Selective anti-platelet aggregation synergism between a prostacyclin-mimetic, RS93427 and the nitrodilators sodium nitroprusside and glyceryl trinitrate

Impaired relaxing response to isoprenaline in isolated thoracic aorta of nephrotic rats: decrease in release of EDRF from endothelial cells

Isoprenaline-induced relaxation was investigated in aortas from control and daunomycin-induced nephrotic rats. In the endothelium-intact aortas precontracted with phenylephrine, the isoprenaline-induced relaxation and cyclic adenosine monophosphate (AMP) accumulation were significantly less in nephrotic rats than in control animals. Removal of the endothelium, pretreatment with methylene blue (MB), a guanylate cyclase inhibitor, or NW-nitro-L-arginine methyl ester (L-NAME), a nitric oxide (NO) synthase inhibitor, markedly reduced the relaxation induced by isoprenaline in nephrotic and control animals. The increase in cyclic AMP content induced by isoprenaline also was inhibited by these treatments. In addition, the difference in the isoprenaline-induced relaxation and cyclic AMP accumulation between nephrotic and control preparations was abolished by these treatments. The tissue cyclic guanosine monophosphate (GMP) level was not affected by isoprenaline. In the presence of zaprinast (Zap), a cyclic GMP phosphodiesterase inhibitor, the cyclic GMP level in the endothelium-intact tissues was significantly lower in nephrotic rats than in control animals. Removal of endothelium or pretreatment with MB or L-NAME markedly reduced cyclic GMP content in nephrotic and control animals. In the endothelium-denuded tissues, the isoprenaline-induced relaxation and cyclic AMP accumulation were markedly potentiated by a low concentration of nitroprusside (NP). In the endothelium-intact aortas precontracted with phenylephrine, relaxations induced by dobutamine, salbutamol, and forskolin in nephrotic rats were not significantly different from those in control animals. In the endothelium-intact aortas precontracted with KCl, the isoprenaline-induced relaxation also was significantly less in nephrotic rats than in control animals. Pretreatment with prazosin, but not yohimbine, abolished this difference. These results indicate that nephrosis decreases the relaxing response of the endothelium-intact aortas to isoprenaline. In addition, these results suggest that the endothelium-derived relaxing factor (EDRF) released from the endothelial cells markedly enhances isoprenaline-induced increase in the tissue level of cyclic AMP. The decreased relaxing response to isoprenaline in nephrotic rats may be caused by the decrease in the endothelial-dependent cyclic GMP release caused by alpha 1-adrenoceptor activation by isoprenaline.

Regulation of PAF-induced platelet responses by cyclic nucleotides

The existence of cross-talk mechanisms between the cyclic nucleotide system and other transduction systems involved in PAF-activated platelets is described in this study. A protein of 125 kDa, identified as pp 125(FAK), is tyrosine phosphorylated by PAF in a time- and concentration-dependent manner. The presence of a cAMP- or a cGMP-elevating agent, used alone or in combination, together with PAF diminished tyrosine phosphorylation. The sensitivity to cAMP shown by PAF-induced ppl25 phosphorylation on tyrosine residues was similar to PAF-induced phosphorylation of a 47-kDa protein (pp47) on serine and threonine. In contrast, the latter was not affected in the presence of a cGMP-elevating agent, although it was able to enhance synergistically the inhibitory effect of forskolin. Data reported herein also show that pp47 phosphorylation and serotonin secretion are not closely correlated. Accordingly, sodium nitroprusside (SNP) did not have any effect on phosphorylation of pp47, but it was able to inhibit serotonin secretion when added alone, and it showed a synergistic inhibitory action with forskolin.

Inhibition of platelet aggregation by transdermal glyceryl trinitrate

OBJECTIVE

To determine the optimum conditions for the demonstration of an antiplatelet effect of nitric oxide and to use these conditions to elucidate the effects of a transdermal glyceryl trinitrate patch on platelet aggregation in normal volunteers.

METHODS

An open prospective crossover study. The effects of nitric oxide on platelet aggregation in whole blood and platelet rich plasma as stimulated by adenosine diphosphate and U46619 was assessed in the presence and absence of iloprost and MB22948. Optimum conditions for the demonstration of an antiplatelet effect of nitric oxide were then applied to whole blood from normal volunteers in the presence and absence of a transdermal glyceryl trinitrate patch.

SETTING

University hospital.

SUBJECTS

Eight normal volunteers.

MAIN OUTCOME MEASURES

Platelet aggregation in the presence and absence of transdermal glyceryl trinitrate.

RESULTS

The optimum conditions for the demonstration of an antiplatelet effect of nitric oxide in whole blood were collecting blood into a tube containing MB22948 and citrate and stimulating platelet aggregation with adenosine diphosphate in the presence or absence of iloprost. Using this method a significant effect of transdermal glyceryl trinitrate on platelet aggregation was shown (P < 0.03) in the presence and absence of iloprost.

CONCLUSIONS

These results are consistent with an inhibitory effect on platelet aggregation of nitric oxide liberated by transdermal glyceryl trinitrate. Optimum test conditions are needed to show this effect. The clinical significance of the antiplatelet effect of transdermal glyceryl trinitrate is unknown.

Synergistic phosphorylation of the focal adhesion-associated vasodilator-stimulated phosphoprotein in intact human platelets in response to cGMP- and cAMP-elevating platelet inhibitors

The mechanism underlying the synergistic inhibition of platelet activation by cGMP- and cAMP-elevating vasodilators was investigated using washed human platelets and platelet-rich plasma. With both types of human platelet preparations, low concentrations of sodium nitroprusside increased the cAMP-elevating potency of low concentrations of prostaglandin E1 (PG-E1). Using threshold concentrations of both sodium nitroprusside and PG-E1, the NO-donor potentiated the effect of PG-E1 with respect to the phosphorylation of the focal adhesion-associated vasodilator-stimulated phosphoprotein (VASP) at serine157. In contrast, threshold concentrations of cell-membrane permeant selective activators of the platelet cGMP-dependent protein kinase or the cAMP-dependent protein kinase had only additive effects on VASP serine157 phosphorylation in washed human platelets. The data demonstrate that low intracellular levels of cGMP effectively inhibit type III cGMP-inhibited phosphodiesterase in human platelets despite the high levels of cGMP-dependent protein kinase present in this cell type. This study provides the first evidence that the simultaneous activation of both cGMP- and cAMP-dependent protein kinase results in additive effects on VASP serine157 phosphorylation, whereas the supra-additive effects observed with the combination of sodium nitroprusside and PG-E1 are due to cGMP-mediated inhibition of type III phosphodiesterase. VASP phosphorylation at serine157 may be an important component underlying the synergistic inhibition of human platelets by cGMP-and cAMP-elevating agents.

Effects of forskolin and organic nitrate on aggregation and intracellular cyclic nucleotide content in human platelets

1. The present study investigated the effect of a combination between forskolin, a naturally occurring diterpene which directly activates adenylyl cyclase, and glyceryl trinitrate (GTN), which enhances intraplatelet cyclic guanosine monophosphate levels, on human platelet aggregation and intracellular content of cyclic nucleotides 3',5'-cyclic adenosine monophosphate (cAMP) and 3',5'-cyclic guanosine monophosphate (cGMP). 2. Forskolin inhibited, in a dose-dependent way, platelet aggregation in response to collagen and adrenaline in platelet-rich plasma. In whole blood samples, forskolin inhibited collagen-stimulated aggregation. In presence of forskolin the intraplatelet cAMP levels were significantly increased. 3. GTN directly decreased the platelet response to collagen in whole blood samples (IC50 = 122 mumol/l) and it increased the intraplatelet levels of both cGMP and cAMP. 4. GTN at 20 and 40 mumol potentiated the inhibitory effects of forskolin on platelet aggregation in both platelet-rich plasma and whole blood. 5. Our results suggest a synergistic effect of the simultaneous increase of both cAMP and cGMP on the biochemical steps involved in the inhibition of the platelet response.

Synergistic platelet antiaggregatory effects of the adenylate cyclase activator iloprost and the guanylate cyclase activating agent SIN-1 in vivo

The aim of our study was to evaluate the platelet antiaggregatory and hemodynamic effects of the stable prostacyclin analog iloprost and the NO-donor SIN-1, an active metabolite of molsidomine. The number of circulating platelets was determined in anesthetized male Wistar rats as a measure of in vivo platelet aggregation. Platelet count decreased from 648 +/- 25 to 476 +/- 15 x 10(3) platelets/microliter and from 578 +/- 36 to 411 +/- 40 (mean +/- SEM) in response to two repetitive injections of collagen (70 micrograms/kg body weight). Treatment with SIN-1 bolus injections (0.3 or 1 mg/kg bw) and/or continuous i.v. infusion of iloprost (0.2 or 0.4 micrograms/kg bw/min) was initiated 15 min before the second collagen bolus. As a single agent, SIN-1 did not influence platelet count. Iloprost at 0.2 micrograms/kg/min reduced platelet aggregation (PA) by 15.5%, and at 0.4 micrograms/kg/min by 27.1% (p = n.s.). When iloprost (0.2 micrograms/kg) and SIN-1 (0.3 mg/kg) were administered simultaneously, PA was suppressed by 56.2%, iloprost at 0.4 micrograms/kg/min and SIN-1 at 0.3 mg/kg were even more effective and inhibited PA synergistically by 64.9% (p < 0.05). A statistically significant decrease in mean arterial blood pressure (MABP) was seen in response to all SIN-1 and iloprost groups. However, the vasorelaxant effect of both agents given simultaneously was not synergistic but less than additive. In conclusion, iloprost and SIN 1 exert synergistic platelet inhibitory effects in vivo. In contrast, the decrease in MABP is less then additive. These results are of major interest for the therapeutic regimen with NO-donors (nitrovasodilators).

Effects of zaprinast and rolipram on platelet aggregation and arrhythmias following myocardial ischaemia and reperfusion in anaesthetized rabbits

1. This study was designed to compare the effects of two selective inhibitors of certain phosphodiesterase (PDE) isoenzymes on arrhythmias induced by coronary artery occlusion and reperfusion. The drugs used were zaprinast which inhibits guanosine 3':5'-cyclic monophosphate (cyclic GMP)-specific PDE (PDE V) and rolipram which inhibits cyclic GMP-insensitive, adenosine 3':5'-cyclic monophosphate (cyclic AMP)-specific PDE (PDE IV). 2. Pretreatment of anaesthetized rabbits with zaprinast (300 micrograms kg-1 plus 30 micrograms kg-1 min-1) had no significant effect on ischaemia- or reperfusion-induced ST-segment changes, or arrhythmias. In contrast, rolipram (30 micrograms kg-1 plus 3 micrograms kg-1 min-1) and (100 micrograms kg-1 plus 10 micrograms kg-1 min-1) increased the severity of arrhythmias. With the higher dose of rolipram, ST-segment changes were increased in magnitude and mortality due to ventricular fibrillation during ischaemia or reperfusion was increased to 80% compared with 30% in controls (n = 10 per group). 3. Zaprinast caused small but significant increases in heart rate and arterial blood pressure whereas rolipram decreased diastolic arterial pressure, increased left ventricular (LV) dP/dtmax and substantially increased heart rate. 4. At the end of each experiment platelet aggregation was measured ex vivo. Pretreatment of rabbits with either dose of rolipram had no significant effect on platelet aggregation induced by adenosine diphosphate (ADP), collagen, arachidonic acid or thrombin or on isoprenaline- or prostacyclin-induced inhibition of aggregation. Aggregatory responses to ADP and collagen were increased in platelets obtained from rabbits which had received zaprinast. 5. These results indicate that in the dose used here, the PDE V inhibitor zaprinast had no significant effect on arrhythmias. The effects of the PDE IV inhibitor rolipram on haemodynamics, combined with its lack of antiplatelet activity, may have contributed to the exacerbation of arrhythmias observed during myocardial ischaemia and reperfusion.

Comparison of vasodilatory prostaglandins with respect to cAMP-mediated phosphorylation of a target substrate in intact human platelets

The recent purification of a vasodilator-stimulated phosphoprotein (VASP) from human platelets and the development of a specific antiserum against VASP made it possible to study the quantitative effects of cAMP-elevating prostaglandins on cAMP-mediated phosphorylation of VASP in intact human platelets. Prostacyclin (PG-I2), prostaglandin-E1 (PG-E1) and the stable prostacyclinanalog Iloprost, all agents used for the treatment of peripheral vascular disease, induced rapid, stoichiometric and reversible phosphorylation of VASP in human platelets mediated by the cAMP-dependent protein kinase. However, there were substantial differences between these three cAMP-elevating prostaglandins with respect to their effects on extent, duration and reversibility of VASP phosphorylation. Maximal VASP phosphorylation was induced both by PG-I2 and Iloprost, but the PG-I2 effect was only of short duration in comparison to that of Iloprost. The extent of PG-E1-induced VASP phosphorylation was less than that observed with PG-I2 and Iloprost. In endothelial cell-platelet coincubations, an endothelial cell-derived, indomethacin-sensitive factor caused a rapid elevation of platelet cAMP level and VASP phosphorylation. These results provided direct evidence that human endothelial cells are capable of producing biologically active quantities of cAMP-elevating prostaglandins sufficient to induce stoichiometric cAMP-mediated protein phosphorylation in human platelets. VASP-phosphorylation induced by PG-I2 and PG-E1 was completely reversible after removal of the prostaglandins whereas this was only partially the case with Iloprost. In addition, evidence is presented that the prostaglandin-regulated adenylate cyclase system but not the cAMP-mediated protein phosphorylation desensitizes in human platelets after prolonged treatment with cAMP-elevating prostaglandins. VASP phosphorylation is proposed as a marker for quantitating aspects of vessel wall-platelet interaction.

Synergistic actions of nitrovasodilators and isoprenaline on rat aortic smooth muscle

Previous studies have established that nitrovasodilators potentiate the inhibition of platelet function by activators of adenylyl cyclase, but uncertainty exists as to whether a comparable effect is seen in vascular smooth muscle. We initially studied the effects of the nitrovasodilators, sodium nitroprusside (SNP) and 3-morpholinosydnonimine (SIN-1), on the relaxation by isoprenaline of rat aortic smooth muscle that had been precontracted by phenylephrine. Concentrations of SNP (0.25 nM) and SIN-1 (30 nM) that relaxed aortic smooth muscle less than 30% alone, caused significant (3-fold) decreases in the IC50 values for isoprenaline. The cAMP phosphodiesterase inhibitors, cilostamide (20 nM) and Ro 20-1724 (10 microM), caused comparable reductions in the IC50 values for isoprenaline. At these concentrations, each of the four compounds also increased the maximum relaxation achieved with isoprenaline. Even more marked synergistic interactions were observed between isoprenaline and either the nitrovasodilators or the cAMP phosphodiesterase inhibitors when these compounds were added simultaneously before contraction of aortic smooth muscle by phenylephrine. Thus, concentrations of SNP (5 nM), SIN-1 (1 microM), cilostamide (1 microM) and Ro 20-1724 (100 microM) that inhibited contraction by less than 30% decreased the IC50 values for isoprenaline by 8- to 10-fold. At the above concentrations, these compounds each caused a supra-additive inhibition of contraction when added with 100 nM isoprenaline. Thus, synergism between nitrovasodilators and isoprenaline, an activator of adenylyl cyclase, could be detected in vascular smooth muscle and was particularly marked when inhibition of contraction was studied. This action of nitrovasodilators resembled that of inhibitors of cAMP phosphodiesterase.