The platelet inhibiting activity of endothelium‐derived relaxing factor (EDRF) released by the perfused thoracic aorta of the rabbit was investigated. The aortic effluent superfused a ring of the abdominal aorta without endothelium in order to bioassay EDRF. Aliquots of effluent were collected on rabbit washed platelets and aggregation induced by U‐46619 was measured after 1 min. Prostacyclin (PGI2) was monitored by radioimmunoassay of 6‐oxo‐prostaglandin F1α. Acetylcholine (ACh) caused a dose‐dependent secretion of EDRF, PGI2 and anti‐aggregating activity. Plasma and methylene blue suppressed the platelet inhibition by the effluent. The PGI2 content of the effluent was not sufficient to account for all the anti‐aggregating activity. However, the platelet inhibition disappeared when PGI2 formation was blocked with indo‐methacin. Compression of the thoracic aorta increased the EDRF content in the effluent. A transient secretion of anti‐aggregating activity was then observed in aortic effluent in the absence of PGI2. This activity coincided with the presumed EDRF peak in the effluent. Superoxide dismutase enhanced the ACh‐induced EDRF content and revealed secretion of an anti‐aggregating substance when PGI2 formation was blocked. Pretreatment of the platelets with subthreshold concentrations of PGI2, or the cyclic GMP phosphodiesterase inhibitor RX‐RE 56, also revealed the release of a labile platelet inhibitor in response to ACh. The results indicate that EDRF released by fresh aortic endothelium may suppress platelet aggregation, particularly when PGI2 is present.

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SummaryIn a placebo-controlled double blind cross-over experiment the adenosine uptake inhibitor dipyridamole (400 mg/day) did not affect ex vivo platelet aggregation induced by collagen or adenosine-diphosphate (ADP) in an electronic whole blood aggregometer (WBA). Dipyridamole was also inactive in vitro, unless red blood cell injury was deliberately enhanced, thereby increasing the level of free adenine nucleotides. Since dipyridamole also inhibits cyclic guanosine monophosphate (GMP) phosphodiesterase (PDE), we used platelet rich plasma (PRP) to study its interaction with authentic and endothelium-derived nitric oxide (NO). The latter inhibits platelets by increasing cyclic GMP. Dipyridamole (1 to 30 εM), either alone or in combination with a subthreshold concentration of prostacyclin (PGI2), was inactive. However, when combined with a subthreshold concentration of NO, dipyridamole caused a concentration-dependent platelet suppression, which became more pronounced when PGI2 was present as well. It is concluded that dipyridamole could reduce the threshold for platelet suppression by NO through inhibition of cyclic GMP PDE.

No abstract

Objective  The strength of the fibrous cap of an atherosclerotic plaque is maintained by smooth muscle cells (SMCs). 7‐Ketocholesterol (7‐KC), and other oxysterols formed inside plaques, are known to induce SMC death, thereby increasing the risk of rupture and myocardial infarction (MI). Statins are known to protect patients from MI, but paradoxically they induce apoptosis in vascular SMCs as well. The aim of this study was to investigate the cell death effects of statins in 7‐KC‐treated SMCs. Methods  Rabbit aorta SMCs (50 000; passage 2–6) were seeded in 24 well plates in HAM F10 with 10% foetal bovine serum (FBS). The next day, they were exposed to 25 μM 7‐KC and 10−8–10−4 M of fluvastatin (fluva), simvastatin (simva) or pravastatin (prava) in the presence of 2% FBS. Uptake of neutral red (0.01%, added 16 h later) was measured for 2 h as viability index. Results  SMC viability decreased from 100% to 55 ± 3% (n = 28) upon 18 h exposure to 25 μM 7‐KC. High concentrations of fluva (≥ 10 μM) or simva (≥ 1 μM) also induced a concentration‐dependent loss of SMC viability; prava was always without effect. Interestingly, fluva and simva partly reversed the viability loss evoked by 7‐KC. Similar effects were seen when mitochondrial function was assessed using the succinate dehydrogenase substrate MTT. Fluva and simva alone were without effect, but they attenuated the mitochondrial dysfunction induced by 7‐KC. In contrast, both statins and 7‐KC enhanced the conversion of the caspase substrate DEVD‐R110 by SMC monolayers. Conclusions  The results confirm that 7‐KC and high concentrations of lipophilic statins induce SMC death and activate caspases. Yet, in 7‐KC treated SMCs statins paradoxically attenuated viability loss and mitochondrial dysfunction. This clearly illustrates that drug effects seen in normal SMCs are not necessarily predictive of their action in an atherosclerotic setting.