Nitrate resistance in platelets from patients with stable angina pectoris

BACKGROUND

Hemodynamic resistance to nitrates has been previously documented in congestive heart failure. In patients with stable angina pectoris (SAP), we have observed a similar phenomenon: decreased platelet response to disaggregating effects of nitroglycerin (NTG) and sodium nitroprusside (SNP).

METHODS AND RESULTS

In blood samples from normal subjects (n=32) and patients with SAP (n=56), we studied effects of NO donors (NTG and SNP) on ADP-induced platelet aggregation and on intraplatelet cGMP. NTG and SNP inhibited platelet aggregation in patients to lesser extents than in normal subjects (P<0.01). The cGMP-elevating efficacy of NTG and SNP was diminished in platelets from patients in comparison with those from normals (P<0.001). Inhibition of the anti-aggregatory effects of NTG and SNP by ODQ, a selective inhibitor of NO-stimulated guanylate cyclase, was significantly less pronounced in patients than in normal subjects. Content of O2- was higher in blood samples from patients than in those from normal subjects (P<0. 01). In blood samples from patients with SAP, but not in normal subjects, the O2- scavenger superoxide dismutase (combined with catalase) suppressed platelet aggregation (P<0.01) and increased the extent of anti-aggregatory effect of SNP (P<0.01).

CONCLUSIONS

In patients with SAP, platelets are less responsive to the anti-aggregating and cGMP-stimulating effects of NO donors; this may reflect both reduction in guanylate cyclase sensitivity to NO and inactivation of the released NO by O2-. The implied impairment of anti-platelet efficacy of endogenous NO (in the form of EDRF) may contribute to platelet hyperaggregability associated with angina pectoris.

Inhibition of long-chain fatty acid metabolism does not affect platelet aggregation responses

A number of anti-anginal agents (perhexiline, amiodarone, trimetazidine) have been shown to inhibit myocardial carnitine palmitoyltransferase-1, which controls access of long-chain fatty acids to mitochondrial sites of beta-oxidation. In view of clinical data suggesting that perhexiline improves symptomatic status in unstable angina pectoris, and the known role of mitochondrial beta-oxidation in platelet metabolism, we compared the platelet carnitine palmitoyltransferase-1 inhibitory and putative anti-aggregatory effects of perhexiline, amiodarone and trimetazidine with those of specific carnitine palmitoyltransferase-1 inhibitors: etomoxir and hydroxyphenylglyoxylate in both normal subjects and patients with stable angina. All of the compounds examined inhibited platelet carnitine palmitoyltransferase-1 activity; rank order of potency etomoxir > malonyl-CoA > hydroxyphenylglyoxylate > amiodarone > or = perhexiline > trimetazidine. However, only perhexiline, amiodarone and trimetazidine inhibited platelet aggregation. We conclude that (a) the carnitine palmitoyltransferase-1 inhibitors perhexiline, amiodarone and trimetazidine exert significant anti-aggregatory effects which may be therapeutically relevant and, (b) these effects are independent of carnitine palmitoyltransferase-1 inhibition.

Nitroglycerin tolerance at the platelet level in patients with angina pectoris

Suppression of platelet aggregation may be an important component of the therapeutic effect of nitroglycerin (NTG). Because of the phenomenon of hemodynamic tolerance to NTG, we tested the hypothesis that the anti-platelet effects of NTG in humans are also subject to tolerance induction. In patients with stable angina who had not received nitrates for at least 24 hours before study, sublingual administration of NTG (300 microg; n = 17) attenuated the reversal of adenosine diphosphate-induced platelet aggregation by NTG applied in vitro. Three minutes after in vivo NTG administration, concentration of NTG producing 50% reversal of aggregation (C50) increased from 7.9 +/- 1.9 x 10(-5) to 5.5 +/- 0.3 x 10(-4) M (p <0.01); this change persisted for at least 60 minutes. There was no concomitant change in C50 values for sodium nitroprusside applied in vitro. Basal activity of platelet guanylate cyclase and its response to sodium nitroprusside were not affected after administration of NTG. Brief intravenous infusion of NTG (10 microg/min for 10 minutes) produced no significant changes in platelet responses to NTG in vitro. However, prolonged infusion of NTG (5 microg/min for 24 hours, patients with unstable angina pectoris, n = 11) caused suppression of in vitro platelet response to NTG. Platelets from patients receiving prophylactic nitrates (n = 19) were less responsive to the antiaggregatory effects of NTG in vitro than those from patients who had not received nitrates in the previous 24 hours (n = 21). Thus, clinical exposure to NTG, even in very low doses, induces tolerance to antiaggregatory effects of NTG. This phenomenon is not associated either with cross tolerance to sodium nitroprusside or with down-regulation of platelet guanylate cyclase.

Suppressed anti-aggregating and cGMP-elevating effects of sodium nitroprusside in platelets from patients with stable angina pectoris

Platelet hyperactivity plays an important role in the pathogenesis of cardio-vascular diseases. In patients with stable angina pectoris, we have recently demonstrated that nitroglycerin suppressed the increased platelet aggregability. The anti-aggregating effect of NTG and other nitrovasodilators is mediated by platelet guanylate cyclase, which generates cyclic GMP (cGMP) in response to nitric oxide (NO) liberated from the nitrovasodilator molecule. In the current study we utilised a more "direct" NO donor, sodium nitroprusside (SNP), to examine reversal of ADP-induced platelet aggregation in comparison with intraplatelet cGMP elevation in platelets from normal subjects (n = 22) and patients with stable angina pectoris (n = 23). Concentrations of SNP associated with 50% reversal of aggregation were 2.7 +/- 0.4 x 10(-7) mol/L with normal subjects and 4.5 +/- 0.5 x 10(-6) mol/L with patients (P < 0.01). SNP produced a concentration-dependent elevation of intraplatelet cGMP content: with 10(-4) mol/L SNP this was 17-fold for normals and 5-fold for patients (P < 0.01). An increase in cAMP content was seen only with 10(-4) mol/L SNP, and was 157 +/- 11% of baseline in platelets from normal subjects and 138 +/- 14% in patients. There was a strong interrelationship between cGMP-stimulating and anti-aggregating effects of SNP. The decrease in cGMP responsiveness to SNP was not related to a dysfunction of platelet guanylate cyclase; neither basal nor SNP-stimulated activity of the enzyme varied significantly between normal subjects and patients. Lipophilic derivatives of cGMP (db-cGMP) and cAMP (db-cAMP) caused reversal of aggregation; there was a nonsignificant trend towards decreased responsiveness of platelets from patients to both db-cGMP and db-cAMP. The observed decrease in responsiveness of platelets from angina patients to anti-aggregating effects of the exogenous NO donor, SNP, can therefore be attributed to suppressed cGMP accumulation. These results imply reduced platelet sensitivity to endogenous NO (endothelium-derived relaxing factor): this might contribute to platelet hyperaggregability observed in angina pectoris.

Impaired responsiveness of platelets from patients with stable angina pectoris to antiaggregating and cyclicAMP-elevating effects of prostaglandin E1

The antiplatelet effects of prostacyclin (PGI2) and prostaglandin E1 (PGE1) are mediated by the same receptor and are secondary to intraplatelet cyclicAMP formation. Therefore, any dysfunction in PGI2/PGE1-stimulated cyclicAMP generation might lead to pathologically increased platelet aggregation. This possible consequence has not yet been studied. We examined antiaggregating effects of PGE1 in comparison with its cyclicAMP-elevating potency in platelets obtained from normal subjects and patients with stable angina pectoris; platelet hyperaggregability in such patients has been documented by us previously. ADP-induced aggregation was measured in platelet-rich plasma (PRP); PGE1 was added to platelets 0.5 min after ADP for assessment of reversal of incipient aggregation. Concentrations of PGE1 associated with 50% reversal of aggregation (C50) were 2.4 +/- 0.3 x 10(-8) M in normal subjects and 6.3 +/- 1.6 x 10(-7) M in patients (p < 0.01). PGE1 produced a concentration-dependent increase in intraplatelet cyclicAMP, and there was a strong correlation between cyclicAMP-stimulating and antiaggregating effects of PGE1. Maximal increases in cyclicAMP with PGE1 10(-4) M were 330 +/- 10% for normal subjects and 220 +/- 20% for patients (p < 0.01). Thus, the observed decrease in PGE1-induced reversal of platelet aggregation in patients can be attributed to a suppressed cyclicAMP response to PGE1. These results are likely also to imply reduced platelet sensitivity in vivo to endogenous PGE1 and PGI2, which in turn might contribute to platelet hyperaggregability observed in cardiovascular diseases.

[Changes in the radiosensitivity of cortical thymocytes during differentiation]

To determine the radiosensitivity of cortical thymocytes at different stages of their differentiation, that took three days, estimated was the part of DNA of labelled thymocytes within the salt soluble fragments of the chromatin formed 6 h after irradiation of rats at different times during three days following the initial labelling of thymic blast DNA. The radiosensitivity of cells, which were at the terminal stages of differentiation, was twice as high as that of lymphocytes which were at the initial stage of differentiation.

[Characteristics of the development of interphase death of hematopoietic tissue in dogs in response to gamma irradiation]

In experiments with 2-3 week dogs it was shown that whole-body gamma-irradiation with a dose of 3 Gy causes an insignificant absolute rise in the amount of degrading chromatin of the haemopoietic organs during the first 24 h following irradiation. After 48 h, this cell death parameter is normalized. A considerably lower radiosensitivity of lymphoid cells of dogs compared to small laboratory animals is indicated by a stable DNA content per 1 g of thymus, intactness of its structure after irradiation, and the absence of an increase in thymidine content of blood of young and adult dogs under the effect of 3-3.7 Gy radiation.

[Inhibition of cell proliferation in the thymus: possibility of detecting natural cell death within the organ and its contribution to the phenomenon of induced interphase cell death]

Parallelism was noted between the suppression of proliferation and the amount of cells dying in mouse thymus after the effects inducing cell destruction. However, inhibition of DNA synthesis under the effect of nontoxic doses of arabinoside cytosine increased insignificantly the number of dying cells as compared to normal. This indicated the absence of the masking effect of reutilization of degradation products of dying cells, minor amounts of cells normally dying in the thymus, and their insignificant contribution to the induced cell death after the effects leading to inhibition of cell proliferation.