On the multiplicity of platelet prostaglandin receptors. I. Evaluation of competitive antagonism by aggregometry

Methods for the evaluation of competitive interactions at receptors associated with platelet activation and inhibition using aggregometry of human PRP have been developed. The evidence supports the suggestion that PGE1 and PGI2 share a common receptor for inhibition of platelet reactivity, but only a portion (if any) of the aggregation stimulation associated with PGE2 is the result of PGE2 binding (without efficacy) to this receptor. PGE2 (at .3-20 microM) is an effective antagonist of PGE1, PGI2, and PGD2 producing a shift of about one order of magnitude in the IC50-values obtained from complete aggregation inhibition dose response curves. The antagonism of PGD2 inhibition is particularly notable, 80 nM PGE2 levels are detectable. This and other actions of PGE2 indicate another platelet receptor for PGE2. PGE1 acts at both the PGE2 and PGI2 receptor. Other substances showing PGI2-like actions only at high doses (1-30 microM), display additive responses with PGI2 indicative of decreased affinity for the I2/E1 receptor and the absence of PGE2-like aggregation stimulation activity. PGI2 methyl ester has intrinsic inhibitory action not associated with in situ ester hydrolysis. The methyl ester is dissaggregatory showing particular specificity for inhibition of release and second wave aggregation.     

Reversal of indomethacin-induced inhibition of tubuloglomerular feedback by prostaglandin infusion

Experiments were performed in rats to study the effect of infusion of PGI₂, PGE₂, and PGF_2α on tubuloglomerular feedback responses (i.e. the change of SNGFR in response to a change of loop of Henle flow rate) in the presence and absence of simultaneous inhibition of endogenous PG synthesis with indomethacin. Infusion of PGI₂ or PGE₂ at rates that did not alter arterial blood pressure did not significantly modify the magnitude of feedback responses (PGI₂) 8.5 μg/hr, PGE₂ 85 μg/hr). Some inhibition of feedback responses was seen when PGI₂ and PGE₂ were administered at higher rates were associated with a reduction of blood pressure (PGI₂ 20 μg/hr, PGE₂ 200 μg/hr). PGI₂ (8.5 μg/hr) and PGE₂ (85 μg/hr) largely prevented feedback inhibition induced by indomethacin. When given subsequent to indomethacin PGI₂ and PGE₂ restored feedback responsiveness almost to normal. In contrast, PGF_2α did not influence feedback inhibition caused by indomethacin. Infusion of PGI₂ induced partial restoration of feedback responses in DOCA-salt treated animals in which the feedback system is virtually completely inactive. Our results indicate that availability of PGI₂ or PGE₂ is necessary for the normal operation of the tubuloglomerular feedback mechanism for control of nephron filtration rate.     

Influences of prostaglandins on electrophoretic mobility and aggregation of rabbit platelets

Influences of prostaglandin(PG)s on electrophoretic mobilities and aggregation of rabbit platelets were studied. The PGs studied (PGI₂, PGE₁, PGD₂, PGE₂, PGF_2α, PGA₂ and PGA₁) had no effect on platelet electrophoretic mobility. However, both PGE₁ and PGI₂ in 0.3 and 3.0 μM inhibited ADP-induced aggregation and ADP-induced decrease in the mobility. PGD₂ in 0.3 and 3.0, and PGE₂ in 30 μM inhibited the aggregation but did not depress the ADP-induced decrease in the mobility. PGF_2α, PGA₂ and PGA₁ had no effect on the decrease in electrophoretic mobility and on the aggregation caused by ADP.     

Effect of dipyridamole of prostaglandin generation by human platelets and vessel walls

These experiments were conducted to determine the effects of dipyridemole on human platelet aggregation, platelet thromboxane A₂ (TXA₂) and human vessel wall prostacyclin (PGI₂) generation. Dipyridamole in varying concentrations (5 to 50 μg/ml) had no direct effect on ADP-induced platelet aggregation in vitro, but it potentiated PGI₂-induced platelet aggregation inhibition at these concentrations. Dipyridamole also inhibited arachidonic acid-induced platelet TXA₂ generation at these concentrations. In continuously perfused umbilical vein segments, dipyridamole treatment resulted in stimulation of PGI₂ release determined by bioassay and by measurement of its stable metabolite 6-keto-PGF_1α. Minimum concentration of dipyridamole causing PGI₂ release was 50 μg/ml. These in vitro studies suggest that anti-thrombotic effects of dipyridamole in man are mediated mainly by potentiation of PGI₂ activity and to some extent by TXA₂ suppression. Stimulation of PGI₂ release by human vessels may not be seen in usual therapeutic concentrations.     

The use of stable prostaglandins to investigate prostacyclin (PGI2)-binding sites and PGI2-sensitive adenylate cyclase in human platelet membranes

Prostacyclin, (PGI₂) is a potent but unstable inhibitor of platelet aggregation, probably acting through stimulation of adenylate cyclase.A stable analogue of prostacyclin with antiaggregatory properties, 5,6-dihydro-PGI₂ (6β-PGI), and PGE₁ can compete for the binding sites labelled by ³H-PGI₂ in human platelet membranes (the affinity being PGI₂ > PGE₁ > 6β -PGI₁). Both 6β-PGI₁ and PGE₁, as well as PGI₂, bind to two classes of binding sites. 6β -PGI₁ and PGE₁ activate adenylate cyclase to the same extent as PGI₂,with a rank order of potency which parallels that observed in binding experiments. The stimulation of this enzyme is brought about by interaction of each these prostanoids with two different classes of components. The comparison of binding and adenylate cyclase data suggests that the sites to which PGI₂, 6β -PGI₁ and PGE₁ bind might be coupled to the activation of adenylate cyclase. Since 6β-PGI₁ seems to act through the same molecular mechanisms as PGI₂, because of its stability it is an useful tool to investigate the mode of action of prostacyclin in platelets.     

Potentiation of anti-aggregating activity of PGI2 by 7-ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1(2H)-phthalazinone (EG-626) in rabbit platelets in vitro

Anti-aggregating activity of 7-ethoxycarbonyl-6,8-dimethyl-4-hydroxymethyl-1(2H)-phthalazinone (EG-626) was tested using rabbit platelets in vitro. EG-626 alone, when added before, prevented platelet aggregation induced by ADP, as did PGI₂, papaverine and dipyridamole. Spontaneous disaggregation was also accelerated when EG-626 was added after the maximal aggregation induced by ADP. EG-626 alone also inhibited platelet aggregation induced by collagen and arachidonic acid. ID₅₀s of these agents in ADP-induced aggregation were 7–9 nM for PGI₂, 223 μM for EG-626, 266 μM for papaverine and 957 μM for dipyridamole. When EG-626 was used in combination with PGI₂, a threshold dose (50 μM) of EG-626 potentiated the anti-aggregating effect of subthreshold dose (3 nM) of PGI₂ upto 100% inhibition in collagen-induced platelet aggregation. The marked potentiating effect of EG-626 was accompanied by an accumulation of cyclic AMP in the platelets. These effects might be due to inhibition of phosphodiesterase. Papaverine and dipyridamole, other phosphodiesterase inhibitors, also potentiated the anti-aggregating activity of PGI₂. The activity of papaverine, however, was one eighth of EG-626 and that of dipyridamole was much less. The most effective combination of PGI₂ and EG-626 to induce 50% inhibition was obtained with 20% of ID₅₀ of each agent, whereas that of PGI₂ and papaverine or dipyridamole was 39 or 41%, respectively.     

A comparison of some pharmacological actions of prostaglandin E1, 6-OXO-PGE1 and PGI2

Some pharmacological actions of prostaglandin E₁ (PGE₁), 6-oxo-PGE₁ and PGI₂ have been studied. 6-oxo-PGE₁ and PGI₁ relaxed guinea-pig tracheal muscle in vitro and increased nasal patency in normal volunteers and in subjects with vasomotor rhinitis whereas PGI₂ produced opposite effects. All three compounds produced bronchodilatation in the anaesthetised guinea-pig and relaxed human respiratory tract muscle in vitro.PGI₂ was several times more potent than either 6-oxo-PGE₁ or PGE₁ against ADP-induced aggregation of human and baboon platelets in vitro. Intravenous 6-oxo-PGE₁ in the baboon caused an ex vivo inhibition of platelet aggregation, but the EC₅ was 7.8 times that of PGI₂. As a vasodepressor in the baboon 6-oxo-PGE₁ and PGE₂ were equipotent. Thus with the exception of the vasodepressor effect, the actions of 6-oxo-PGE₁ qualitatively and quantitatively resembled those of the structurally related PGE₁ rather than those of PGI₂.     

Effects of 13, 14-dehydroprostacyclin methyl ester on the feline intestinal vascular bed

The effects of a stable PGI₂ analog, 13, 14-dehydro-PGI₂ methyl ester and several vasoactive hormones were compared in the feline intestinal vascular bed under conditions of controlled blood flow so that changes in perfusion pressure directly reflect changes in vascular resistance. The PGI₂ analog decreased perfusion pressure in a dose-dependent fashion when injected in the range of dose of 0.03–3 μg and was quite similar to PGE₂ whereas isoproterenol was somewhat more potent as a vasodilator in the feline intestinal vascular bed. The present data show that 13, 14-dehydro-PGI₂ methyl ester has potent vasodilator activity in the intestinal vascular bed.     

Selective antagonism of the systemic vasodepressor response to PGE1 by d,1–11, 15-bisdeoxy PGE1

Several bisdeoxy PGE₁ analogs are potent, competitive antagonists of PGE₁-induced colonic contractions in the gerbil. The efficacy of these analogs in antagonizing PGE₁-mediated systemic vasodepression has not been previously demonstrated. In this study, serial doses of PGs were administered before, during and after infusion of d,1–11, 15-bisdeoxy PGE₁. Bolus injections of PGE₁ (3.0 μk/kg), PGE₂ (3.0 μg/kg) and PGI₂ (0.3 μg/kg) were administered via the right external jugular vein to male Wistar rats. PGE₁, PGE₂ and PGI₂ decreased systemic arterial pressure 41%, 38% and 38%, respectively. The PGE₁ analog was infused (200 μg/kg/min) through the right common carotid artery. The analog itself had no effect on mean systemic arterial pressure, but maximum reversible inhibition (51%) of PGE₁-mediated vasodepression occurred following a 50 minute infusion. No significant effect of the PGE₁ analog was observed on PGE₂ or PGI₂-mediated vasodepression. These data demonstrate the ability to antagonize PGE₁-mediated vasodepression, and to differentiate the vascular responses to PGE₁ and PGE₂ or PGI₂.     

Physiological concentrations of ADP stimulate the release of prostacyclin from bovine aortic endothelial cells

ADP (0.2−200 μN) stimulated the synthesis of prostacyclin (PGI₂), as reflected by the release of 6-keto-prostaglandin F_1α (6-K-PGF_1α), in endothelial cells cultured from bovine orta. This effect of ADP was mimicked by ATP, whereas AMP and adenosine were completely inactive. The release of 6-K-PGF_1α triggered by ADP was rapid in onset (within 5 min), transient (10 min) and followed by a period of refractoriness to a new ADP challenge. Growing and confluent cells were equally responsive to ADP. ADP stimulated the release of free arachidonic acid from the endothelial cells. ADP could be thus exert two opposite actions on platelet aggregation in vivo: a direct stimulation and an inhibition mediated by PGI₂. This last action might contribute to limit thrombus formation to areas of endothelial cell damage.     

The effects of a 5-lipoxygenase inhibitor, AA-861, on PGI2-like substance production in guinea-pig lungs

To determine the effects of AA-861 on PGI₂ production in guinea-pig lungs, 3 g of guinea-pig lung was chopped in 4 ml of buffer (control group), in buffer with 4 μg/ml indomethacin (indomethacin group) and in buffer with 2.5 × 10⁻⁵M AA-861 (AA-861 group). The chopped lungs were incubated for 30 min. 250 μl of incubation medium from each group was assessed before and after 3, 5, 10, 15, 20, 25 and 30 min of incubation. The incubation medium was centrifuged and the supernatant was tested for a PGI₂-like substance (PGI₂) by platelet aggregation inhibition. PGI₂ was produced mainly during the initial 3–5 min of incubation and was decreased thereafter. PGI₂ production was almost completely inhibited in the indomethacin group at all of the incubation times and was partially inhibited in the AA-861 group during the initial 3–5 minutes. Endogenous 5-lipoxygenase products generated in the early stages of incubation seem to be involved in PGI₂ production in guinea-pig lungs.     

Binding to protaglandin (PG) receptors and activation of adenyl cyclase by 20-isopropylidene-PGS in rabbit platelet membranes

20-Isopropylidene-PGE₁ (Isop-PGE₁) was about 10 times more potent than PGE₁ in inhibition of thrombin-induced aggregation of rabbit washed platelets. Likewise, 20-isopropylidene-17(R)-methyl-carbacyclin (CS-570), a stable PGI₂ analogue, was more potent than carbacyclin in the anti-aggregatory activity. In order to define the platelet-prostaglandin interactions, a binding assay was done using platelet membranes with [³H]-PGE₁ as a radioligand. Isop-PGE₁ (IC₅₀μM) bound to the PG receptors more potently than PGE₁ (IC₅₀ = 2.1 μM). CS-570 (IC₅₀=0.39 μM) was more potent than carbacyclin (C₅₀=1.9 μM). These indicate that introduction of an isopropylidene group to the carbon 20 of PGs increases the binding ability to the receptors. These PGE₁ and PGE₂ analogues activated platelet membrane adenyl cyclase and increased intracellular cAMP levels with the same potency series obtained in the binding experiments. All these results suggest that the binding to the receptors by these PGs is coupled to the activation of adenyl cyclase, followed by the increase in cAMP levels in platelets and the inhibition of platelet aggregation. Thus, the increased anti-aggregatory activity of 20-isop-PGs may be explained by their increased affinity for the PG receptors and stimulation of adenyl cyclase.15-Epimeric-20-isopropylidene-PGE₁ (15-Epi-isop-PGE₁), which has an unnatural configuration of the 15-hydroxyl group, was much less potent than isop-PGE₁ in the binding experiment and the other three investigations. This indicates that the configuration of the 15-hydroxyl group is important for the binding to the PG receptors and the consequent activities in platelets.     

Microsomal preparations of normal bovine iris-ciliary body generate prostacyclin-like but not thromboxane-A2-like activity

Indomethacin-treated bovine iris-ciliary body microsomes (IBIM) have been studied for their ability to convert PG endoperoxides into either thromboxance-A₂ (TxA₂)-like or prostacyclin (PGI₂)-like activity. The biological activity of the ocular tissue microsomes were compared with either indomethacin-treated human platelet microsomes (for TxA₂-like activity) or rabbit aorta microsomes (for PGI₂-like activity) under appropriate incubation conditions. No evidence could be found for the formation of TxA₂-like activity from PG endoperoxides by the IBIM. In contrasts, when the IBIM were incubated with PGH₂ for 1 min at 22°C without cofactors, PGI₂-like activity was produced, causing profound relaxation of the isolated dog coronary artery preparation without contracting the rabbit aorta and inhibiting arachidonic acid-induced platelet aggregation. Equivalent quantities of boiled IBIM failed to aleter the biological activity of PGH₂ under identical conditions. Tranylcypromine (500 μg/ml) completely abolished the appearance of PGI₂-like activity. Furthermore, the PGI₂-like activity found was stable for 10 min at 22°C at pH 8.5 but completely lost under similar conditions at pH 5.5. It is concluded than microsomal preparations of normal bovine iris-ciliary body can synthesize PGI₂-like activity in substantial amounts but not TxA₂-like activity.     

Duration of activity of the acid, methyl ester and amide of an orally active platelet aggregation inhibitory prostanoid in the rat

The prostanoid 3-oxa-4,5,6-trinor-3,7-inter- -phenylene PGE₁ (OI-PGE₁) has been shown to be a more potent inhibitor of ADP-induced human platelet aggregation than PGE₁. OI-PGE₁ inhibits ex vivo ADP-induced platelet aggregation for 60 minutes after an oral dose of 20 mg/kg to rats. Present studies compare duration of ex vivo inhibition to ADP-induced platelet aggregation in the rat by OI-PGE₁, its methyl ester and amide after administration by various routes. All oral (p.o.) and intraduodenal (i.d.) doses were 20 mg/kg and all intravenous (i.v.) doses were 1 mg/kg. OI-PGE₁ and its methyl ester had the same duration of activity after i.v. (60 min.) and p.o. (60 min.) administration, however, the methyl ester, when administered i.d., had a longer duration of activity than the free acid i.d. (>90 min. vs. 60 min.). OI-PGE₁-amide had significantly longer duration than the acid or methyl ester after i.v. (>120 min.), p.o. (>240 min.) or i.d. (>240 min.) administration. Present data suggest that in the rat (1) intestinal absorption of OI-PGE₁-methyl ester is more efficient than it is for the free acid and (2) due to metabolic and/or distributional differences between OI-PGE₁ and its amide, the amide has a much greater duration of activity.     

Comparative gastric antisecretory and antiulcer effects of prostaglandin E1 and its methyl ester in animals

The influence of methyl esterification of the carboxyl group of PGE₁ on the gastric antisecretory and antiulcer activities were studied. The gastric antisecretory effects of PGE₁ free acid and PGE₁ methyl ester (PGE₁ME) were studied in the Heidenhain pouch dog. Secretion was stimulated with constant intravenous infusion of histamine dihydrochloride. When a steady-state plateau of gastric secretion had been reached, the prostaglandins were administered either by a single intravenous bolus (10.0 μg/kg) or by continuous infusion (1.0 μg/kg/min). PGE₁ME was found to be slightly more potent and longer-acting than PGE₁ when administered by a single i.v. bolus. PGE₁ME was also shown to be more potent than PGE₁ when infused intravenously for a two-hour period. PGE₁ME caused a significant alteration in gastric juice concentration of hydrogen and sodium ions in an inverse relationship. Potassium and chloride concentration were not altered from pre-existing steady-state values following administration of either form of prostaglandin. Similarly, PGE₁ME was also found to possess significantly greater antiulcer activity in the rat forced-exertion ulcer test. These findings support the hypothesis that methyl esterification of the prostaglandin molecule will increase some of the biological actions of PGE₁ through inhibition of metabolic β-oxidation of the carboxylic side chain.     

Comparative gastric antisecretory and antiulcer effects of prostaglandin E1 and its methyl ester in animals

The influence of methyl esterification of the carboxyl group of PGE₁ on the gastric antisecretory and antiulcer activities were studied. The gastric antisecretory effects of PGE₁ free acid and PGE₁ methyl ester (PGE₁ME) were studied in the Heidenhain pouch dog. Secretion was stimulated with constant intravenous infusion of histamine dihydrochloride. When a steady-state plateau of gastric secretion had been reached, the prostaglandins were administered either by a single intravenous bolus (10.0 μg/kg) or by continuous infusion (1.0 μg/kg/min). PGE₁ME was found to be slightly more potent and longer-acting than PGE₁ when administered by a single i.v. bolus. PGE₁ME was also shown to be more potent than PGE₁ when infused intravenously for a two-hour period. PGE₁ME caused a significant alteration in gastric juice concentration of hydrogen and sodium ions in an inverse relationship. Potassium and chloride concentration were not altered from pre-existing steady-state values following administration of either form of prostaglandin. Similarly, PGE₁ME was also found to possess significantly greater antiulcer activity in the rat forced-exertion ulcer test. These findings support the hypothesis that methyl esterification of the prostaglandin molecule will increase some of the biological actions of PGE₁ through inhibition of metabolic β-oxidation of the carboxylic side chain.     

Prevention of blockage of partially obstructed coronary arteries with prostacyclin correlates with inhibition of platelet aggregation

Coronary arteries (circumflex or left anterior descending) of anesthetized dogs were partially obstructed to approximately 5% of the normal lumen size by fitting a plastic cylinder around the vessel. Under these conditions, blood flow in the artery was not maintained but, instead, gradually declined over a few minutes until the vessel was completely blocked. Shaking the plastic obstructor restored blood flow temporarily, however, flow gradually declined again to zero. Sometimes flow was spontaneously restored by immediate increases that occurred at irregular intervals while, on other occasions, blood flow had to be restored by shaking the obstructor every time the rate declined to near zero. Intravenous infusion of prostacyclin (PGI₂) at 15 to 150 ng/kg/min reversed and prevented the blockage of the coronary arteries. The efficacy of PGI₂ in preventing blockage correlated with inhibition of ADP-induced platelet aggregation in platelet rich plasma prepared from blood samples withdrawn from the dogs during PGI₂ infusion. Other coronary vasodilators, nitroglycerin and PGE₂, that have no antiaggregatory effects, failed to prevent blockage whereas PGE₁ and indomethacin, which do block aggregation, also prevented blockage of the vessels. PGI₂ or its precursor, PGH₂, dripped topically on the obstructed site prevented the blockage of the artery. This local effect of PGI₂ could be obtained with amounts too small to cause systemic inhibition of platelet aggregation. The results show that PGI₂ prevents blockage of partially obstructed coronary arteries and this effect correlates with inhibition of platelet aggregation. Furthermore, the data suggest that locally produced PGI₂ may have a local antiaggregatory effect without inhibiting platelet aggregation in the general circulation.     

Prostaglandin I2 stimulation of granulosa cell cyclic AMP production

The ability of prostaglandin I₂ (PGI₂) to stimulate cyclic AMP production by granulosa cells, isolated from intact immature rats, has been demonstrated in vitro. The minimal effective dose was 15 ng/ml, which was comparable to the minimal effective dose for PGE₂. However, a concentration of 15 μg/ml PGI₂ was required to stimulate cyclic AMP production maximally, compared to a concentration of 1 μg/ml PGE₂, which produced the maximum response. It therefore appears that PGI₂ is not more effective than PGE₂ in stimulating cyclic AMP production in granulosa cells, and is possibly less effective. Submaximal concentrations of PGI₂ appeared to be able to modify the stimulation of cyclic AMP production by follicle- stimulating hormone (FSH), but whether or not PGI₂ plays any role in follicular function remains to be established.     

Comparison of the effects of prostacyclin (PGI2), prostaglandin E1 and D2 on platelet aggregation in different species

The activity of prostacyclin (PGI₂), PGE₁ or PGD₂ as inhibitors of platelet aggregation in plasma from human, dog, rabbit, rat, sheep and horse was investigated. Prostacyclin was the most potent inhibitor in all species. PGD₂ was a weak inhibitor in dog, rabbit and rat plasma whereas PGE₁ and prostacyclin were highly active. Theophylline or dipyridamole potentiated the inhibition of human platelet aggregation by prostacyclin, PGE₁ or PGD₂. Compound N-0164 abolished the inhibition by PGD₂ of human platelet aggregation but did not inhibit the effects of PGE₁ or prostacyclin. The results suggest that prostacyclin and PGE₁ act on similar sites on platelets which are distinct from those for PGD₂.     

Prostacyclin as neuromodulator in the sympathetically stimulated rabbit heart

Prostaglandin E₂ (PGE₂) has previously been shown to inhibit sympathetic neurotransmission in different organs and species. Based on this inhibitory effect and on its reversal by cyclo-oxygenase inhibitors, PGE₂ has been claimed to be a physiological modulator of in vivo release of norepinephrine (NE) from sympathetic nerves. It is now recognized that prostacyclin (PGI₂) is the main cyclo-oxygenase product in the heart. We therefore addressed the question whether PGI₂, within the same preparation, is formed in increased amounts during sympathetic nerve stimulation and has neuromodulatory activity.The effluent from isolated rabbit hearts subjected to sympathetic nerve stimulation or to infusion of NE or adenosine (ADO) was collected, and its content of PGE₂ and 6-keto-PGF_1α (dehydration product of PGI₂) was analyzed using gas chromatography/mass spectrometry, operated in the negative ion/chemical ionization mode. Other hearts were infused with PGI₂ and nerve stimulation induced outflow of endogenous NE into the effluent was analyzed using HPLC with electrochemical detection. Nerve stimulation at 5 or 10 Hz (before but not after adrenergic receptor blockade), as well as infusion of NE (10⁻⁶–10⁻⁵M) or ADO (10⁻⁴M) increased the cardiac outflow of 6-keto-PGF_1α. Basal and nerve stimulation induced efflux of 6-keto-PGF_1α was approximately 5 times higher than the corresponding efflux of PGE₂. PGI₂ dose-dependently inhibited the outflow of NE from sympathetically stimulated hearts, the inhibition at 10⁻⁶M being approximately 40%.On the basis of these observations we propose that PGI₂ is a more likely candidate than PGE₂ as a potential modulator of neurotransmission in cardiac tissue in vivo.