Recovery of prostacyclin production by de-endothelialized rabbit aorta. Critical role of neointimal smooth muscle cells

Arterial 6-keto-PGF1 alpha and TxB2 release in ex vivo perfused canine vessels: effects of pulserate, pulsatility, altered pressure and flow rate

Certain experimental conditions are known to influence the release of prostacyclin and thromboxane from the vessel wall. The specific effects of altered pulsatility, pressure, and flow rate on intraluminal release of 6-keto-PGF1 alpha and thromboxane B2 were assessed in canine arteries perfused ex vivo for five 15 min periods with arachidonic acid (AA) added during the last period. Control arteries were perfused at 100 mmHg with pulsatile flow of 90 ml/min. Experimental arteries were perfused at 7, 50 and 200 mmHg with pulsatile flow of 90 ml/min, and at 100 mmHg pressure with pulsatile flow of 20, 60, 130 and 180 ml/min, as well as at 100 mmHg with 90 ml/min nonpulsatile flow. Perfusion pump rates of 44 and 96 beats/min were also assessed. The lowest perfusion pressure, 7 mmHg, resulted in a lesser initial release of prostacyclin compared to higher pressures, and there was a tendency to a higher release of prostacyclin with increasing pressures. There was also a tendency for a lesser response to AA in arteries perfused at 200 mmHg, perhaps due to endothelial cell damage. Nonpulsatile flow was associated with a decreased initial release of prostacyclin, and diminished release following addition of AA when compared to pulsatile flow. Altered flow rate elicited no difference in prostacyclin release, although there was a tendency towards a lesser release when perfused at 20 ml/min compared to 130 ml/min or 180 ml/min. Thromboxane release was decreased by nonpulsatile flow but was otherwise unaffected by the experimental conditions tested. It is concluded that pulsatility enhances release of prostacyclin from arteries.(ABSTRACT TRUNCATED AT 250 WORDS)

Regional variability of prostacyclin biosynthesis

To investigate the regional variability in arterial and venous endothelial prostacyclin (PGI2) biosynthesis, we obtained 1-cm segments of carotid arteries, external jugular veins, femoral arteries and veins, iliac arteries and veins, inferior venae cavae (IVC), and aortas from 17 dogs. Vessel luminal PGI2 production was measured in the basal state by radioimmunoassay of 6-keto-prostaglandin F1 alpha (6-keto-PGF1 alpha). A total of 90 arterial specimens (57, 19, and 14 segments, respectively, of femoral/carotid arteries, iliac arteries, and aorta) and 41 venous specimens (15, 10, and 16 segments, respectively, of femoral/jugular veins, iliac veins, and IVC) were analyzed. Overall, arterial endothelial 6-keto-PGF1 alpha was higher than venous (8.1 +/- 0.5 ng/ml vs. 4.9 +/- 0.7 ng/ml, p less than 0.0004); 6-keto-PGF1 alpha levels were greater in the arteries than in their corresponding veins [femoral/carotid arteries (6.3 +/- 0.4 ng/ml) vs. femoral/jugular vein (2.1 +/- 0.4 ng/ml), p less than 0.0002; iliac arteries (9.3 +/- 1.0 ng/ml) vs. iliac veins (4.8 +/- 0.9 ng/ml), p less than 0.005; aorta (14.0 +/- 1.6 ng/ml) vs. IVC (7.5 +/- 1.4 ng/ml), p less than 0.006].(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a bidirectional prostaglandin endoperoxide shunt between platelets and the bovine coronary artery

While platelets have been shown to be capable of supplying prostaglandin (PG) H2 to endothelial cells in culture for PGI2 synthesis, endothelial cells have been shown unable to supply PGH2 to platelets for thromboxane (TX) A2 synthesis. We incubated rings of the bovine coronary artery (BCAR) with human platelets treated with aspirin (to inhibit cyclooxygenase) or CGS 13080 (to inhibit TXA2 synthase) in the presence of 20 microM arachidonic acid. BCAR, with damaged endothelium, produced significantly less PGI2 than that with intact endothelium. However, co-incubation with CGS 13080-treated platelets resulted in an increase in PGI2 independent of endothelium, demonstrating a shunt of PGH2 from platelets to BCAR. Co-incubation of BCAR with aspirin-treated platelets resulted in a net increase in TXA2 demonstrating a shunt of PGH2 from BCAR to platelets. Employing [14C]PGH2 as substrate, BCAR with and without intact endothelium produced similar amounts of 6-keto-[14C]PGF1 alpha. Likewise, homogenates (50 micrograms protein) of intimal and subintimal regions of BCAR and BCAR converted similar amounts of PGH2 to 6-keto-PGF1 alpha. These data suggest that vascular production of PGH2 is more dependent on an intact endothelium than is the conversion of PGH2 to PGI2. These data also suggest a potential for a bidirectional exchange of PGH2 between platelets and vascular wall during platelet-vascular wall interactions.

Factors inducing endothelium-dependent relaxation in the guinea-pig basilar artery as estimated from the actions of haemoglobin

1. Factors inducing dilatation of guinea-pig basilar artery were investigated in intact and endothelium-denuded tissues by measurement of isometric tension and by electrophysiological methods. 2. The amplitudes of contractions induced by 9,11,epithio-11,12-methanothromboxane A2 (STA2) and by high K+ were enhanced by haemoglobin (oxyhaemoglobin, Hb) in a concentration-dependent fashion (above 1 microM). For the high K+-induced contraction, the initial tonic component was enhanced to a greater extent than the secondary phasic component. Mechanical responses evoked by STA2 and by high K+ were greater in endothelium-denuded tissues, but Hb (below 10 microM) had no effect on them. 3. Hb (10 microM) had no effect on the contractile proteins as estimated from the actions of Hb on Ca2+-induced contractions in skinned muscle tissues. Further, Hb had no effect on the release of Ca2+ from intracellular stores but it accelerated the Ca2+ accumulation into the sarcoplasmic reticulum as judged from the caffeine- or STA2-induced contraction generated in intact tissues. 4. Acetylcholine (ACh) relaxed tissues that were precontracted by STA2 but Hb prevented this relaxation, in a concentration-dependent fashion. The ACh-induced relaxation was sustained for over 10 min in the absence of Hb, but following application of Hb, ACh caused only a transient relaxation. 5. STA2 (up to 100 nM) did not modify the resting membrane potential of smooth muscle cells of the basilar artery. ACh (10 microM) caused transient hyperpolarization which was only slightly inhibited by Hb (10 microM) whether or not STA2 was present. The hyperpolarization induced by ACh required the presence of endothelial cells. 6. A23187 (0.01-1 microM) relaxed tissues which were precontracted by STA2, in a concentration-dependent fashion but had no effect on the membrane potential. 7. These results suggest that in guinea-pig basilar artery, ACh induces relaxation of tissues that were precontracted by STA2 by causing release of both endothelium-derived relaxing (EDRF) and endothelial dependent hyperpolarizing factor (EDHF) (sustained and initial transient relaxation, respectively), but via different mechanisms. Hb inhibits the former and to a lesser extent, the latter. Since A23187 produced relaxation of pre-contracted tissue but caused no detectable change in the membrane potential, this agent may release EDRF but not EDHF.

The interplay between platelet and vessel-wall mediators in coronary artery occlusion

Myocardial infarction (MI) is associated with platelet aggregation and coronary vasospasm. Endogenous mediators produced by platelets and the vessel wall alter platelet function and smooth muscle tone and may be involved in the infarctive process. The synthesis and actions of these mediators is largely determined by interactions between platelets and the vessel-wall. MI occurs at sites of endothelial cell damage where the balance of mediators is shifted in favour of aggregation and vasospasm. Therapeutic intervention should aim at restoring the balance of mediators, and this will involve manipulation of endothelium-derived nitric oxide or its intracellular second messengers.

Effect of monocytopenia on trauma-induced atherosclerotic lesions in the rabbit ear artery

In a trauma model of atherosclerosis (repeated mechanical injury of the rabbit ear artery), rabbits were pretreated either with etoposid (inducing a monocytopenia) or with prednisolone (inhibiting monocyte function) to investigate the role of monocytes in traumatically induced plaque formation. Three weeks after the last injury the arteries were carefully examined. While a profound monocytopenia during the period of injuries did not at all influence the size of the plaque formation, this was almost completely inhibited in the prednisolone-treated rabbits. Obviously, the effect of prednisolone must be attributed to other pharmacological properties. Monocytes appear to be of less importance in purely trauma atherosclerosis models.

Effects of autologous mesothelial cell seeding on prostacyclin production within Dacron arterial prostheses

Canine abdominal aortas have been replaced with Dacron arterial prostheses to assess the effects of mesothelial cell seeding on graft prostacyclin and thromboxane A2 release. At both 2 weeks and 6 weeks after surgery, three seeded and two unseeded control grafts were examined for prostacyclin release. In addition, thromboxane release was assessed in one seeded and one unseeded graft. Sections of aorta and graft were removed and incubated in PBS containing either 10 microM calcium ionophore A23187 or 20 microM arachidonic acid. The incubation mixture was sub-sampled at 5 min intervals over a 20 min period to assess the progressive release of prostacyclin and thromboxane A2 using a radioimmunoassay for 6-keto-prostaglandin F1 alpha and thromboxane B2 respectively. In seeded grafts, 6-keto-prostaglandin F1 alpha release averaged 15 per cent compared with aorta at 2 weeks and 45 per cent compared with aorta at 6 weeks. By contrast, release from unseeded grafts was undetectable at 2 weeks; however, by 6 weeks there was some release amounting to 15 per cent compared with aorta. There was a statistically significant increase in the release of 6-keto-prostaglandin F1 alpha from mesothelial cell seeded grafts at 6 weeks compared with unseeded grafts (P less than 0.01). Thromboxane release from the graft sections was variable and unrelated to whether the grafts had been seeded or not. These preliminary results, showing that grafts seeded with autologous peritoneal mesothelial cells release more prostacyclin than unseeded grafts, further highlight the role of the mesothelial cell as an alternative to the endothelial cell for improving the patency of arterial Dacron prostheses in the early postoperative days.

Comparison of static incubation versus physiologic perfusion techniques for quantitation of luminal release of prostacyclin and thromboxane in canine arteries and veins

Intraluminal release of 6-keto-PGF1 alpha and TxB2 in ex vivo canine arteries and veins was assessed during five consecutive 15-min periods using static incubation and physiologic perfusion techniques. Arterial segments were perfused with 90 ml/min pulsatile flow at 100 mm Hg and vein segments with 90 ml/min nonpulsatile flow at 7 mm Hg. During the final 15-min period vessels were stimulated with arachidonic acid (AAS). Perfusion of vein segments caused a higher release of 6-keto-PGF1 alpha during the first 30 min (P less than 0.05) and following AAS (P less than 0.05) than did static incubation. Perfused arterial segments exhibited a higher release than did incubated segments of 6-keto-PGF1 alpha for 45 min (P less than 0.01) as well as following AAS (P less than 0.01). TxB2 release was higher during the entire observation period in perfused arteries and veins compared to incubated vessels (P less than 0.01 and less than 0.05, respectively). There was no correlation between the amounts of 6-keto-PGF1 alpha or TxB2 released when comparing values obtained by one technique to values obtained by the other (P greater than 0.1). These data suggest that flow related shear stress alters vascular prostanoid production, and that such should be accounted for when interpreting results of studies on prostacyclin and thromboxane release from intact vessels.

Mechanisms underlying pressure-related natriuresis: the role of the renin-angiotensin and prostaglandin systems. State of the art lecture

It has long been known that increments in renal perfusion pressure can induce an elevation of urine sodium excretion without changing renal blood flow or glomerular filtration rate. The mechanism underlying this pressure-related natriuresis remains undefined, although the interest in its elucidation has been stimulated by the notion that it may constitute the central phenomenon through which the kidney regulates blood volume and, thereby, blood pressure. Recently, the use of novel experimental techniques has disclosed some important clues about changes in renal hemodynamics that, along with changes in renal humoral regulators, allow us to visualize a possible sequence of events responsible for pressure-related natriuresis. According to this hypothesis, the autoregulatory responses responsible for maintaining glomerular filtration rate are elicited in preglomerular vasculature by changes in renal perfusion pressure. These myogenic responses are coupled through Ca2+ entry in juxtaglomerular cells with inversely related changes in the release of renin and, consequently, with the amount of angiotensin II generated in renal interstitium. The release of renin from juxtaglomerular cells is modulated by the synthesis of prostaglandin I2 from the adjacent endothelial cells. Interstitial angiotensin II could influence sodium tubular reabsorption directly by stimulating sodium transport in proximal renal tubules and indirectly by altering medullary blood flow and, thereby, medullary interstitial pressure. In the renal medulla, the effects of interstitial pressure on sodium reabsorption can be amplified by the release of prostaglandin E2 from interstitial cells. A deficient regulation of this relationship could result in a shift of the pressure-natriuresis curve, leading to hypertension.

Prostacyclin production in free versus native IMA grafts

The long-term patency of the internal mammary artery (IMA) in coronary artery surgery has been superior to that of other grafts. We tested the IMAs as free grafts in an experimental study with 8 dogs. The right IMA was dissected free as a naked artery without surrounding tissue, and the left IMA was dissected with surrounding tissue using electrocautery. Both free IMA grafts were anastomosed first to the left IMA one after another and then to the left anterior descending coronary artery. The grafts were harvested 6 months later, and the prostacyclin (PGI2) production of the free IMA grafts and of the intact IMAs was measured. Radioimmunoassay was used to measure 6-keto-prostaglandin F1 alpha, the stable metabolite of PGI2. The PGI2 production of the right free IMA without surrounding tissue was 21.1 +/- 8.5 pg/mg and of the left IMA with surrounding tissue was 17.9 +/- 7.9 pg/mg (expressed in picograms of 6-keto-prostaglandin F1 alpha per milligram of tissue wet weight). The control value of the intact IMA was 18.3 +/- 6.5 pg/mg. Thus, PGI2 production of both free IMAs did not differ significantly from each other and from controls. This study showed that free IMAs were able to produce as much PGI2 as were control IMAs and that the type of dissection did not affect the result.

Vascular smooth muscle cells synthesize, secrete and express coagulation factor V

Expression of cellular procoagulant activity may be one of the more important responses to vascular injury. Because factor V, a coagulation cofactor in the prothrombinase complex, catalyzes the conversion of prothrombin to thrombin, it may be a key to understanding this response. Therefore, we have investigated the synthesis, secretion and expression of factor V by vascular smooth muscle cells, which proliferate at sites of vascular injury. Cultured aortic vascular smooth muscle cells constitutively secreted Factor V activity, as determined by a functional assay. Labeled factor V was immunoprecipitated from conditioned medium of [35S]methionine-labeled cells, indicating that the secreted factor V was synthesized by vascular smooth muscle cells. Treatment of vascular smooth muscle cells with tunicamycin prevented secretion of factor V, suggesting that its secretion was dependent on the presence of N-linked carbohydrate. Factor V activity was also expressed on the vascular smooth muscle cell surface, as indicated by the ability of cultured cells to promote factor Xa-catalyzed prothrombin activation. These data suggest that the proliferation of smooth muscle cells in response to vascular injury may be one mechanism that links vascular disease with thrombosis.

Recovery of prostacyclin synthesis by rabbit aortic endothelium and other tissues after inhibition by aspirin

The effect of aspirin on prostacyclin (PGI2) and thromboxane B2 (TXB2) synthesis was studied in rabbits. Tissues were removed from animals killed at intervals after injection of aspirin, and incubated with Hanks' solution. PGI2 synthesis was monitored by radioimmunoassay of its hydrolysis product, 6-oxo-prostaglandin F1 alpha (6-oxo-PGF1 alpha). TXB2 production in clotted blood, also measured by radioimmunoassay, was determined as an index of platelet cyclo-oxygenase activity. 6-oxo-PGF1 alpha and TXB2 production 0.5 h after aspirin were similarly inhibited to less than 5% of control in all incubations. Subsequent recovery of PGI2 synthesis occurred more rapidly in aortic endothelium than in other tissues, including aorta denuded of endothelium. Recovery of TXB2 production was slower than that of PGI2. Intravenous cycloheximide prevented the partial recovery of PGI2 synthesis that otherwise occurred 6 h after aspirin, while intravenous epidermal growth factor increased recovery. It is concluded that in the rabbit, cyclo-oxygenase is synthesized more rapidly in aortic endothelium than in deep layers of aorta, or in the other tissues studied.

Studies on the prostaglandin synthesizing capacity of several blood vessels from male and female rats

Prostaglandin (PG) I2 (measured as 6-keto-PGF1 alpha) was the major PG synthesized by homogenates of the aorta from male rats, followed by lesser quantities of PGF2 alpha and PGE2. The amounts of 6-keto-PGF1 alpha synthesized by homogenates of the vena cava, mesenteric artery and femoral artery were much less than synthesized by the aorta, and were similar to the amounts of PGF2 alpha and PGE2 synthesized. The amounts of 6-keto-PGF1 alpha synthesized by homogenates of the aorta were 30% lower in female rats than in male rats. The amounts of 6-keto-PGF1 alpha and PGF2 alpha synthesized by homogenates of the aorta from female rats were similar, but the amounts of PGE2 synthesized were lower. The amounts of 6-keto-PGF1 alpha synthesized by homogenates of the aorta and femoral artery of female rats were similar, but the amounts of 6-keto-PGF1 alpha synthesized by homogenates of the mesenteric artery and vena cava were lower and were similar to the amounts of PGF2 alpha and PGE2 synthesized. The amounts of 6-keto-PGF1 alpha synthesized by the vena cava were significantly lower (P less than 0.05) and the amounts of 6-keto-PGF1 alpha and PGF2 alpha synthesized by the femoral artery and aorta, respectively, were significantly higher (P less than 0.05) in female rats than in male rats. The total amount of 6-keto-PGF1 alpha, PGF2 alpha and PGE2 synthesized by the aortae of male and female rats did not differ, indicating that the higher output of 6-keto-PGF1 alpha from the aorta of male rats compared to female rats is not due to a higher concentration of PGH2 synthetase in the aortic tissue of male rats. However, the shift in the relative amounts of 6-keto-PGF1 alpha and PGF2 alpha synthesized by the aorta of female rats in favour of PGF2 alpha may be responsible for the lower output of 6-keto-PGF1 alpha from the aorta of female rats. Oestradiol and progesterone had no effect on the amounts of 6-keto-PGF1 alpha, PGF2 alpha and PGE2 synthesized by homogenates of the aorta and vena cava from intact and ovariectomized, female rats. However, ovariectomy increased the amounts of 6-keto-PGF1 alpha and PGE2 synthesized by the aorta and vena cava, respectively, an effect not reversed by oestradiol and progesterone treatment.

Effect of preoperative antiplatelet drugs on vascular prostacyclin synthesis

Patients undergoing aortocoronary bypass using autogenous saphenous veins were randomly divided into three comparable groups. Group 1 (n = 10) acted as a control, Group 2 (n = 14) received 80 mg of aspirin at midnight before the operation, and Group 3 (n = 12) received 80 mg of aspirin and 75 mg of dipyridamole at midnight and an additional 75-mg dose of dipyridamole at 6 AM. The purpose was to determine which regimen would maximally inhibit platelet function without depressing vascular prostacyclin synthesis. Serum thromboxane A2, saphenous vein wall and aortic wall prostacyclin, platelet aggregation, and bleeding time were measured in all patients. None was restarted on a regimen of aspirin or dipyridamole postoperatively. Aspirin alone and in combination with dipyridamole significantly inhibited thromboxane A2 and platelet aggregation in all treated patients but spared venous prostacyclin synthesis. Aortic prostacyclin synthesis was partially inhibited in both treated groups. Chest tube drainage was comparable in all three groups. These results indicate that the combination of aspirin and dipyridamole offers no measurable advantage over aspirin alone in the perioperative period.

Pharmacology of platelet inhibitors

Although many drugs have inhibitory effects on platelet function, none of them inhibits all of the mechanisms that may be involved in the various forms of thrombosis. Choice of suitable drugs is hampered by lack of full knowledge concerning the reactions that make the major contributions to the formation of arterial thrombi at sites of repeated vessel wall injury or on atherosclerotic lesions. Drugs such as aspirin that inhibit the arachidonate pathway in platelets can only be expected to be effective against thromboembolic events in which the generation of thromboxane A2 plays a major part. If thrombin and fibrin formation are dominant, oral anticoagulant agents or heparin should be beneficial; thus, experimental evidence indicates that with repeated vessel wall injury, the formation of platelet fibrin thrombi on the vessel wall is probably influenced more by inhibitors of thrombin generation than by the subendothelial constituents such as collagen. Agents like prostacyclin that raise platelet cyclic adenosine monophosphate (AMP) levels in platelets by stimulating adenylate cyclase are potent inhibitors of the reaction of platelets to all aggregating and release-inducing stimuli, but these agents are not suitable for long-term administration. The effect of dipyridamole on platelet cyclic AMP levels is weak, and this drug may act through other effects on platelets or on other cells. Indeed, several of the drugs that have been tested in clinical trials may exert their effects through unrecognized mechanisms. Many combinations of drugs have been used to affect platelets or platelets and coagulation. This practice has been based on the theory that because several mechanisms may be involved in thrombus formation, combinations of drugs that inhibit different mechanisms may be beneficial.

Effects of prostaglandin I2 synthesized in the endothelium and in the smooth muscle on mechanical properties of the canine thoracic aorta

In circular-cut strips prepared from canine thoracic aorta, acetylcholine (ACh) and A23187 relaxed endothelium-intact tissues [E(+) preparations] pre-contracted with noradrenaline or excess concentrations of K. These relaxations were associated with marked increases in the amount of 6-keto PGF1 alpha. After removal of the endothelium [E(-) preparations] the relaxation ceased, and the amounts of 6-keto PGF1 alpha were markedly reduced. In E(+) preparations, application of indomethacin attenuated the increase in 6-keto PGF1 alpha induced by ACh or A23187 in the presence of noradrenaline or high K, but not the endothelium-dependent relaxations. In E(-) preparations, ACh (0.1-10 microM) neither increased the amount of 6-keto PGF1 alpha nor produced a contraction. In dispersed single endothelial cells, A23187 markedly increased but 118 mM K did not modify the amount of 6-keto PGF1 alpha. Both noradrenaline and high K increased the production of 6-keto PGF1 alpha in the E(-) preparations but to a lesser extent than that in the E(+) preparations. This action was attenuated by indomethacin. The amplitude of the noradrenaline- and K-induced contractions was enhanced with indomethacin pretreatment in both E(+) and E(-) tissues. PGI2-Na (10 nM), reduced the amplitude of noradrenaline-induced contractions, concentration dependently and to the same extent in both E(+) and E(-) preparations. These results indicate that synthesis of PGI2 in the endothelium is not causally related to the endothelium dependent relaxation. PGI2 synthesized in the endothelium may not act directly on the muscle tissue, but PGI2 synthesized in the smooth muscle tissue may produce an inhibition of contraction.

Prostacyclin production by the bovine aortic smooth muscle

It is well known that cultured aortic smooth muscle cells, the phenotype of which has modulated from contractile to synthetic, are able to release prostacyclin (PGI2). We have studied the release of PGI2 from cultured explants of bovine aortic media, which represent an homogeneous population of smooth muscle cells with a contractile phenotype. These explants released spontaneously huge amounts of PGI2, which was the major eicosanoid produced. PGI2 release was stimulated by serum and by serotonin. This experimental model seems useful to evaluate the contribution of smooth muscle to the biosynthesis of PGI2 by the arterial wall.

Production of eicosanoids by deendothelialized rabbit aorta: interaction between platelets and vascular wall in the synthesis of prostacyclin

Production of eicosanoids by deendothelialized aorta in response to continuous infusions of arachidonic acid and platelet suspensions was determined in a rabbit aorta perfusion model. 6-keto-PGF1 alpha production was stimulated by AA infusion in a dose-related manner. Infusion of AA at 4 micrograms/ml/min led to an initial production rate of 0.64 +/- 0.29 ng/min which gradually increased to 0.93 +/- 0.11 ng/min at the 20th min of infusion. When the concentration of AA infusion was increased to 10 micrograms/ml/min, 6-keto-PGF1 alpha production increased to 1.14 +/- 0.86 ng/min initially but declined with time. PGE2 production in response to AA 10 micrograms/min/ml was steady at around 5 ng/min while PGF2 alpha and TXB2 production were only slightly above the control. Perfusion of rabbit washed platelet suspensions at a rate of 3 X 10(8) plt/ml/min raised 6KPGF1 alpha production. The production was further increased when platelets were pretreated with 1-benzylimidazole (5 mM), along with a concurrent reduction in TXB2 release. Pretreatment of platelets with aspirin, on the other hand, abolished the increase in 6KPGF1 alpha production. Our data indicated that the vascular smooth muscle cells can efficiently utilize PGH2 produced by platelets to synthesize PGI2.

Biochemical (functional) adaptation of "arterialized" vein grafts

Canine venous autografts and allografts were interposed in the femoral and carotid arterial positions in 29 dogs; grafts were harvested at three postoperative intervals (1-2 weeks, 4-6 weeks, and 8-10 weeks) for light and scanning electron (SEM) microscopy and lumenal surface prostacyclin (PGI2) production. Normal veins and arteries were used as controls. Radioimmunoassay for tritiated 6-k-PGF1 alpha, the stable metabolite of PGI2, was performed using a flow surface template incubation chamber during basal and arachidonic acid stimulated conditions. Using SEM, the autografts revealed normal endothelial cell (EC) surfaces at all time intervals; conversely, allografts exhibited extensive EC loss at 1-2 weeks with gradual reparation by 10-12 weeks (such that the EC surface was virtually indistinguishable from that of control veins or autografts). PGI2 production was significantly greater in control arteries than veins (p = 0.0001). At 1-2 weeks and 4-6 weeks, lumenal production of PGI2 in both the autografts and allografts was not significantly different from control vein; however, PGI2 production after 10-12 weeks was identical to normal arterial levels (and significantly [p less than 0.0044] higher than venous levels) in both basal and stimulated conditions. Although the mechanisms responsible for this functional (biochemical) "arterialization" process remain conjectural, increased biosynthesis and/or release of PGI2 by endothelial cells, acute phase inflammatory cells (allografts) mediated by interleukin-1 or myointimal cells seems most likely. Further elucidation of these sources of PGI2 is necessary, but these data demonstrate for the first time that venous grafts placed in the arterial circulation undergo complete functional adaptation (in addition to the well known morphological changes).

Effect of thromboxane synthetase inhibition on canine autogenous vein grafts

This study examined the effect of an orally active thromboxane (TXA2) synthetase inhibitor (TSI) on the patency, TXA2 production, and platelet accumulation of reversed autogenous vein grafts. Ten dogs received TSI (U-63557A) 10 mg/kg po q8 hr for 6 weeks, beginning 24 hr prior to surgery, while 15 control dogs were untreated. One jugular vein was harvested and stored in 37 degrees C saline for 1 hr to induce mild endothelial injury (stored). Normal and stored jugular vein grafts (8 cm) were then implanted in opposite femoral arteries while 3-cm segments of the same veins were implanted in the carotid arteries. Femoral graft flow was restricted with a 5 Fr distal arterial stenosis and patency determined by arteriography at 1, 2, 4, and 6 weeks. Vein graft endothelial surface TXB2 production was measured by RIA at graft implantation and in carotid grafts harvested at 1 week. 111In-labeled platelets were given iv 24 hr prior to carotid graft harvest to determine graft-platelet deposition. TSI treatment improved early (1 week) femoral vein graft patency from 63 to 89% (P less than 0.05), a trend that persisted for 6 weeks. Warm saline storage reduced 1-week graft patency from 83 to 63% (P less than 0.05), a difference that decreased with time. TSI treatment resulted in a marked decrease in TXB2 production, but was not associated with decreased 111In-labeled platelet deposition in carotid vein grafts. Warm saline storage increased graft-platelet deposition which was predominant at the arterial anastomoses. TSI treatment may improve early vein graft patency during the transient period of endothelial injury.

Development of nonthrombogenicity of injured rabbit aortas despite inhibition of platelet adherence

After removal of the endothelium from normal rabbit aortas or after injury to the neointima, the injured surfaces rapidly become nonreactive to circulating platelets. Experiments were done to determine whether prevention of the initial interaction of platelets with the surfaces would influence the loss of vessel wall reactivity. Inhibition of platelet accumulation on the subendothelium by the infusion of PGI2 (850 ng/kg/min) or the administration of dipyridamole (12.5 mg/kg initially followed by 5 mg/kg/hr) for periods of less than 8 hours inhibited platelet accumulation of platelets on the surfaces when the infusions were stopped. If the animals were treated for 8 hours, platelets did not accumulate on the surface when the drugs were discontinued. Thus, an injured vessel wall can develop a nonthrombogenic surface even when platelet adherence is prevented, although approximately 8 hours are required before the surface loses its ability to interact with platelets.

A blood vessel model constructed from collagen and cultured vascular cells

A model of a blood vessel was constructed in vitro. Its multilayered structure resembled that of an artery and it withstood physiological pressures. Electron microscopy showed that the endothelial cells lining the lumen and the smooth muscle cells in the wall were healthy and well differentiated. The lining of endothelial cells functioned physically, as a permeability barrier, and biosynthetically, producing von Willebrand's factor and prostacyclin. The strength of the model depended on its multiple layers of collagen integrated with a Dacron mesh.

Adrenergic stimulation of prostacyclin production in the rat tail artery. I. Response to agonists and antagonists

The release of prostacyclin (PGI2) as measured by the production of 6-keto-PGF1 alpha was assessed in vitro in rat tail artery segments after exposure to norepinephrine (NE). Time course studies demonstrated a three fold increase in 6-keto-PGF1 alpha production in response to 6 X 10(-5) M NE that persisted for approximately one hour. This production was almost completely prevented by the cyclooxygenase inhibitors indomethacin and ibuprofen and the phospholipase inhibitor mepacrine. Vigorous rubbing of the vessel intima did not prevent the response to NE. Previously stimulated tissues were unresponsive to further addition of NE. However, the addition of arachidonic acid following NE resulted in further increases in PGI2 production which exceeded that of the addition of arachidonic acid alone. NE stimulation was significantly inhibited by phentolamine and prazosin pretreatment of the tissues but was not affected by yohimbine or propranolol. Alpha-1 adrenergic agonists (cirazoline, methoxamine, and phenylephrine) were stimulatory whereas alpha-2 agonists in high doses (tramazoline, BHT-920, xylazine, and UK-14304) were without effect. It is concluded that norepinephrine stimulates PGI2 production in the rat tail artery via an alpha-1 adrenergic receptor. NE also enhances the conversion of arachidonic acid to PGI2. Exhaustion of the response appears to be due to consumption of available arachidonic acid. The fact that procedures which generally remove most of the endothelium did not prevent NE-induced stimulation points to the vascular smooth muscle as the likely source of adrenergically mediated prostacyclin generation in this tissue.

The effect of suppression of prostaglandin synthesis on renal function in rats with intact and reduced renal mass

The present study was undertaken to assess the role of prostaglandin system in the compensatory response to reduced nephron population, respective to renal function and electrolyte excretion. Intact and 5/6 nephrectomized rats were divided in 4 groups: 1) rats pretreated with indomethacin, 2) rats pretreated with the vehicle of indomethacin, 3) rats pretreated with sulindac, and 4) rats pretreated with the vehicle of sulindac. In normal rats, indomethacin administration resulted in a mild decrease in creatinine clearance and a significant reduction of the urinary Na excretion. In the rats with reduced renal mass treated with indomethacin, the creatinine clearance did not differ from that in the control group. The 24 h urinary sodium excretion and the fractional excretion of sodium, however, were significantly lower in the indomethacin treated animals than in the control rats. No change in the creatinine clearance or in the sodium excretion was observed in all groups pretreated with sulindac. The urinary PGE2 and thromboxane excretion was significantly lower in the indomethacin treated intact rats and the rats with reduced renal mass. Sulindac induced a slight decrease in urinary excretion of PGE2 in intact rats. No significant change in urinary excretion of PGE2 or thromboxane was seen after sulindac in the rats with reduced renal mass. The antinatriuretic effect of indomethacin was dissociated from changes in urine flow in all groups of animals, suggesting that the increase in Na reabsorption took place in a water impermeable segment of nephron. These results suggest that the compensatory increase in urinary Na excretion per nephron in rats with reduced nephron population at least partly depends on an intact prostaglandin synthesis.

Arterial prostaglandins and lysosomal function during atherogenesis. I. Homogenates of diet-induced atherosclerotic aortas of rabbit

Homogenates of control and diet-induced atherosclerotic aortas of rabbit were prepared and the levels of DNA, protein, free and esterified cholesterol, and six enzymes known to be associated with various subcellular organelles [N-acetyl-beta-glucosaminidase, beta-galactosidase (lysosomes); cytochrome oxidase (mitochondria); neutral alpha-glucosidase (endoplasmic reticulum); 5'-nucleotidase (plasma membrane); catalase (peroxisomes)] were compared between control and atherosclerotic preparations. The levels of prostaglandins I2, E2, and F2 alpha, based on DNA, also were measured by radioimmunoassay. Atherosclerotic aortas were significantly enriched in catalase activity (440%) and in each of the acid hydrolases (395 and 630%), based on DNA, as well as in free (630%) and esterified cholesterol (930%), based on tissue wet weight, compared to control aortas. The control level of prostaglandin I2 was 10-fold higher than that of prostaglandin E2, which was 3-fold higher than that of prostaglandin F 2 alpha. Prostaglandin I2 doubled in amount with advanced atherosclerosis, while prostaglandin E2 increased over 10-fold, resulting in twice the amount of prostaglandin I2 than E2 in advanced atherosclerosis; the level of prostaglandin F2 alpha did not appear to change significantly with atherosclerosis. Increased levels of prostaglandins I2 and E2 were correlated significantly with increased aortic total cholesterol content (based on DNA) but not increased serum cholesterol levels. N-Acetyl-beta-glucosaminidase activity also was correlated significantly to aortic total cholesterol content and beta-galactosidase activity, as well as to the level of prostaglandin I2; in contrast, N-acetyl-beta-glucosaminidase was not significantly correlated to prostaglandin E2. The association of prostaglandins I2 and E2 with aortic total cholesterol suggests the participation of prostaglandins in the response of arterial cells to lipid accumulation in atherosclerosis. The specific association of aortic prostaglandin I2 level and N-acetyl-beta-glucosaminidase activity further suggests a possible role for this prostaglandin during arterial intralysosomal cholesterol accumulation.

Effect of hydrogen peroxide on prostaglandin production and cellular integrity in cultured porcine aortic endothelial cells

Oxidative damage to the vascular endothelium may play an important role in the pathogenesis of atherosclerosis and aging, and may account in part for reduced vascular prostacyclin (PGI2) synthesis associated with both conditions. Using H2O2 to induce injury, we investigated the effects of oxidative damage on PGI2 synthesis in cultured endothelial cells (EC). Preincubation of EC with H2O2 produced a dose-dependent inhibition (inhibitory concentration [IC50] = 35 microM) of PGI2 formation from arachidonate. The maximum dose-related effect occurred within 1 min after exposure although appreciable H2O2 remained after 30 min (30% of original). In addition, H2O2 produced both a time- and dose-dependent injury leading to cell disruption, lactate dehydrogenase release, and 51Cr release from prelabeled cells. However, in dramatic contrast to H2O2 effects on PGI2 synthesis, loss of cellular integrity required doses in excess of 0.5 mM and incubation times in excess of 1 h. The superoxide-generating system, xanthine plus xanthine oxidase, produced a similar inhibition of PGI2 formation. Such inhibition was dependent on the generation of H2O2 but not superoxide in that catalase was completely protective whereas superoxide dismutase was not. H2O2 (50 microM) also effectively inhibited basal and ionophore A23187 (0.5 microM)-stimulated PGI2 formation. However, H2O2 had no effect on phospholipase A2 activity, because ionophore A23187-induced arachidonate release was unimpaired. To determine the effects on cyclooxygenase and PGI2 synthase, prostaglandin products from cells prelabeled with [3H]arachidonate and stimulated with ionophore A23187, or products formed from exogenous arachidonate were examined. Inhibition of cyclooxygenase but not PGI2 synthase was observed. Incubation of H2O2-treated cells with prostaglandin cyclic endoperoxide indicated no inhibition of PGI2 synthase. Thus, in EC low doses of H2O2 potently inhibit cyclooxygenase after brief exposure whereas larger doses and prolonged exposure are required for classical cytolytic effects. Surprisingly, PGI2 synthase, which is known to be extremely sensitive to a variety of lipid peroxides, is not inhibited by H2O2. Lipid solubility, enzyme location within the EC membrane, or the local availability of reducing factors may explain these results, and may be important determinants of the response of EC to oxidative stress.

Prostacyclin production by the deendothelialized rabbit aorta

The acute effect of in vitro deendothelialization on the production of prostacyclin (PGI2) by the rabbit aorta has been investigated. The effectiveness of removing endothelium by rubbing it against filter paper or scraping it with a scalpel was demonstrated by scanning electron microscopy and en face examination after silver staining. Endothelium removal produced an immediate stimulation of PGI2 release, resulting in 408% of the control after rubbing and 367% of the control after scraping, during the first 30-min period of incubation. This increased production of PGI2 gradually declined over time to reach values similar to the control after 2h. At that time, the deendothelialized aorta was totally unresponsive to the stimuli that increase PGI2 release in the intact aorta (acetylcholine, ADP, ionophore A23187, and arachidonic acid). The enhanced production of PGI2 in the deendothelialized aorta was associated with an increased release of free arachidonic acid (353% of the control): in contrast with PGI2, this stimulation was maintained for at least 150 min. A transient exposure of the deendothelialized aorta to ibuprofen (250 microM) was followed by a rebound of PGI2 production, which was also prolonged by BW-755C (3-10 microM). In conclusion, removal of the endothelium triggered an immediate and sustained mobilization of free arachidonic acid in the rabbit aorta: the resulting increase of PGI2 production was short-lived, probably as a consequence of cyclooxygenase self-inactivation. Our results indicate that the subendothelium has a significant capacity to produce PGI2, but that this capacity is expressed only briefly.

Cumulative inhibitory effect of low-dose aspirin on vascular prostacyclin and platelet thromboxane production in patients with atherosclerosis

The relationship between the antithrombotic and antiplatelet effects of aspirin is complex, since aspirin influences other systems that protect against thrombosis as well as inhibiting platelet function. We investigated possible cumulative effects of low-dose aspirin on vascular production of prostacyclin in patients with documented atherosclerotic cardiovascular disease. Candidates for coronary artery vein graft bypass ingested 20 mg of aspirin daily during the week before surgery, and platelet aggregation, platelet formation of thromboxane A2 (TXA2), aortic and saphenous vein production of prostacyclin (PGI2), and hemostatic status were measured at the time of the bypass surgery. Low-dose aspirin markedly inhibited platelet aggregation responses and reduced TXA2 generation by greater than 90%, effects similar to those observed with much higher doses of aspirin. Both aortic and saphenous vein production of PGI2 were inhibited by 50% compared with PGI2 produced by vascular tissues of control subjects who received no aspirin preoperatively (51 +/- 10 pg 6-keto-PGF1 alpha/mg aortic wet weight [mean +/- SEM] in aspirin-treated subjects vs 130 +/- 16 pg/mg in control subjects, and 71 +/- 8 pg/mg saphenous vein wet weight vs 131 +/- 17 pg/mg). Blood loss at surgery was not significantly increased by preoperative low-dose aspirin as measured by chest tube drainage (754 +/- 229 ml in aspirin-treated subjects vs 645 +/- 271 ml in control subjects), hematocrit nadir (31.2 +/- 1.9% vs 31.8 +/- 1.7%), or transfusions (2.2 +/- 1.3 units of red blood cells vs 2.2 +/- 1.7 units).(ABSTRACT TRUNCATED AT 250 WORDS)

Response of normal and diabetic rat aorta to deendothelialization in terms of 6-keto-prostaglandin F1 alpha synthesis and cholesterol accumulation

The response of rat aorta to balloon catheter-induced denudation in terms of 6-keto-PGF1 alpha synthesis and cholesterol accumulation was studied in normal and streptozotocin-induced diabetic animals. Initial studies in control rats showed that 1 day after deendothelialization, aortic 6-keto-PGF1 alpha was significantly reduced (31%), and this was normalized at 4 days after injury. However, endothelial regeneration as indicated by Evans blue staining was complete only at 14 days after denudation. Diabetic aorta showed a more severe reduction of 6-keto-PGF1 alpha production (50%) at 1 day after deendothelialization, but this was also normalized at 4 days after injury. Deendothelialization did not increase the accumulation of cholesterol in diabetic aorta. It is concluded that (a) recovery of PGI2 production in deendothelialized aorta does not parallel endothelial regeneration, indicating substantial contribution of subendothelial layer, in rats and (b) short-term aortic injury in diabetic rat will not influence the arterial repair process and cholesterol accumulation and is unlikely to be a contributing factor to increased atherogenesis.