Cyclic AMP and platelet prostaglandin synthesis

Prostaglandins and platelet aggregation

Prostaglandins may induce or inhibit platelet aggregation and constrict ro dilate blood vessels. Recent interest has focused on prostaglandins which are derivatives of arachidonic acid including prostaglandin, endoperoxides, thromboxane A2, prostaglandin E2, prostaglandin D2 and prostacyclin. Prostacyclin (PGI2) is a potent vasodilator and inhibitor of platelet aggregation whose enhanced production by vessel walls should be beneficial. It now appears that the circulating levels of PGI2 in man are extremely low and little is known about the manner in which to increase them. Furthermore, aspirin, in doses of as little as 4 mg/kg inhibits prostacyclin as well as thromboxane formation. Thromboxane A2 may be involved in coronary ischemia because it is a potent vasoconstrictor that is biosynthesized during platelet aggregation. Although thromboxane A2 is very unstable indirect evidence obtained by using thromboxane A generating systems or a stable analogue called carbocyclic thromboxane A2 (CTA2) suggests that it exacerbates ischaemic damage because of a selective increase in vascular resistance due to coronary vasospasm and platelet aggregation which acts to decrease myocardial blood flow. The stable prostaglandins PGD2 and PGE2 are also of interest as both are formed during platelet aggregation. Like PGI2, PGD2 inhibits platelet aggregation.

Oxidized low-density lipoprotein (LDL) enhances thromboxane A(2) synthesis by platelets, but lysolecithin as a product of LDL oxidation has an inhibitory effect

Oxidation of low-density lipoprotein (LDL) by copper sulfate led to a significant increase in lysophosphatidylcholine (lyso PC) at the expense of phosphatidylcholine. Incubation of different concentrations of oxidized LDL (oxLDL) (32-650 microg protein/ml) with platelets for 1 h at 37 degrees C increased lyso PC content. The increase was dependent on oxLDL concentration. Incubation of platelets with various concentrations of lyso PC in solution for 5 or 15 min showed that lyso PC percentage was increased in the platelet membrane and the increase was dose dependent. Platelets incubated with various concentrations of lyso PC (2-100 microM) for 5 or 15 min and then triggered with thrombin also showed a significant decrease of thromboxane A(2) (TXA(2)) release as lyso PC concentration reached 10 microM or 6 microM, respectively. The decrease of TXA(2) release was more significant as lyso PC concentration was increased. The present study showed that this inhibition of TXA(2) release by lyso PC was due to 1) inhibition of phospholipase A(2) and the decrease of free arachidonic acid liberation from platelet phospholipid and 2) inhibition of cyclooxygenase. These inhibitory effects of lyso PC were discussed in relation to its effect on membrane fluidity. Lyso PC at concentrations of 30, 50, and 100 microM caused a sudden drop in TXA(2) release and a sudden increase of lactic dehydrogenase loss from the platelets due to their lysis and inhibition of cyclooxygenase enzyme. The present study shows that oxLDL contains high levels of lyso PC that are transferable to the platelets and can weaken their responsiveness to thrombin and decrease TXA(2) release. In our previous study, we found that oxLDL also contained high levels of oxysterols and thiobarbituric acid reactive substances (TBARS), which enhanced platelet reactivity to thrombin and increased TXA(2) release. We conclude that the net effect of oxLDL on platelets will depend on its degree of oxidation and the ratio between oxysterols plus TBARS/lyso PC. Variations in this ratio may explain some of the contradictions cited in the literature concerning the effect of oxLDL on platelet activation.

Development and review of radioimmunoassay of 12-S-hydroxyheptadecatrienoic acid

For more than 25 years 12-S-hydroxyheptadecatrienoic acid (HHT) has been known to be a product of thromboxanesynthase (TX-Syn) when synthesized with thromboxane A2 (TXA2). Although there are some hints that HHT has anti-aggregatory effects, to date, it has neither been shown to have any specific pathological relevance nor is there much information about its physiological role. This review presents a summary of the physicochemical properties of HHT, its chemical synthesis, the impact of various biological systems on its enzymatic and non-enzymatic production and its physiological function and metabolization, as well as a survey of the most important methods for analyzing this unsaturated hydroxy-fatty acid. Due to the low antibody-raising potency expected in HHT, no immunological system for HHT quantification has been developed so far. In our report we present the development and validation of a sensitive and reliable, competitive radioimmunoassay (RIA) suitable for the quantitative determination of HHT. HHT was produced by an enhanced enzymatic method using platelet-rich plasma (PRP). With an effective and modified liquid-liquid and solid-phase extraction method we were able to produce highly purified HHT (97% purity by GC/MS) in sub-milligram ranges. These fractions were used for the synthesis of BSA-antigen-conjugates and for immunization of rabbits. The tritiated tracer was synthesized using prostaglandin H synthase for the production of prostaglandin H2 (PGH2) followed by an aqueous reaction with Fe(2+)-solution to rear-range PGH2 to HHT. The dynamic range of the assay was from 30-400 pg/tube, with a sensitivity of approximately 40 pg/tube. The evaluation of the assay was performed by a HPLC-RIA method as well as by correlation with a quantitative HPLC method and correlation with TXB2 concentrations in a blood coagulation study. The assay may be useful for the quantification of HHT in several tissues and body fluids under various physiological conditions and may also help to understand the possible physiological role of HHT in biological processes.

Platelet prostanoid receptors

Prostaglandins (PGs) and thromboxanes are important modulators of platelet activation, and there is strong evidence to support the existence of distinct thromboxane, prostacyclin, PGD2 and PGE2 receptors on the platelet plasma membrane. In this review, each of these platelet prostanoid receptors is discussed in detail, with respect to their receptor pharmacology, molecular biology and signal transduction, and as to any therapeutic implications of the development of specific agonists and/or antagonists. In addition, it considers the possibility that there are separate vascular receptors for 8-epi PGF2 alpha, which are not present on the platelet.

The effects of milrinone on platelets in patients undergoing cardiac surgery

Although amrinone produces thrombocytopenia, no information is available regarding the acute effects of milrinone on platelets. Therefore, we evaluated the effects of milrinone on platelet number and function in cardiac surgical patients. Twenty-seven patients were studied during cardiac surgery requiring cardiopulmonary bypass (CPB). Patients were randomized to receive no milrinone (n = 10), or milrinone (n = 17) at a loading dose of 50-75 micrograms/kg in the CPB circuit followed by 0.5-0.75 micrograms.kg-1.min-1 for 12-24 h. Bleeding times and blood samples for coagulation studies were obtained prior to induction, and at 2 and 24 h after CPB. In both groups, platelet counts decreased significantly from the baseline at 2 and 24 h after CPB, and bleeding time increased significantly from the baseline at 2 and 24 h after CPB. No significant thromboelastoplasty (TEG) changes were observed in either group, and there were no significant differences in platelet aggregation or chest tube drainage between the groups. Acute milrinone administration did not cause significant changes in platelet number or function in patients undergoing cardiac operations requiring CPB, beyond the usual adverse effects of cardiac surgery and CPB.

Cytotoxicity of activated platelets to autologous red blood cells

Gel-filtered human platelets exerted lytic activity on autologous red blood cells (RBC) when they were coincubated at 37 degrees C with platelet-activating agents, such as thrombin, collagen, ADP, LPS or PMA in the absence of plasma. Lysis of activated platelets themselves did not occur during the incubation period examined. Morphological observations showed that RBC exposed to thrombin-activated platelets were fragmented and/or transformed into spherocytes. This haemolytic reaction by thrombin-activated platelets did not occur at 4 degrees C, or in the presence of agents which inhibited glycolysis or elevated intracellular levels of cAMP, indicating that energy-dependent and cAMP-regulated platelet metabolism was required for this reaction. When platelets and RBC were incubated in the same vessel, but were prevented from coming into direct cell to cell contact by means of a membrane barrier, their cytotoxicity was reduced but not eliminated completely. No cytotoxic activity against RBC was detected in platelet-free supernatants obtained by centrifugation after activation of platelets with thrombin. On the contrary, activated and washed platelets retained the activity. These observations suggested that the cytotoxic activity was carried by some diffusible and easily inactivated factors, which were continuously produced and liberated from activated platelets. Cyclo-oxygenase inhibitors inhibited the haemolytic activity of thrombin-activated platelets, suggesting a role for some products of platelet-cyclo-oxygenase pathway in platelet-mediated haemolysis. These results provide the first evidence for a direct role of activated platelets in mediation of RBC-damage in the absence of any plasma factors.

Phosphodiesterase-inhibitors enoximone and piroximone in cardiac surgery: influence on platelet count and function

OBJECTIVE

Some phosphodiesterase (PDE)-inhibitors are believed to alter platelet count and function due to changes in intracellular cAMP. Whether newly developed (specific) PDE-inhibitors negatively influence platelet function in cardiac surgery should be investigated in a randomized study.

METHODS

Eighty patients undergoing aorto-coronary bypass grafting were divided into 4 groups and received either the new PDE-III-inhibitor piroximone (group 1), the PDE-III-inhibitor enoximone (group 2), epinephrine (group 3) or no inotropic support (control). PDE-III-inhibitors were given as a bolus followed by infusion until starting of cardiopulmonary bypass (CPB). In addition to platelet count and a thrombelastogram, platelet function was assessed by aggregometry (ADP, epinephrine, collagen). Measurements were done before, during and after CPB until the 1st postoperative day.

RESULTS

Platelet count and postoperative blood loss did not differ between the groups within the entire investigation period. Maximum aggregation and maximum gradient of platelet aggregation to all stimuli were not changed by either PDE-inhibitor enoximone or piroximone. CPB resulted in a significant decrease of all aggregation variables which was without differences due to treatment. Platelet aggregation recovered in the post-bypass period and exceeded baseline values on the 1st postoperative day.

CONCLUSION

It is concluded that enoximone and the new PDE-III-inhibitor piroximone do not affect platelet function and can be used before CPB without risking platelet-related bleeding in cardiosurgical patients in the perioperative period.

Effect of prostaglandin E2 and 3-morpholinosydnonimine (SIN-1) on arachidonic acid metabolism in fMLP-stimulated rat neutrophils and on thrombin-induced human platelet aggregation

The effects of prostaglandin (PG) E2 and the nitric oxide (NO) donor SIN-1 on leukotriene (LT) release from formyl-methionyl-leucyl-phenylalanine (fMLP) (100 nM)-stimulated rat peritoneal neutrophils (RPN) and on thrombin-induced aggregation of washed human platelets were investigated. Both PGE2 (1-100 nM) and SIN-1 (30-300 microM) inhibited release of LTB4 and cysteinyl-LT from RPN in a concentration-dependent manner. The combined effects of PGE2 and SIN-1 were not greater than expected by summation. On the other hand, the inhibitory effect of SIN-1 (0.5 or 1.0 microM) on platelet aggregation was potentiated by PGE2 (0.3-5 microM) in a concentration-dependent manner, while PGE2 alone in the concentrations used had only marginal effects. The results suggest differential regulation of platelet and leukocyte functions by the mediators PGE2 and NO, which could be relevant for various physiological and pathophysiological conditions.

Stimulation of dense tubular Ca2+ uptake in human platelets by cAMP

Elevation of intracellular cAMP is shown to increase the rate (V) and maximal extent of Ca2+ uptake by the dense tubules in intact human platelets. Elevation of [cAMP] was accomplished by preincubation with the adenylate cyclase activator forskolin or with dibutyryl-cAMP (Bt2-cAMP). The free concentration of Ca2+ in the dense tubular lumen ([Ca2+]dt) was monitored using the fluorescence of chlorotetracycline (CTC) according to protocols developed in this laboratory. The free cytoplasmic Ca2+ concentration ([Ca2+]cyt) was monitored in parallel experiments with quin2. Both [Ca2+]cyt and [Ca2+]dt were analyzed in terms of competition between pump and leak mechanisms in the plasma membrane (PM) and dense tubular membrane (DT). When platelets are incubated in media with approx. 1 microM external Ca2+, [Ca2+]cyt is approx. 50 nM and [Ca2+]dt is very low. When 2 mM external Ca2+ is added, [Ca2+]cyt rises to approx. 100 nM and the process of dense tubular Ca2+ uptake can be resolved. Forskolin (10 microM) and Bt2-cAMP increase the rate of dense tubular Ca2+ uptake (V) to 2.1 +/- 0.60 and 1.70 +/- 40 times control values (respectively). The agents also increase the final [Ca2+]dt to 1.70 +/- 0.21 and 1.72 +/- 0.60 times control values (respectively). Titrations with ionomycin (Iono) showed that the increase was due to an increase in the Vm of the dense tubular Ca2+ pump. With [Iono] = 500 nM, [Ca2+]cyt was raised to greater than or equal to 1.0 microM and Vm of the dense tubular pump was elicited. (At [Iono] = 1.0 microM, the final [Ca2+]dt values were degraded 15% due to shunting of Ca2+ uptake.) Analysis showed that forskolin (10 microM) and Bt2-cAMP (1 mM) increase the Vm by a factors of 1.56 +/- 40 and 1.56 +/- 40, respectively. Analysis showed that neither agent changed the Km of the pump significantly from its control value of 180 nM. Neither agent changed the rate constant for passive leakage of Ca2+ across the DT membrane (1.7 min-1).

Cyclic AMP stimulates Ca(2+)-ATPase-mediated Ca2+ extrusion from human platelets

The effect of cAMP on active Ca2+ extrusion across the plasma membrane of intact human platelets was studied using quin2, a fluorimetric indicator of free Ca2+ in the cytoplasmic compartment ([Ca2+]cyt). Elevations of cAMP were achieved by incubation with dibutyryl-cAMP or by forskolin, which was found to selectively elevate cAMP without affecting cGMP levels. Progress curves of Ca2+ extrusion from quin2-overloaded platelets were measured. The rate vs. [Ca2+]cyt characteristic was calculated as previously described (Johansson, J.S. and Haynes, D.H. (1988) J. Membr. Biol. 104, 147-163). Forskolin, at a maximally effective concentration of 10 microM, was shown to stimulate Ca2+ extrusion by increasing by a factor of 1.6 +/- 0.5 the Vm of a saturable component, previously identified with a Ca(2+)-Mg(2+)-ATPase located in the plasma membrane. Neither the Km (80 nM) or Hill coefficient (1.7 +/- 0.3) of the Ca(2+)-ATPase was affected. Forskolin had no effect on the linear, non-saturable component of extrusion (previously identified with a Na+/Ca2+ exchanger) over the [Ca2+]cyt range examined (50-1500 nM). Dibutyryl-cAMP (Bt2-cAMP, 1 mM) stimulated the Ca(2+)-Mg(2+)-ATPase component of Ca2+ extrusion by a factor of 2.0 +/- 0.6. Separate experiments showed that 10 microM forskolin reduces the resting [Ca2+]cyt from 112 nM to 96 nM. Mathematical analysis showed that this can be accounted for by the above-mentioned increase in Vm of the pump, countered by a 37-74% increase in the rate constant for passive Ca2+ leakage across the plasma membrane. The results suggest two mechanisms by which prostacyclin-induced elevation of cAMP inhibits platelet aggregation: (a) lowering of resting [Ca2+]cyt and (b) increasing the rate of Ca2+ extrusion after the initial influx or triggered release event.

Inherited platelet-storage pool deficiency associated with a high incidence of acute myeloid leukaemia

A family with an inherited bleeding disorder extending over four generations, and multiple cases of myeloblastic and myelomonoblastic leukaemia was studied. Ten members of the family had, by history, a haemorrhagic diathesis. There were three documented cases of myeloblastic leukaemia, two documented cases of myelomonoblastic leukaemia and two more cases of leukaemia by history. In four of the cases the bleeding diathesis clearly antedated the leukaemia, in two by many years. The bleeding disorder is characterized by a long bleeding time, abnormal platelet aggregation, low platelet ADP and decreased numbers of platelet dense bodies consistent with a dense granule storage pool deficiency. The number of dense granules was decreased by immunofluorescence employing quinacrine or using an antibody to the dense granule membrane protein, granulophysin, confirming an absolute decrease in dense granule numbers rather than the presence of empty granule sacs. This congenital storage pool deficiency is associated with a high incidence of acute myeloid leukaemia in this family.

Forskolin stimulates prostaglandin synthesis in rabbit heart by a mechanism that requires calcium and is independent of cyclic AMP

Infusion of forskolin, an adenylate cyclase activator, in concentrations (2 microM) that do not alter basal prostaglandin (PG) synthesis inhibit synthesis of PG elicited by isoproterenol in rabbit heart. This inhibitory action of forskolin appears to be dependent on cyclic AMP (cAMP). Bolus injection of forskolin (75 nmol), however, was found to stimulate PG synthesis in rabbit heart. The purpose of this study was to elucidate the mechanism of the stimulatory action of forskolin on PG synthesis (prostaglandin I2 measured as 6-ketoprostaglandin F1 alpha [6-keto-PGF1 alpha]) in isolated perfused rabbit heart. Forskolin enhanced PG production in a dose-dependent manner. 1,9-Dideoxyforskolin, a forskolin analogue devoid of adenylate cyclase-stimulating activity, also enhanced PG synthesis. The cAMP analogue chlorophenylthio-cAMP failed to stimulate output of 6-keto-PGF1 alpha, although this agent produced dose-related changes in mechanical function in rabbit heart. Furthermore, the adenylate cyclase inhibitor (-)-N6-(R-phenylisopropyl)adenosine potentiated, whereas the phosphodiesterase inhibitor cilostamide attenuated, forskolin-stimulated PG production. (-)-N6-(R-Phenylisopropyl)adenosine and cilostamide had no effect on the mechanical actions of chlorophenylthio-cAMP, suggesting selectivity of these agents for adenylate cyclase and phosphodiesterase, respectively. 6-Keto-PGF1 alpha output elicited by forskolin was abolished by reduction of calcium in the perfusion fluid as well as by the calcium channel blocker diltiazem. The intracellular calcium antagonists TMB-8 and ryanodine also abolished forskolin-stimulated PG synthesis in rabbit heart. PG synthesis stimulated by 1,9-dideoxyforskolin was also prevented by reduced extracellular calcium, diltiazem, and ryanodine. The calmodulin antagonists trifluoperazine, W-7, and calmidazolium failed to significantly alter PG production in response to forskolin. These results indicate that forskolin-stimulated PG synthesis in rabbit heart is independent of cAMP and requires calcium from both extracellular and intracellular sources.

Influence of tea and coffee beverages on prostacyclin synthesis by the rat aorta

Influences of 2.5 and 5% (w/v) aqueous tea and coffee beverages administered ad lib. to rats for two weeks on PGI2 synthesis by the rat thoracic aorta in vitro were investigated using a rat platelet antiaggregatory bioassay and HPLC methods. The 2.5% beverages did not affect PGI2 synthesis; however, the 5% beverages significantly decreased PGI2 synthesis. The observed decreases were significantly abolished in presence of exogenous arachidonic acid suggesting a beverage-induced inhibition of precursor release. The ability of the beverages to inhibit PGI2 synthesis may partly contribute towards better understanding of the biochemical mechanisms underlying some of the beverages-induced actions in vivo.

Histamine alters prostaglandin output from diestrous rat uteri. Involvement of H2-receptors and 9-keto-reductase

The effects of exogenous histamine (H) on prostaglandin (PG) generation and release in uteri isolated from diestrous rats and the influences of H2-receptors blockers (cimetidine and metiamide) on the output of uterine PGs, were explored. Moreover, the action of H on the uterine 9-keto-reductase, was also studied. Histamine (10(-4) M) failed to alter the basal output of PGE1 but reduced significantly the generation and release of PGE2 and augmented the output of PGF2 alpha. On the other hand, cimetidine (10(-5) M) enhanced the basal release of PGE2 but had no action on the outputs of PGs E1 or F2 alpha. The enhancing effect of H on the production and release of PGF2 alpha was abolished in the presence of cimetidine. Also, the antagonist reversed the influence of H on the output of PGE2. Metiamide, another H2-receptor antagonist, did not alter the basal control generation and release of uterine PGs, but antagonized the augmenting influence of H on PGF2 alpha uterine output, as much as cimetidine did, and prevented the depressive action of H on the release of PGE2 from uteri. Histamine (10(-4) M) significantly stimulated uterine formation of cyclic-adenosine monophosphate, an action which was antagonized by the presence of cimetidine (10(-5) M), a blocker of H2 receptors. Also, histamine (10(-5) M) and dibutyrylcyclic-adenosine monophosphate (DB-cAMP) at 10(-3) M, enhanced significantly the formation 3H-PGF2 alpha from 3H-PGE2. Results presented herein demonstrate that H is able to diminish the generation of PGE2 in uteri from rats at diestrus augmenting the synthesis of PGF2 alpha, apparently via the activation of H2-receptors, enhancing adenylate-cyclase. These effects appear to increase uterine 9-keto-reductase activity which transforms PGE2 into PGF2 alpha. Relationships between the foregoing results and those evoked by estradiol, are also discussed.

Effect of amniotic fluid on platelet thromboxane production

We have studied the effect of amniotic fluid on thromboxane A2 (TXA2) production as an initial step in an evaluation of the role of this metabolite as the mediator of the pulmonary hypertension that accompanies perinatal aspiration. Term amniotic fluid enhanced platelet thromboxane B2 (TXB2) production in the presence of the aggregating agents thrombin and arachidonic acid, activity being confined to the lipid fraction. Compared with a baseline production of 1.4 +/- 0.45 pmol TXB2/10(6) platelets in response to thrombin (1 U/ml), unfractionated amniotic fluid or its lipid fraction enhanced TXB2 production to 2.87 +/- 0.53 and 2.81 +/- 0.62 pmol, respectively (P less than 0.01). Values for the aqueous extract were no different from buffer control values (1.14 +/- 0.5). No enhancement of platelet TXB2 production was observed in amniotic fluid obtained at 15 to 17 weeks. Similar activity was observed with either adult or neonatal platelets. This thromboxane enhancing property of amniotic fluid appears to be distinct from its thrombin generating property. Following perinatal aspiration, in situ production of thrombin and proaggregatory TXA2 could recruit more platelets, enhance local TXA2 production, and be responsible for the platelet thrombi that have been documented at autopsy in the pulmonary microcirculation in infants with perinatal aspiration syndrome.

Platelet granule disorders

The present review has cataloged the inherited and acquired disorders of platelet granules. Unfortunately, a mere listing of different conditions in which dense bodies, alpha granules, or both are decreased, absent, or fused does little to define their importance in human platelet physiology or as a causative factor in hemorrhagic disease. The inherited disorders serve as the best index of granule involvement in platelet hemostatic function. Our experience with storage pool deficiency in patients with Hermansky-Pudlak syndrome has suggested that in many individuals virtual absence of dense bodies and their contents does not present a serious threat to hemostasis. Placing HPS patients on aspirin did not cause spontaneous hemorrhage, suggesting that secretion of dense body contents and synthesis of endoperoxides and thromboxane A2 are not absolutely essential for platelet function. However, the literature strongly suggests that many patients with HPS and SPD face a serious risk from bleeding, and hemorrhage may cause death. We can only conclude that some patients with HPS have platelet defects or other hemostatic problems that render SPD a far more serious threat than in other patients who appear to have the same disease. Dense bodies of and by themselves do not appear absolutely required for platelet function. Isolated deficiency of alpha granules presents the same enigma. Only a few patients with this rare inherited disease have been reported. They are generally considered to have mild to severe hemorrhagic problems. However, the past medical history of our two patients with GPS has recently been reviewed and platelet function studies repeated. Despite the mild thrombocytopenia, they are free of any significant bleeding episodes and their platelet function appears virtually normal. Our findings do not support the concept that alpha granules are essential for platelet function. The only condition that seems to support a critical role for storage organelles in hemostasis is the combined alpha-granule, dense body deficiency in one patient reported by Weiss. This patient does have bleeding problems. However, it is difficult to draw conclusions based on a single patient, and the discovery of other patients will help to clarify the hemostatic problem of patients with dual storage organelle deficiencies. In the meantime, we have prepared platelets from normal individuals free of storage granules by sedimentation through gradients containing cytochalasin B. The function of the normal agranular platelets is compromised, but they do respond to some aggregating agents.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation of permeabilized platelets by inositol-1,4,5-trisphosphate

The effect of inositol 1,4,5-trisphosphate (IP3) was studied using human platelets permeabilized with saponin and suspended in a high potassium, Ca2+-free buffer containing 40 microM EGTA and 1.2 mM magnesium. Under these conditions IP3 stimulated aggregation at a concentration of 0.5 microM with maximum aggregation at 5.0 microM. Aggregation was associated with phosphorylation of myosin light chain and a 47,000 dalton protein, and with a change in platelet shape including granule centralization and pseudopod formation similar to changes seen when cytoplasmic calcium is raised by other means. IP3 stimulated [14C]-serotonin release from platelet dense granules, [14C]-arachidonic acid release from platelet phospholipids and production of thromboxane B2. Preincubation of platelets with aspirin which blocked thromboxane formation also inhibited protein phosphorylation, serotonin secretion and partially inhibited aggregation. These results support the concept that IP3 is a major intracellular messenger in platelets and suggests that its effects are mediated both through Ca2+ flux and thromboxane formation.

Effect of bromocriptine on prostacyclin release and cyclic nucleotides on rat aortic and uterine tissues

The effect of bromocriptine mesylate on cyclic nucleotides and PGI2 release by rat aortic and uterine tissues was investigated. Treatment of rats with bromocriptine (10 mg kg-1 I.P. daily for 14 days) increased PGI2 release by the thoracic aorta from 0.67 +/- 0.02 to 1.4 +/- 0.03 ng/mg wet tissue (P less than 0.001; n = 6). This increase was antagonized by treatment with sulpiride (15 mg kg-l). Incubation of the arterial tissue with bromocriptine (50 micrograms ml-1) in vitro also stimulated PGI2 release. Mepacrine (160 micrograms ml-1) significantly decreased both basal and stimulated PGI2 release. Incubation of myometrial tissue from pregnant rats with bromocriptine (50 micrograms ml-1) in vitro significantly decreased PGI2 release from 1.25 +/- 0.07 to 0.60 +/- 0.08 ng/mg wet tissue (P less than 0.05, n = 6). It also elevated uterine cAMP from 40 +/- 2 to 64 +/- 3 pmoles/100 mg wet tissue. Both effects were antagonized by sulpiride. Bromocriptine did not affect uterine cGMP or the cyclic nucleotides in the aorta. It is concluded that the increase in aortic PGI2 was mediated via activation of dopamine D-2 receptors that stimulate phospholipase A2 enzyme. The decrease in myometrial PGI2 release may be related to the increase in uterine cAMP resulting from activation of dopamine D-1 receptors. Previous studies suggested a role for PGI2 in implantation in the rat. The results suggest that the inhibitory effect on uterine PGI2 may underlie the reported inhibition of bromocriptine on implantation. On broad basis, the decrease in uterine PGI2 together with the reported luteolytic effect of bromocriptine point to a potential role for the compound in postcoital contraception.

Dipyridamole: a critical evaluation

Dipyridamol is a vasodilator that is used primarily in clinical practice as an antiplatelet agent. It increases coronary blood flow and was originally introduced as an antianginal agent. An ability to prolong a shortened platelet survival has been used to justify its value in preventing thromboembolic complications. Conditions characterized by a reduction in platelet survival and where dipyridamole has been used include heart valve replacement, arterial grafting, cerebrovascular disorders, and disorders of peripheral circulation. The in vivo effect of dipyridamole on platelet aggregation has not been well defined and may depend on additional factors. Prostaglandins appear to have important roles in platelet homeostasis; their relationships to the action of dipyridamole are discussed. Dipyridamole usually is combined with aspirin for synergistic anti-aggregatory purposes. However, the nature of the interaction has not been elucidated and benefit from the addition of dipyridamole has not been demonstrated in clinical studies. A review of clinical studies using dipyridamole indicates that it currently has limited value.

Oxygenation products of arachidonic acid: third messengers for insulin release

Although an association between membrane phospholipid turnover and exocytotic hormone release has long been recognized, a causal relationship has not been firmly established. Recent studies suggest that glucose (and probably other insulin secretagogues) activates phospholipases and thereby releases membrane-bound arachidonic acid (AA). AA is then converted through islet 12-lipoxygenase to mediators or modulators of insulin release (tentatively identified as peroxides and epoxides of arachidonate). These products may be critical links in stimulus-secretion coupling, since blockade of either AA release or lipoxygenation abrogates insulin release induced by glucose and many other (but not all) stimuli. Cogeneration of prostaglandins from AA through the cyclooxygenase pathway may directly or indirectly modulate the formation and/or effect of lipoxygenase products. A critical role for lipoxygenase products (and possibly metabolites of AA synthesized by other pathways, such as P-450-dependent monooxygenases) may extend to many secretory cells in addition to pancreatic beta cells. The phasic release of AA described in many cells could explain the biphasic pattern of insulin release induced by glucose. Since some phospholipases and lipoxygenases are Ca++ activated, the release of AA in conjunction with its oxygenation appears to be a concerted system generating "third messengers" for hormone release.

Relationship between platelet phospholipase activity and plasma in ischemic heart disease

Platelet phospholipase plays an important role in the metabolic responses of platelets to exogenous stimuli. The platelet phospholipase activity (PLA) was therefore studied in 38 patients with ischemic heart disease (IHD) and in 26 age-matched normal subjects who served as controls. The mean platelet PLA in the IHD group was 12.72 +/- 1.03 nmol/mg protein/30 sec which was significantly (p less than 0.005) higher than that of the normal controls (8.72 +/- 0.76). When they were classified into acute stage, such as unstable angina or acute myocardial infarction (AMI), and chronic stage, such as stable angina or old myocardial infarction (OMI), there was no significant difference between them. On the other hand, about two-fold activation of platelet PLA was observed in acute stage IHD, and 20-30% inhibition of it was demonstrated in chronic stage IHD following the addition of autologous plasma to washed platelet suspensions, suggesting that certain plasma factor(s) are responsible for such phenomena. In an attempt to identify these plasma factor(s), various substances such as serum albumin, high density lipoprotein, prostaglandin E1 (PGE1) and E2 (PGE2), and platelet activating factor were assessed by in vitro experiments. Only PGE1 and PGE2 revealed a significant effect on the platelet PLA. The relationship between plasma and platelet activity in terms of platelet PLA deserves attention since it varies according to the type and stage of IHD.

Theophylline inhibits platelet aggregation, prostaglandin and thromboxane production by a mechanism which is independent of cyclic AMP

Theophylline (0.05-1.0 mM) greatly enhanced the inhibition of platelet aggregation by prostaglandin E1 and at higher concentrations (1.0-4.0 mM) theophylline alone inhibited platelet aggregation, by a mechanism which did not involve significant changes in platelet cAMP levels. These concentrations of theophylline and theophylline in combination with prostaglandin E1, inhibited the synthesis of thromboxane A2, prostaglandin E2 and F2a in stimulated platelets. Theophylline also inhibited arachidonic acid-stimulated platelet aggregation suggesting an effect on cyclooxygenase activity. The calcium ionophore A23187 prevented and reversed the inhibition of platelet aggregation by theophylline, suggesting that theophylline inhibits the rise in intracellular calcium levels which occurs in response to the synthesis of prostaglandin endoperoxides and thromboxane A2.

TXA2-antagonistic properties of agents affecting prostaglandin synthesis or the cyclic nucleotide system in human platelets

Prostaglandins (PG) E1 and E2 as well as 3-isobutyl-1-methylxanthine, nitroprusside, dibutyryl cyclic AMP and N-0164 inhibited platelet aggregation induced by the thromboxane (TX) A2-mimetic prostaglandin endoperoxide analogue U46619. Non-steroidal anti-inflammatory agents - acetylsalicylic acid, indomethacin, tolfenamic acid, flumizole, nictindole and proquazone - did not demonstrate any antagonistic actions on U46619-induced aggregation at concentrations causing inhibition of prostaglandin/thromboxane synthesis-dependent forms of platelet aggregation. Comparing with the effects of the different test substances on ADP-or arachidonic acid-induced platelet aggregation, it can be suggested that PGE2 as well as 3-isobutyl-1-methylxanthine, nitroprusside, and dibutyryl cyclic AMP are functional antagonists and N-0164 is a receptor level antagonist of TXA2 in platelets.

Platelets and vascular smooth muscle: abnormalities of phosphodiesterase, aggregation, and cell growth in experimental and human diabetes

Platelets appear to be involved in both the maintenance of homeostasis and the regulation of proliferation of vascular smooth muscle cells. Anomalies of platelet function may be responsible in part for the pathogenesis of vascular disease in experimental and human diabetes. In a search for an appropriate animal model, we have studied platelet function and the properties of platelet cyclic NCL-PDE in rats with streptozocin-induced diabetes, spontaneous diabetes, and human insulin-dependent diabetes mellitus (IDDM). It appears that opposite abnormalities in both aggregation and phosphodiesterase activity exist in the two animal models. In human IDDM, similar abnormalities to those seen in the BB model were observed. Vascular smooth muscle cells in culture can be used as a model for studies of the effect of circulating growth factors in animals and humans. A growth inhibitory factor was found in plasma and serum from STZ and human IDDM but not BB. In humans we observed increased growth-promoting activity of diabetic platelets, but this phenomenon was absent in both animal models. It remains to be evaluated whether these differences may account for the fact that diabetic rats appear resistant to development of vascular complications. It also remains to be established which animal model is the best choice for studying growth abnormalities in diabetes.

Synergistic activation by collagen and 15-hydroxy-9 alpha,11 alpha-peroxidoprosta-5,13-dienoic acid (PGH2) of phosphatidylinositol metabolism and arachidonic acid release in human platelets

Collagen stimulates the activation of phosphatidylinositol (PI)-specific phospholipase C (EC 3.1.4.10) in human platelets, as manifested by the disappearance of PI, the transient formation of diacylglycerol (DG), and release of myoinositol. Platelets exposed to collagen also form lysophosphatidylinositol (LPI). Maximum formation of DG occurs within 60 s of the addition of collagen and is in proportion to the concentration of collagen provided, up to 100 micrograms/2 x 10(9) platelets/ml. Hydrolysis of PI, formation of DG, and release of arachidonic acid are all inhibited approximately 68% by aspirin or indomethacin, both of which inhibit platelet cyclooxygenase. This inhibition is reversed by the product of cyclooxygenase activity, 15-hydroxy - 9 alpha,11 alpha - peroxidoprosta - 5,13 - dienoic acid (PGH2), or by the PGH2 analogue and agonist, U-46619. The counteracting effects of either PGH2 or the PGH2 analogue can be blocked, in turn, by a PGH2 antagonist, U-51605. Neither PGH2 nor its stable analogue is, by itself, an efficient stimulus for PI breakdown to DG and LPI in platelets. However, in conjunction with collagen, these agents synergistically promote the net breakdown of PI and the release of arachidonic acid in aspirin-treated platelets. Our findings thereby imply that PGH2 has an important role in regulating both the release of its precursor, arachidonic acid, and the metabolism of PI induced by collagen. Dibutyryl cyclic AMP or prostaglandin D2 (PGD2), a prostaglandin that elevates concentrations of cAMP in platelets by stimulating adenylate cyclase, inhibits the hydrolysis of PI induced by collagen by 70%. The activation of PI metabolism by collagen appears to be inhibited by cAMP independently of any effects of this inhibitor on the formation of PGH2.

Prostacyclin prolongs viability of washed human platelets

The functional viability of stored human platelets, washed in the presence and absence of prostacyclin, was examined over a 96 h period. Platelet counts, aggregation responses and cyclic AMP levels were monitored as well as the spontaneous generation of thromboxane B2 and the liberation of labelled oleate from cellular phosphatides. In suspensions prepared without prostacyclin in the medium, platelet counts declined rapidly as did the sensitivity to aggregating agents. In addition, substantial amounts of thromboxane B2 were generated during preparation and storage and oleate liberation occurred at a rapid rate. In contrast, in prostacyclin-washed platelets, aggregation was maintained throughout the study period and there was little generation of thromboxane B2. Moreover, only a gradual decrease in platelet count and a slow increase in the rate of oleate liberation was observed when compared with controls. However, cyclic AMP levels rapidly declined when platelets were resuspended in prostacyclin-free medium.

A hypothesis for the mechanism of superoxide production by phagocytic cells

When arachidonic acid is reacted with Fe2+, the Fe2+ is oxidized to Fe3+, and lipid peroxides are formed as detected by absorption at 232 nm. However, there is a large discrepancy between the amount of arachidonic acid converted to stable oxidized products and the amount of Fe2+ converted to Fe3+. We suggest that under appropriate conditions, electrons from the ferrous iron may reduce oxygen to superoxide in the presence of arachidonic acid. The results are consistent with evidence using bromophenacyl-bromide suggesting that release of fatty acids from cell phospholipids might be required for phagocytes to synthesize superoxide.

Lithium stimulates thromboxane B2 formation in human platelets

The aggregation of human platelets is enhanced after preincubations with lithium salts. Previously it was shown that lithium inhibits adenylate cyclase activity in human platelets. Enhancement of aggregation and inhibition of adenylate cyclase by lithium was not observed in rabbit platelets. In this paper it is shown that in human platelets lithium enhances considerably the synthesis of thromboxane B2, whereas the synthesis of PGE2 and PGF2 alpha was enhanced to a lesser extent. In rabbit platelets lithium had no effect on prostaglandin synthesis. It is concluded that lithium specifically stimulates the synthesis of thromboxanes in human platelets, probably due to the previously reported inhibition of adenylate cyclase activity, resulting in an increased aggregability.

Effect of endotoxin on arachidonic acid release and thromboxane B2 production by human platelets

Plasma thromboxane A2, which is elevated during endotoxemia, has previously been shown to be a major factor contributing to the mortality and morbidity that occurs in endotoxin shock in the experimental animal. Using a minimal dose of Escherichia coli endotoxin (1 microgram/ml), we have demonstrated that the preincubation of human platelets with endotoxin induces changes in platelet arachidonic acid release and the subsequent conversion of the released arachidonic acid to thromboxane B2, and stable end product of thromboxane A2. In paired experiments, in the presence of endotoxin, the addition of the aggregating agent thrombin (0.5 U/ml) caused human platelets to release 29.1 +/- 3.4% of 14C-arachidonic acid from prelabeled platelet phospholipids. This value was significantly elevated (p less than 0.02) when compared with the release of 14C-arachidonic acid from platelets in the absence of endotoxin (21.9 +/- 3.6%). Similarly, comparison of the results of the conversion of the released arachidonic acid to platelet thromboxane B2 (TxB2) revealed that TxB2 production was significantly increased (P less than 0.01) when human platelets were preincubated with endotoxin prior to the addition of thrombin (6.1 +/- 0.6%) when compared with TxB2 formation observed in the absence of endotoxin (3.4 +/- 0.5%). The absolute amount of released arachidonic acid that was converted to TxB2 in the presence of endotoxin (1.8 +/- 0.3%) was also significantly higher (P less than 0.01) than the value observed in its absence (0.8 +/- 0.2%) was also significantly higher (P less than 0.01) than the value observed in its absence (0.8 +/- 0.2%). This study suggests that one of the tissue sources of the proaggregatory vasoconstrictor thromboxane A2 during endotoxemia is the platelet.

Stimulation of platelet protein phosphorylation by arachidonic acid and endoperoxide analogs

The present study has investigated the influence of arachidonate, endoperoxide analogs, and the calcium ionophore A23187 on platelet aggregation and on the phosphorylation of platelet proteins. Following stimulation of platelets by these agents a rapid increase in phosphorylation of three proteins was observed which began at the same time as the initial formation of platelet aggregates. These three proteins were the 260,000 dalton actin-binding protein, a 40,000 dalton protein in unknown function, and the 20,000 dalton myosin light chain. When extensive aggregation was reached, the extent of phosphorylation returned toward baseline. Pretreatment of platelets with aspirin completely inhibited both aggregation and protein phosphorylations induced by arachidonate, but had only partial inhibitory effects on endoperoxide analogs or A23187. Since endoperoxide analogs and A23187 may trigger endogenous production of prostaglandin endoperoxides and thromboxane A2, in addition to having a direct effect of their own, it is probable that the partial inhibition seen was due to inhibition of that component of their effect due to this endogenous production, though other effects of aspirin can not be entirely ruled out. Since recent evidence shows that phosphorylation of myosin light chain results from calcium stimulation of a protein kinase in the presence of calmodulin, the results are consistent with mobilization of calcium as the primary role of the arachidonate-endoperoxide-thromboxane pathway.

Interactions of zinc and arachidonic acid

To probe the interaction of zinc with polyunsaturated fatty acids we have studied the effect of zinc on the cooxygenation of ferrous iron and arachidonic acid. Zinc inhibited the process of cooxygenation in a concentration dependent fashion. Further evaluation of the interaction of zinc and arachidonic acid gave spectroscopic evidence that zinc, oxygen and arachidonic acid can form an unstable hydroperoxide-like complex similar to that postulated earlier for iron, oxygen and arachidonic acid. However, in the case of zinc the complex will not proceed further to form stable peroxides and the unstable complex falls apart to give zinc and arachidonic acid intact. The findings have implications for the role of zinc in enzyme reactions and for antioxidant reactions of zinc within the cell. The influence of zinc on platelet aggregation was also evaluated. Zinc was found to inhibit cell-cell aggregation. However, in contrast to the known ability of zinc to inhibit prostaglandin synthesis in a broken cell preparation, zinc did not inhibit prostaglandin or thromboxane synthesis in the intact platelet. Inhibition of platelet aggregation by zinc must result from some other action of this cation.

The phosphatidylinositol cycle and the regulation of arachidonic acid production

An increase in the metabolism of phosphatidylinositol occurs in a wide variety of tissues by the action of specific ligands^1–3. In platelets, the interaction of thrombin with its receptor initiates the degradation of phosphatidylinositol by the action of a specific phospholipase C (refs 4–8). In normal conditions of stimulation, the resultant 1,2-diacylglycerol is rapidly and completely phosphorylated to phosphatidic acid^4–11. The formation of phosphatidic acid precedes the release of arachidonic acid from the phospholipids of stimulated platelets⁵. This early appearence of phosphatidate might result in the initial production of arachidonic acid and lysophosphatidic acid by the action of a phospholipase A₂ specific for phosphatidate¹². Phosphatidate/lysophosphatidate could induce calciumgating^13–15 and subsequently stimulate phospholipases of the A₂-type⁸, that degrade phosphatidylcholine, phosphatidyl-ethanolamine and a further fraction of phosphatidylinositol⁶. Alternatively, the lysophosphatidate produced may serve as a substrate for the transfer of arachidonate directly from other phospholipids^16,17 to form new phosphatidate which in turn can release more arachidonate. Overall, such a sequence would be equivalent to phospholipase A₂ activation of other phospholipids. Our present data indicate that when the release of arachidonic acid is completely inhibited by cyclic AMP or quinacrine, phosphatidic acid is redirected entirely to phosphatidylinositol and there is no production of arachidonate. In these conditions, the availability of calcium might be profoundly restricted. The correlation in platelets of a phosphatidylinositol by a specific phospholipase A₂ might suggest that these phenomena are applicable to activations in other cell systems.

Synthesis and biological evaluation of 2.3-diazabicyclo[2.2.1]heptane derivatives as prostaglandin endoperoxide analogs

Three prostaglandin endoperoxide analogs that possess the 2,3-diazabicyclo[2.2.1]heptane skeleton were synthesized and evaluated for activity in human platelets and in the rat fundus. All three compounds were inactive in inhibiting or stimulating platelet aggregation. One compound possessed weak contracting activity on the rat fundus.

Influence of trifluoperazine on platelet aggregation and disaggregation

Calmodulin has been shown to activate platelet phospholipase A2 and initiate the metabolism of arachidonic acid. Trifluoperazine (stelazine), an agent which selectively binds calmodulin, inhibits phospholipase activity induced by calcium. We have evaluated the effect of this agent on the aggregation, disaggregation and reaggregation of human platelets. Results of our study have shown that stelazine is a selective inhibitor of the second wave response of platelets to the stimulation of agonists. The compound causes dissociation of aggregated platelet clumps and induces a refractory state in dispersed cells. Epinephrine reverses the refractory state and potentiates the response of dissociated cells to the action of thrombin, arachidonate and ionophore A23187. Stelazine elevates cAMP levels to some extent and inhibits release of labeled arachidonic acid from platelet membranes in response to thrombin induced stimulation. Epinephrine may correct stelazine induced inhibition by acting through alpha-adrenergic mechanism, by inducing changes in membrane permeability to calcium, by releasing membrane associated calcium or by directly promoting deacylation of phospholipids.

Biochemical studies of two patients with the gray platelet syndrome. Selective deficiency of platelet alpha granules

The biochemistry of platelets from two unrelated patients with the gray platelet syndrome, a deficiency of platelet alpha-granules, has been evaluated. Ultrastructural studies of their platelets revealed the number of alpha-granules to be less than 15% of normal, whereas the number of dense bodies was within normal limits. Platelets from both patients had severe deficiencies of platelet factor 4 and beta-thromboglobulin (less than 10% of normal). Sodium dodecyl sulfate-polyacrylamide gel electrophoresis showed a marked deficiency of thrombin-sensitive protein in both patients. Analysis of the platelet-derived growth factor in one patient showed it was also markedly reduced. Levels of lysosomal enzymes, adenine nucleotides, serotonin, and catalase, and conversion of arachidonic acid by the lipoxygenase and cyclo-oxygenase enzymes, were within normal limits. The results provide important evidence to define the contents of alpha-granules and to differentiate these contents from the contents of lysosomal granules, dense bodies, and peroxisomes. Functional studies of these platelets showed deficiencies in ADP, thrombin, and collagen aggregation. The results suggest that alpha-granules or their contents make a contribution to normal platelet aggregation.

1-arachidonyl-monoglyceride causes platelet aggregation: indirect evidence for an acylglycerol acylhydrolase involvement in the release of arachidonic acid for prostaglandin synthesis

Phosphatidylcholine liposomes containing 1-arachidonyl-monoglyceride were found to cause aggregation of human platelets. In contrast, addition of phosphatidylcholine liposomes, 1-arachidonyl-monoglyceride, or phosphatidylcholine liposomes containing I-oleoyl-monoglyceride to a similar platelet preparation had no effect. Aggregation stimulated by 1-arachidonyl-monoglyceride was inhibited by 100 microM aspirin or 1 microM indomethacin, suggesting that the arachidonic acid is first released by; a platelet acylglycerol acylhydrolase and then converted to PGG2 and thromboxane A2 which initiate the platelet aggregation. Changes in platelet morphology in response to 1-arachidonyl-monoglyceride were similar to those reported previously to occur following stimulation of platelets by arachidonic acid or PGG2 providing further support for this concept. EDTA inhibited aggregation of platelets but no shape change or granule centralization in response to 1-arach-idonyl-monoglyceride. PGE1 and theophylline inhibited both aggregation and morphological changes. These results with inhibitors are similar to the effects of these inhibitors on PGG2 and provide further evidence for similarity between the action of 1-arachidonyl-monoglyceride and PGG2. The results provide important evidence to support the concept that an acylglycerol acylhydrolase may be involved in arachidonic acid release and platelet aggregation.