepted. For initial studies, the rat may be attractive, since in this animal species arterial thrombogenesis can conveniently be quantified (15). Moreover, the PGI -synthesizing capacity of its aorta can easily be measured and seems to 8 e higher than that of any other arterial ti,ssue described so far. However, any finding obtained with rats has to be confirmed in other species. Note After this short communication was submitted, Moncada and coworkers published a study in which they showed that more than 60% of the vascular prostacyclin synthetase activity is located in the non-endothelial part of the blood vessel (16). Although their results differ from ours quantitatively - which is most probably due to methodological differences - they nevertheless confirm the reality of substantial subendothelial prostacyclin formation which seriously complicates the evaluation of the thromboregulatory role of the vessel wall.
< - ; .R . An enzyme BCYcI:x, s. and l"~;;l;r, isolated from arteries transforms prastaglandin endsperoxides to an unstable substance that inhibits platelet.aggregatica. Xature 263,633, 1975.
. “OSCrnA,
2.
s . , GF.YGL%'SKI R J ,..,I
G?.YGLZWSKI, R.J., BCYTING, S., XO?iCADA, S., FLO:ZR, R.J. and l"'AxE, J.R. thrO&Di
Arterial walls are protected
agslns:
denojition of platelet
5:; a substance (prostaglandin X) which they make from prostaglandin
endoperoxidcs. Prostaglandins 12,583, 1975. XOSCXDA, S., GRYGLEWSKI, R.J., BLD?TIXG, S. and VUE, J.R. .\ lipid peroxide inhibits the enzyme in blood vessel nicrosomes that generates from prostaglandin endoperoxides the substance (prostaglandin K) which , prevents platelet aggregation. Prostaglandins ll,7ij, 1970. :.
BlJS'II?;G, S., GRYGLE;;SKI, ii.J., >:OSi:XDA, S. and %'%E, .J.R. Arterial wallS generate from prostaglandin endoperosides a substance (nrostaglnndin I<) which relases strips of mesenteric and coeliac arteries and in!,iibits platelet aggregation. Prostaglandins I?, S97, !976.
3.
?IONC.kDX,S., HIGGS, E.A. and VAX, J.R. Human arteriai and venous tissues generate prostacyclin (prostaglandin Xi, a potent inhibitor of platelet aggregation. Lancet i, 18, 1977.
h.
Editorial: Nomenclature Announcement. Prostac~cl~h (PGI,). Prostaglandins 13, 375, 1977.
7.
DUTILH, C.E., C'ENDE~L~S-STZ~~B~RC, A. and TE?; ROOR, F. production by cells in culture. In preparation.
s.
HORTON, E.W.
Molecular insight into thrombosis. Sature 263, 637, 1976.
9.
HORXSTRA, G. 1973, 1974.
Dietary fats and arterial thrombosis. Haemostasis 2, 21,
Prostacyclin
10.
HORXSTRA, G. and HADDEXAN, E. Effects of dietary fats on the role of platelets in arterial thromboembolism. In: D.C.E. !lills and F.I. Pareti (Eds): Platelets and Thronbosis, Academic Press, London, New York, San Francisco (in press).
II.
BALJNGARTYER, H.R. The subendothelial surEace and thrombosis. Thromb. Diath. Haemorrh. Suppl. 59, 91, 1974.
I?-.
HAYBERG, >I., SVENSSOS, ;. and SAXUELSSOX, B. Thronboxanes: 3 new group of biologically active $ompounds derived from prostaglandin endoperoxides. Proc. Xat. Acad. Sci. USA 72, 2994, 1975.
I?. BULT, H. and BOSTA, I.L. Rat platelets aggregate in the absence of endogenous precursors of prostaglandin endoperoxides. Nature 264, 449, 1976. 1;. HORXSTRA, G. and HADDENX'i,E. Arachidonic acid metabolites not crucial for thro~boregu~ation? Thromb. Haemostas. 38, 19, 1977.
FROSTACYCLIN
37;
15.
!iO?XST%1, G . and arterial occlusive
:
THRO~~BOREGt-LATIOS
Vo1.12,No.2
‘.~.UDE~~~~_‘;S-ST~~_~~g~RG, A. Indluctlon of zqerizextal thro&i in rats. Atheroscle-osis I Ii, 369, 1?73.
16 . XOXCADA, S., HENAS, A.G., HICGS, E.A. and VtiE, J.R. Differential fomation of prostacyclin (PGX or PGI?) by layers of the arterial wall. An explanation I for the anti-throabotic properties of vascular endothelium. Thronb. Res. 323, 197i.
I,
6.9.1977; Accepted
by
in revised form 19.i2.1977. Editor J.J. Sixma)
ISTRODCCTIOX The interesting finding that blood vessel microsomes are able to convert prostaglandin endoperoxides into an extremely potent anti-aggregating substance, prostacyclin (PGI2, Ref. l-6), may provide an explanation of the unique anti-thrombotic properties of intact vascular endothelium, especially since it was observed that endothelial cells in culture also produce As one of the physiological implications prostacyclin from endoperoxides (7). of this finding, it has immediately been assumed that a mural platelet thrombus only forms at places of functionally damaged or disrupted endothelium, because at that particular site there will be no PCI2-generation to prevent thronbogenesis (3,4,8). However, this theory can be accepted only after it has been shown that vascular PGI2-synthesis is mainly located in viable endothelium and occurs in sub-endothelial tissue to only a negligible extent. We wish to present some evidence that this may not be true. Moreover, we investigated the physiological relevance of PC12 in thromboregulation using rats with different arterial thrombosis tendencies.
XETHODS The production of PGL2-like material was determined as described by Buntin:: et al. (4) with slight modifications. A small piece of tissue, diameter 3 mn, dry weight approximately 100 pg, was punched out of the aorta of rats or rabbits and incubated in 200 ~1 0.02 X TRIS buffer at pH 7 .? and rooz temperature. After various incubation times 50 ~1 of the incubation fluid was transferred into 1 ml titrated platelet rich plasma of normal rats, diluted with saline to a platelet concentration of about 180,00O/al. Aggregation was triggered 20 s later with ADP (final concentration 0.3 x 20 s after administrati>n 10-6 mol/l). Control measurements were performed
367
of (2)
3 of
“1 05 frcsl? .ADP-induced
315 ‘3,ffer. aggregation.
T’ne effect of endothelial iatS, in a rabbit aorta acids (EFA) . EPA-deficient Incubations tendency (9). vascular endothelium by washing.
PGI?-production -
xas
e:xpressed
as
ichi’hiti-n
disruption was investigated in aortas of normal deficient in essential fatty and in aortas of rats, rats have a decreased arterial thrombosis were carried out before and after disruption of carefully rubbing with sand paper and subsequent
In a separate experiment PGI?-formation was measured in rats diets containing 35 calX of sither sunflowerseed oil (SO) or These diets cause a low and coconut oil (HCO) for 2-3 weeks. In this experiment the thrombosis tendency respectively (9). PGI2 generated cas expressed as equivalents of prostaglandin
fed adequate hydrogenated high arterial activity of El-activity.
the
Upon incubation of arterial tissue in TRIS buffer, a substance if formed which inhibits ADP-induced aggregation. The amounts produced depend on the incubation time and the substance is rather labile when kept at room temperature (Fig. 1). Heating at 80°C for a few minutes destroys the aggregation inhibiting potency (not shown). Incubation of aortic tissue with indomethacin (1 ;ig/ml) prevents the whereas subsequent addition formation OE this substance almost completely, of prostaglandin H7 restores the formation (Fig. 2). These findings confirm the results of Bunting et al. (4) and Honcada et al. (5) and indicate that rat aortic tissue can release a prostacyclin-like substance. Disruption of endothelium resulted in an increased and rabbit aorta.
by rubbing release of
with sand paper prostacyclin-like
(Table 1 and Fig. 3) material from rat
TABLE 1 Effect of Disruption of Aortic paper on Vascular prostacyclin as Inhibition (X -+ s.e.m.) of
I
Type
of
Normal
rats
Endothelium production, ADP-induced
(n = 3)
with Sand measured Aggregation
I EFA-deficient
(n = 3)
Endothelium
98 -+ 1.2 11 min
90
+ 4.1
I
15 +
3.7
44 -+ 4.4
Tisatasy
CPGI?)
formatian
j;;
Diets containing 35 caiK SurliLovar3eed :il (.SO, or Hydrogenated Coconut oil (HCil) for 2-3 x~eks. Activity oi PG12 is expressed in equi~;a?cnts 2: 11~ PCEi activity generated per min (n = 8, mea:. A s.e.z.!. For all experiments P3 .> 0.10 fed
PROSTACYCLIS:
I W’
vo1.12,xo.2
THROYB~REGU.ATIO~
?F
alla
4
3
’
tims hn,
FIG.?
.\: 50 :!I
incubation
incubation
fluid
(TRIS
buffer);
after
9 nin
with
I. no dOrCd 2. 1.
nondl aurtd
aorta pretreated
4.
pretreated
5.
no
aorta,
with
aorta only
indomethasin
+ PCH, PCHL
.3 Effect
of
released TRIS A:
endochelial upon
incubation
TRIS
buffer
disruption of
on
aggregation
inhibition
(left)
or
(rigbc)
rabbit
buffer
50
~1
I : no aorta 2 : nonnina arJrt.a 3
8:
rat
ADP.
fins1
concentration
:
aorta 0.3
vith x
disrupted IO -6
mol/l
endothelium
caused aorta
for
by
prostacyclin
4 min
in
light transmission
IA I 0
1 1
f 2
I 3
I 4
I 5
1 6
I 7
, f 8 9 time (min)
FCC.1
I. reference aorta, reference platelets. 2. EFA drficisnt aorta, EFA-deficient platel?cr.
DISCUSSIOX
Itat aorta, when incubated in TRIS buffer, produces a substance which inhibits platelet aggregation very effectively. This substance is rather labile and since its production is inhibited by indomethacin i;hi?e the endoperoxide PGH2 acts as a precursor, it is likely to be prostacyclin (PGI2). However, confirmation by other means is required. :tZter endothelial disruption the aggregation inhibiting vascular incubate had increased considerably. This is an increased prostacyclin content since its occurrence upon pre-Incubation with indomethacin of the vascular of the endothelium. Ftoreover the inactivation at room creasing storage time parallels that of PGIz released
potency of ihe most likely due tg> is completely urevented tissue before d’isruptiJn temperature with infrom non-damaged aor:a.
It is well known that mechanical activation of tissues may enhance their prostaglandin production and we convincingly shoT;ed this to apply also to vascular PGI2-formation (unpublished results). Since scanning electron the rubbing procedure did not completely remove the microscopy showed that endothelial layer, the increased PGI3-formation observed after this treatment
may partly be due to activ;acion of remaining endothelia: :ells. HoTever, this finding alss strongly suggests the ability of snbend3thelium to generate PC17 from endogenous precursors. This should, therefore, be investigated further,
using
Recently it was to convert PGH2 culture do (7), capacity may be
less
trajumstic
procedures.
shown that medial smooth muscle cells in culture are into prostacyclin as effectively as endotheiial cells Therefore the suggested subendotbelial ?GI2-producing activity . due to medial smooth muscle cell
The increased PGI2-production on endothelial damage makes it hard understand why endotbelial removal by ballooning (also a traumatic ment) leads to the immediate formation of mural thrombi (I!).
able in
to treat-
It is thought that the platelet release reaction, which is essential for is mediated by formation of endothe formation of a platelet thrombus, the platelets (12). In peroxides (EP) and thronbosane A2 (TxA.2) within EFA-deficient rats the formation of platelet EP and TxA2 is very low. However, after adequate platelet activation the release reaction - and consequently platelet aggregation - is almost normal, which indicates the existence of a thrombosis mediating system different from the AA-EP-TxA 2 In EFA-deficiency the balance between prostacyclin pathway (10,13,14). formation and the platelet release reaction will, therefore, be very much in favour of the latter. If this balance - as has been suggested (1,2,3) indeed determines thrombosis tendency, this situation will result in a prothrombotic state. This is in line with our finding that a piece of EFA-deficient aorta, brought into contact with a suspension of EFA-deficient of control aorta in platelets induces much more aggregation than 3 piece a suspension of control platelets (Fig. 4). ?Ioreover, similar differences between EFA-deficient and control aortas were observed after “crossing over” the platelet suspensions whereas no differences occurred after pre-treatment of the aortas with indomethacin. Although all these in vitro findings point to an increased arterial thrombosis tendency in EFA-deficiency, in vivo experiments invariably show this tendency to be considerably depressed (9): This is not in line with a major role of vascular PGI2 in chromboregulation. The same conclusion can be drawn from the experiment with rats fed either SO of HCO. Although these animals have different thrombosis tendendies (9) their aortic PGI2-generation is essentially similar. The present findings indicate that more experimental evidence is required before an ultimate thromboregulatory role of the EP-TxAz-PGIZ-tri3d can be accepted. For initial studies, the rat may be attractive, since in this animal species arterial thrombogenesis can conveniently be quantified (15). Moreover, the PGI -synthesizing capacity of its aorta can easily be measured and seems to 8 e higher than that of any other arterial ti,ssue described so far. However, any finding obtained with rats has to be confirmed in other species. Note After this short communication was submitted, Moncada and coworkers published a study in which they showed that more than 60% of the vascular prostacyclin synthetase activity is located in the non-endothelial part of the blood vessel (16). Although their results differ from ours quantitatively - which is most probably due to methodological differences - they nevertheless confirm the reality of substantial subendothelial prostacyclin formation which seriously complicates the evaluation of the thromboregulatory role of the vessel wall.
< - ; .R . An enzyme BCYcI:x, s. and l"~;;l;r, isolated from arteries transforms prastaglandin endsperoxides to an unstable substance that inhibits platelet.aggregatica. Xature 263,633, 1975.
. “OSCrnA,
2.
s . , GF.YGL%'SKI R J ,..,I
G?.YGLZWSKI, R.J., BCYTING, S., XO?iCADA, S., FLO:ZR, R.J. and l"'AxE, J.R. thrO&Di
Arterial walls are protected
agslns:
denojition of platelet
5:; a substance (prostaglandin X) which they make from prostaglandin
endoperoxidcs. Prostaglandins 12,583, 1975. XOSCXDA, S., GRYGLEWSKI, R.J., BLD?TIXG, S. and VUE, J.R. .\ lipid peroxide inhibits the enzyme in blood vessel nicrosomes that generates from prostaglandin endoperoxides the substance (prostaglandin K) which , prevents platelet aggregation. Prostaglandins ll,7ij, 1970. :.
BlJS'II?;G, S., GRYGLE;;SKI, ii.J., >:OSi:XDA, S. and %'%E, .J.R. Arterial wallS generate from prostaglandin endoperosides a substance (nrostaglnndin I<) which relases strips of mesenteric and coeliac arteries and in!,iibits platelet aggregation. Prostaglandins I?, S97, !976.
3.
?IONC.kDX,S., HIGGS, E.A. and VAX, J.R. Human arteriai and venous tissues generate prostacyclin (prostaglandin Xi, a potent inhibitor of platelet aggregation. Lancet i, 18, 1977.
h.
Editorial: Nomenclature Announcement. Prostac~cl~h (PGI,). Prostaglandins 13, 375, 1977.
7.
DUTILH, C.E., C'ENDE~L~S-STZ~~B~RC, A. and TE?; ROOR, F. production by cells in culture. In preparation.
s.
HORTON, E.W.
Molecular insight into thrombosis. Sature 263, 637, 1976.
9.
HORXSTRA, G. 1973, 1974.
Dietary fats and arterial thrombosis. Haemostasis 2, 21,
Prostacyclin
10.
HORXSTRA, G. and HADDEXAN, E. Effects of dietary fats on the role of platelets in arterial thromboembolism. In: D.C.E. !lills and F.I. Pareti (Eds): Platelets and Thronbosis, Academic Press, London, New York, San Francisco (in press).
II.
BALJNGARTYER, H.R. The subendothelial surEace and thrombosis. Thromb. Diath. Haemorrh. Suppl. 59, 91, 1974.
I?-.
HAYBERG, >I., SVENSSOS, ;. and SAXUELSSOX, B. Thronboxanes: 3 new group of biologically active $ompounds derived from prostaglandin endoperoxides. Proc. Xat. Acad. Sci. USA 72, 2994, 1975.
I?. BULT, H. and BOSTA, I.L. Rat platelets aggregate in the absence of endogenous precursors of prostaglandin endoperoxides. Nature 264, 449, 1976. 1;. HORXSTRA, G. and HADDENX'i,E. Arachidonic acid metabolites not crucial for thro~boregu~ation? Thromb. Haemostas. 38, 19, 1977.
FROSTACYCLIN
37;
15.
!iO?XST%1, G . and arterial occlusive
:
THRO~~BOREGt-LATIOS
Vo1.12,No.2
‘.~.UDE~~~~_‘;S-ST~~_~~g~RG, A. Indluctlon of zqerizextal thro&i in rats. Atheroscle-osis I Ii, 369, 1?73.
16 . XOXCADA, S., HENAS, A.G., HICGS, E.A. and VtiE, J.R. Differential fomation of prostacyclin (PGX or PGI?) by layers of the arterial wall. An explanation I for the anti-throabotic properties of vascular endothelium. Thronb. Res. 323, 197i.
I,