Synthesis of thromboxane A2 by non-aggregating dog platelets challenged with arachidonic acid or with prostaglandin H2

Epinephrine correction of impaired platelet thromboxane receptor signaling

This study evaluated the mechanism of epinephrine potentiation of platelet secretion induced by thromboxane A(2) (TXA(2)). Dog platelets that do not secrete in response to TXA(2) alone (TXA(2)-) were compared with dog platelets that do secrete (TXA(2)+) and with human platelets. TXA(2)- platelets had impaired TXA(2) receptor (TP receptor)-G protein coupling, indicated by 1) impaired stimulated GTPase activity, 2) elevated basal guanosine 5'-O-(3-thiotriphosphate) binding, and 3) elevated Galpha(q) palmitate turnover that was corrected by preexposure to epinephrine. Kinetic agonist binding studies revealed biphasic dog and human platelet TP receptor association and dissociation. TXA(2)- and TP receptor-desensitized TXA(2)+ dog and human platelets had altered ligand binding parameters compared with untreated TXA(2)+ or human platelets. These parameters were reversed, along with impaired secretion, by epinephrine. Basal phosphorylation of TXA(2)- platelet TP receptors was elevated 60% and was normalized by epinephrine. Epinephrine potentiates platelet secretion stimulated by TXA(2) by reducing basal TP receptor phosphorylation and facilitating TP receptor-G protein coupling in TXA(2)- platelets and, probably, in normal platelets as well.

Thromboxane-insensitive dog platelets have impaired activation of phospholipase C due to receptor-linked G protein dysfunction

Human platelet thromboxane A2/prostaglandin H2 (TXA2/PGH2) receptors are linked to phosphoinositide-specific phospholipase C (PI-PLC) via a G protein tentatively identified as a member of the Gq class. In contrast, platelet thrombin receptors appear to activate PI-PLC via other unidentified G proteins. Platelets from most dogs are TXA2 insensitive (TXA2-); i.e., they do not aggregate irreversibly or secrete although they bind TXA2, but they respond normally to thrombin. In contrast, a minority of dogs have TXA2-sensitive (TXA2+) platelets that are responsive to TXA2. To determine the mechanism responsible for TXA2- platelets, we evaluated receptor activation of PI-PLC. Equilibrium binding of TXA2/PGH2 receptor agonists, [125I]BOP and [3H]U46619, and antagonist, [3H]SQ29,548, revealed comparable high-affinity binding to TXA2-, TXA2+, and human platelets. U46619-induced PI-PLC activation was impaired in TXA2- platelets as evidenced by reduced (a) phosphorylation of the 47-kD substrate of protein kinase C, (b) phosphatidic acid (PA) formation, (c) rise in cytosolic calcium concentration, and (d) inositol 1,4,5 trisphosphate (IP3) formation, while thrombin-induced PI-PLC activation was not impaired. GTPase activity stimulated by U46619, but not by thrombin, was markedly reduced in TXA2- platelets. Antisera to Gq class alpha subunits abolished U46619-induced GTPase activity in TXA2-, TXA2+, and human platelets. Direct G protein stimulation by GTP gamma S yielded significantly less PA and IP3 in TXA2- platelets. Immunotransfer blotting revealed comparable quantities of Gq class alpha-subunits in all three platelet types. Thus, TXA2- dog platelets have impaired PI-PLC activation in response to TXA2/PGH2 receptor agonists secondary to G protein dysfunction, presumably involving a member of the Gq class.

In vivo characterization of synthetic thromboxane A2 in canine myocardium

Recently, total chemical synthesis of thromboxane was achieved. The in vitro activity of synthetic thromboxane A2 is indistinguishable from biologically generated material. The present study describes the in vivo characterization of synthetic thromboxane A2 on the regional blood flow distribution of the canine heart. Local injections of synthetic thromboxane A2 into the coronary vasculature caused marked reductions in coronary blood flow, measured by both radiolabeled microsphere injection and an electromagnetic flow device. The threshold concentration required to bring about this effect varied greatly between dogs and ranged from 0.125 microgram/kg to 2.0 micrograms/kg. Similarly, the dose of thromboxane A2 required to aggregate dog platelets in vitro varied from 30 ng/ml to 1,000 ng/ml. Bolus injections of 2 micrograms/ml thromboxane A2 into the circumflex or left anterior coronary artery resulted in a simultaneous reduction in platelet count in coronary sinus blood of 83 +/- 5.2% (mean +/- SEM, n = 4, p = .0005). Both flow reduction and platelet effects were transient and localized. The time taken from onset to recovery of the response to control levels was 77 +/- 6.0 seconds (mean +/- SEM) for flow and 70-80 seconds for platelet count. Injections of thromboxane A2 caused a small but significant increase in heart rate with no change in systemic blood pressure. In conclusion, the in vivo actions of synthetic thromboxane A2 are consistent with the vasoconstrictor and platelet aggregatory effects seen in vitro, but dogs vary considerably in their sensitivity.

Synergistic actions of PAF-acether and sodium arachidonate in human platelet aggregation. 1. Studies in normal human platelet rich plasma

The effect of sodium arachidonate and paf-acether in the activation of human platelet was studied. Concentrations of paf-acether which induced a reversible aggregation in normal human platelet rich plasma (0.029-0.0029 microM) and subthreshold concentrations of sodium arachidonate (0.25-0.35 mM), produced full aggregation when added together. Pre-exposition of platelets to paf-acether that renders them insensitive to paf-acether supresed the synergism. With full aggregation a markedly increase of thromboxane synthesis was detected by RIA. In vitro addition of aspirin (200 micrograms/ml) or indomethacin (12 microM) prevented aggregation and thromboxane formation by the joint action of sodium arachidonate plus paf-acether. Specific inhibition of 12-lipoxygenase by esculetin (10 microM) did not affect the synergistic action of paf-ace-ther and sodium arachidonate. These findings suggest that synergism between both agonists is mediated by active derivatives of arachidonic acid via cyclooxygenase.

Inhibition of platelet aggregation by monoclonal antibody reactive with beta 2-microglobulin chain of HLA complex

A mouse monoclonal antibody that reacts with beta 2-microglobulin, the light chain of class I major histocompatibility antigens, inhibited the second wave of human platelet aggregation induced by adenosine diphosphate and epinephrine and blocked aggregation and platelet protein phosphorylation induced by sodium arachidonate. Thrombin-induced platelet aggregation was inhibited at threshold concentrations but not at higher concentrations. The antibody also inhibited aggregation and secretion in response to thromboxane A2 or the stable endoperoxide analog, U46619. These results suggest that beta 2-microglobulin in the histocompatibility complex is intimately associated with transmission of the endoperoxide-thromboxane signal at the platelet membrane.

The lack of correlation between inhibition of aggregation and cAMP levels with canine platelets

Arachidonic acid (AA) induced aggregation of canine platelets can be inhibited by various phosphodiesterase inhibitors (PDIs) with the order of potency IBMX greater than or equal to papaverine greater than Ro 20-1724 greater than theophylline. With aggregation induced by AA plus epinephrine (EPI), only IBMX and papaverine inhibited at 100 microM. None of these PDIs affected the basal cAMP levels but all potentiated the PGE1-stimulated cAMP production, with the order of potency being Ro 20-1724 greater than papaverine greater than IBMX greater than theophylline. PGE1 at 1 microM caused a sharp increase in cAMP and complete inhibition of platelet aggregation induced by AA plus EPI. However, when EPI was added before PGE1, there was no elevation of cAMP yet inhibition of aggregation still occurred. Our results indicated that inhibition of platelet aggregation does not require a measurable increase in cAMP.

Lack of thromboxane A2 involvement in the arrhythmias occurring during acute myocardial ischemia in dogs

Coronary artery occlusion (CAO) followed by reperfusion of the ischemic myocardium has been associated with the onset of ventricular arrhythmias. It has been suggested that platelet aggregates in the ischemic area may release thromboxane A2 (TxA2) which may then be responsible for the arrhythmias that occur during reperfusion. To study this possibility, the effect of TxA2 synthetase inhibition on arrhythmias was examined in anesthetized dogs during occlusion and for 60 minutes following release. Imidazole (30 mg/kg) was infused intravenously for 10 minutes, followed by continuous infusion of 100 mg/kg/hr for 125 minutes. The left anterior descending coronary artery was occluded, 5 minutes after the initial dose, for 60 minutes. Three minutes after release of CAO, TxB2 concentrations were significantly higher in the arterial blood of vehicle-treated animals (2.06 +/- 0.53 pmoles/ml) than in either CAO + imidazole (0.66 +/- 0.16 pmoles/ml) or sham-CAO animals receiving imidazole (0.66 +/- 0.09 pmoles/l). However, CAO dogs whether receiving imidazole or 0.9% NaCl generated a significantly greater number of ectopic beats during and after occlusion than sham-CAO animals. Therefore, release of TxA2 does not appear to be a major causative factor in the generation of reperfusion arrhythmias in dogs following coronary artery occlusion.

Modification of human platelet response to sodium arachidonate by membrane modulation

Previous studies have shown that the majority of mongrel dogs have platelets that do not aggregate when stirred with sodium arachidonate, but respond normally to other aggregating agents. Here we have created a model in which human platelets mimic the unusual behavior of canine cells. Brief exposure of human platelets to low concentrations of prostacyclin will convert them to a physiologic state resembling that of canine platelets without causing elevation of intracellular levels of cAMP. Studies on the conversion of arachidonic acid showed that prostacyclin did not cause any inhibition of thromboxane B2 generation. Exposure of prostacyclin treated platelets to epinephrine restored their sensitivity to arachidonate without causing any detectable changes in levels of cAMP. The selective inhibition achieved by PGI2 could be mimicked by the endoperoxide analog. U44069, a thromboxane inhibitor, U-51605, and an endoperoxide/thromboxane receptor blocker, 13-APA. Inhibition induced by these compounds was also reversed by epinephrine. A calcium channel blocker, verapamil, and an alpha blocker, dihydroergocryptine, effectively blocked the corrective influence of epinephrine on prostacyclin-treated platelets. Results of these studies show that catecholamines and prostaglandin receptors share close sites on the membrane and exhibit a degree of cooperativity in calcium modulation. Dog platelets that do not respond to products of AA metabolism and the human platelets made to behave like canine platelets may have a defect in calcium mobilization and this defect is corrected by adrenaline through an alpha receptor modulation.

Relative effects of aspirin on platelet aggregation and prostaglandin-mediated coronary vasodilatation in the dog

Aspirin, as an inhibitor of platelet aggregation, may be of benefit in ischemic heart disease. However, aspirin blocks not only platelet aggregation but also synthesis of prostacyclin, a vasodilator and platelet deaggregator. The relative sensitivity of prostaglandin-mediated coronary vasodilatation and platelet aggregation to inhibition by aspirin remains uncertain. We therefore investigated the relative dose-response relationship of aspirin on arachidonic acid-induced increments in coronary blood flow and on ADP-induced aggregation of platelets. In 11 open-chest dogs, intracoronary arachidonic acid, 0.1-3.0 mg, produced dose-related increases in coronary blood flow that were inhibited progressively by i.v. aspirin over the dose range 0.3-3.0 mg/kg. Aspirin at 3 mg/kg almost completely obliterated the response to 3 mg of arachidonic acid. Similarly, aspirin doses of 0.3-3.0 mg/kg progressively raised the minimal concentration of ADP necessary for platelet aggregation. The threshold concentration of ADP that produced aggregation of platelets from 10 control dogs ranged from 2.3 x 10(-6) M to 1.2 x 10(-5) M. Aspirin at 3 mg/kg completely inhibited aggregation of platelets from 11 of 12 dogs, even with ADP at 2.3 x 10(-4) M concentration, the maximum tested. Aspirin at 0.1 mg/kg failed to inhibit either ADP-induced platelet aggregation or arachidonic acid-induced increments in coronary blood flow. Thus, the two test systems showed similar sensitivity to inhibition by aspirin with respect to threshold dose and maximal effect. These results show that very low doses of aspirin inhibit arachidonic acid-induced coronary vasodilatation and that aspirin at low doses does not appear to selectively inhibit platelet activity relative to coronary vasodilatation.

Effects of prostaglandins and thromboxane A2 on the coronary circulation of adult dogs and puppies

Intracoronary injections of prostaglandin endoperoxide (PGH2), prostacyclin (PGI2) and thromboxane A2 (TXA2) were given to adult dogs and puppies three months or younger. Both PGH2 and PGI2 caused dose-related coronary vasodilation in the adult as well as in the puppy dogs; and PGH2 was about twice as potent as PGI2. The threshold dose of PGH2 for coronary vasodilation, 0.01-0.02 microgram, was the same for the adult and puppy dogs although control coronary blood flow of the adult dog was 3-10 times higher. In the pupply, maximal coronary vasodilation was effected with high doses (1.0 microgram or more) of PGI2, but not with PGH2. TXA2 prepared from PGH2 up to 1.0 microgram, had no effects on the coronary circulation of the adult dog. In contrast, both the vasocon-stricting and platelet aggregating actions of TXA2 were demonstrated in the puppy. Mechanisms for the observed age-dependent differences of the canine coronary circulation to PGH2 quantitatively, and to TXA2 qualitatively, remain to be determined.

Biotransformation and cardiovascular effects of arachidonic acid in the dog

The biotransformation and cardiovascular effects of arachidonic acid (AA) were studied in the circulation of anaesthetized dogs. Arterial blood was continuously bioassayed for arachidonate metabolites using the blood-bathed organ technique of Vane. AA (5-10 microgram/ml) infused into an incubation coil of flowing blood was converted into a labile substance which contracted the vascular tissues (rabbit aorta, RbA; rabbit coeliac and mesenteric arteries, RbCA and RbMA; bovine coronary artery, BCA) and the gastrointestinal smooth muscle strips (rat stomach strip, RSS; rat colon, RC). These effects could be mimicked by exogenously generated thromboxane A2 (TXA2). Conversion of AA was inhibited by indomethacin and the selective thromboxane synthetase inhibitor, imidazole (100 microgram/ml). The half-life of TXA2 in blood was 30-47 sec, a similar value to that found in aqueous solutions at 37 degrees C. PGH2 was also converted in blood to other product(s) which contracted RSS and RC, relaxed RbCA and RbMA but had little effect on RbA. Intravenous infusion of AA (50-800 microgram kg-1 min-1) caused effects on the bioassay tissues which could be mimicked by prostacyclin. The AA infusion also induced falls in pulmonary and systemic arterial pressures and bradycardia. All effects were abolished by indomethacin (5 mg/kg) or aspirin (200 mg/kg). Radioimmunoassay confirmed that the major product of intravenously infused AA was 6-oxo-PGF1alpha, the chemical degradation product of prostacyclin. Thus, although AA is transformed to the vasoconstrictor TXA2 when incubated for sufficient time with blood alone, on rapid pulmonary transit it is transformed into a prostacyclin-like substance.

The effect of 5,8,11,14-eicosatetraynoic acid on lipid metabolism

The purpose of this presentation is to review the current state of knowledge regarding 5,8,11,14-eicosatetraynoic acid (ETYA, Ro 3-1428) and its effects on lipid metabolism. Accordingly, the topics discussed include hypocholesterolemic and dermatological studies involving ETYA in both animals and man, as well as the effects of ETYA on desaturase enzymes. Metabolic studies involving ETYA are also noted. Primary interest is focused on the effects of ETYA on selected processes of arachidonate metabolism, and the effect of ETYA on inflammation, platelet aggregation and tumor growth are discussed, keeping in mind the relevance of arachidonate metabolism to these processes.