Effect of aspirin and salicylate on platelet-vessel wall interactions in rabbits

COX-2 Inhibition by Use of Rofecoxib or High Dose Aspirin Enhances ADP-Induced Platelet Aggregation in Fresh Blood

Aim:

Increased cardiovascular risk after use of selective or nonselective cyclooxygenase-2 (COX-2)-inhibitors might partly be caused by enhanced platelet aggregability. However, an effect of COX-2 inhibition on platelets has so far not been observed in humans.

Methods:

We tested in healthy volunteers the effect of COX-2-inhibition nearly in-vivo, i.e. immediately after and even during blood sampling.

Results:

Measurement within 2 minutes after venipuncture, but not 60 minutes later, showed that 50 mg of rofecoxib (n=12) or 500 (n=8) or 1000 (n=8) mg of aspirin increased ADP-induced platelet aggregation in a whole-blood aggregometer to, respectively, 152, 176 and 204 % of basal level (p<0.01). No significant differences in aggregability were observed after ingestion of 80 mg of aspirin (n=16), or placebo (n=8). Plasma 6-keto-PGF1α was decreased to 74 % after rofecoxib and to 76 and 70 % after 500 and 1000 mg of aspirin but did not change after low dose aspirin. Continuous photometrical measurement of aggregation in blood flowing from a cannulated vein revealed that high dose aspirin did not elicit aggregation by itself, but increased ADP-induced aggregation in proportion to the decrease in prostacyclin formation (r=0.68, p = 0.004). Since in these experiments thromboxane production was virtually absent, the enhanced aggregation after partial COX-2 inhibition was not caused by unopposed thromboxane formation.

Conclusions:

We conclude that both selective and nonselective COX-2 inhibition enhances ADP-induced platelet aggregation in humans. This effect can only be detected during or immediately after venipuncture, possibly because of the short half-life of prostacyclin.

Intravenous aspirin causes a paradoxical attenuation of cerebrovascular thrombolysis

BACKGROUND AND PURPOSE

Aspirin treatment is recognized as an advantageous adjunct to thrombolytic agents in myocardial infarct patients. In this study we examined the effects of aspirin on the rate of clot lysis and on the frequency and extent of hemorrhagic transformations in rabbit models of embolic stroke.

METHODS

Rabbit models of ex vivo platelet aggregation and cutaneous template bleeding times were used to show the anticoagulant effects of aspirin in our experimental paradigm. We monitored tissue-type plasminogen activator (TPA)-induced clot lysis in two rabbit models of embolic stroke by (1) scintigraphically following the dissolution of a 99mTc-tagged clot or (2) using roentgenography to follow the disappearance of an Sn-tagged clot.

RESULTS

In animals pretreated (18 hours) with a single administration of aspirin (1, 5, or 20 mg/kg IV) or 1 mg/kg per day for 3 days, the aggregation response of platelets to collagen (3.3 micrograms/mL) or arachidonic acid (0.5 mmol/L) was attenuated. High-dose aspirin also increased ear template bleeding time from 1.6 to 2.6 minutes. When aspirin (20 mg/kg) was administered 18 hours before embolism and subsequent lysis with TPA (0.3 mg/kg bolus; 3 mg/kg per hour IV), the pretreatment significantly antagonized the rate and extent of TPA-induced clot lysis by up to 70%. This was confirmed in a second embolic stroke model. The suppression of TPA-induced lysis was reversed by administration of the prostacyclin analogue iloprost (10 micrograms/kg per hour) directly into the cerebral circulation.

CONCLUSIONS

We conclude that aspirin reduces the effects of TPA in embolic stroke models. This effect may be the result of a loss of endothelial prostacyclin production since the effect is reversed by iloprost.

Arterial response to mechanical injury: balloon catheter de-endothelialization

Coronary angioplasty has been used clinically for over a decade. Its initial promise as an alternative to coronary bypass surgery has only partially been fulfilled because of the high rate of post-operative restenosis. A number of animal models have been devised to study this phenomenon and although none is entirely satisfactory, they have, together with recent advances in molecular biology provided an insight into the cellular mechanisms that may contribute to this complication. This knowledge may ultimately lead to a means of therapeutic intervention. This review summarises our present understanding of the pathology of post-angioplasty re-stenosis as revealed by studies using the balloon catheter de-endothelialization model, and discusses some of the intervention strategies that have been attempted.

Relationship between vessel wall 13-HODE synthesis and vessel wall thrombogenicity following injury: influence of salicylate and dipyridamole treatment

We performed studies to determine the relationship between injured vessel wall thrombogenicity, vessel wall 13-hydroxyoctadecadienoic acid (13-HODE) synthesis and cAMP levels in rabbit treated with salicylate or dipyridamole. Injured vessel wall thrombogenicity was measured as the number of 3H-adenine labelled platelets adhered to the subendothelial basement membrane exposed by air injury in carotid arteries of rabbits treated orally with salicylate or dipyridamole. Vessel wall 13-HODE was measured by HPLC and vessel wall cAMP was measured by RIA. Vessel wall thrombogenicity was increased two-fold in rabbits treated with salicylate and decreased by half in rabbits treated with dipyridamole. The levels of vessel wall cAMP levels were correlated both with the plasma dipyridamole levels and increases in 13-HODE synthesis. cAMP levels were unaffected by salicylate treatment, but 13-HODE synthesis was decreased. We conclude that there is a significant relationship between vessel wall cAMP levels and 13-HODE synthesis, which in turn, influences subsequent vessel wall thrombogenicity.

The kinetics of platelet and fibrin deposition on to damaged rabbit carotid arteries in vivo: involvement of platelets in the initial deposition of fibrin

Thrombus formation in the rabbit carotid artery has been studied kinetically in vivo using a minimally invasive technique utilising radioisotopes. Clamping of the carotid artery for 5 min resulted in the simultaneous accumulation of platelets and fibrin at the site of injury over the next 45 min. Under the electron microscope the response was seen to range from platelet monolayer adhesion to mural thrombus formation with fibrin deposition. In animals rendered thrombocytopenic, fibrin deposition was impaired during the first 15-20 minutes after injury. Basic coagulation times and fibrinogen concentration were within normal limits. In addition the injured vessels in these animals accumulated more radiolabelled albumin, but not erythrocytes, than injured vessels in control animals. The results may imply a role for platelets in the enhancement of fibrin deposition during the early part of the response to injury and in contributing to the maintenance of normal permeability following vessel injury.

The role of aspirin and dipyridamole on vascular DNA synthesis and intimal hyperplasia following deendothelialization

Platelets are implicated both in acute thrombotic events and, through platelet-derived growth factor, in the development of intimal hyperplasia. We have investigated, in vivo, the influence of aspirin and dipyridamole on vascular smooth muscle cell proliferation and DNA synthesis following balloon catheter injury. Fifty-eight male, New Zealand white rabbits were divided equally into two groups; the test group was fed aspirin (14 mg/kg/day) and dipyridamole (9 mg/kg/day) from 2 days prior to surgery until sacrifice at 1, 2, 3, 4, 7, 14, or 28 days after injury. All animals were sacrificed 1 h after injection of [3H]thymidine and the smooth muscle cell DNA specific activity and total kinetic activity were determined. Intimal hyperplasia was measured by light microscopy and intimal nuclear proliferation was determined by counting nuclei per millimeter of internal elastic lamina. Nuclear proliferation was maximal at 14 days (25 +/- 1.2) but intimal hyperplasia was still increasing at 28 days. DNA specific activity after 24 hr (test: 4 +/- 2 dpm/micrograms DNA; control: 3.3 +/- 3 dpm/micrograms DNA) was similar to basal levels in uninjured rabbits. DNA synthesis peaked in both groups between the second and third day (test: 177 +/- 27 dpm/micrograms DNA; control: 185 +/- 39 dpm/micrograms DNA) and then declined slowly toward baseline values. There was no significant difference between treated and normal rabbits in either [3H]thymidine incorporation, nuclear proliferation, or development of intimal hyperplasia despite 90% inhibition of platelet aggregation and a significant reduction (78%) in [14C]serotonin release following collagen challenge (6 micrograms/ml).(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of salicylic and acetylsalicylic acid alone and in combination on platelet aggregation and prostanoid synthesis in man

The present study was designed to investigate the effects of salicylate on the antiplatelet action of acetylsalicylic acid as well as on in vivo prostanoid formation and platelet function in healthy volunteers. In the first study six female volunteers received 350 mg acetylsalicylic acid intravenously, with and without previous oral administration of sodium salicylate (1200 mg daily for 3 days). Urinary prostanoid excretion as well as platelet aggregation and thromboxane formation were measured before and during salicylate and after acetylsalicylic acid. In the second study seven female volunteers received sodium salicylate (52.6 mg kg-1) or acetylsalicylic acid (60.7 mg kg-1) for 8 days in a randomized cross-over protocol. Urinary prostanoid excretion, platelet aggregation and thromboxane formation as well as salicylate plasma concentrations were determined before, during and after administration of each drug. Sodium salicylate did not impair the complete suppression of arachidonic acid-induced platelet thromboxane formation and aggregation obtained by the single intravenous dose of acetylsalicylic acid in the first study. Sodium salicylate in the second study did not affect urinary excretion of prostaglandin E2, its major urinary metabolite (7 alpha-hydroxy-5,11-diketo-tetranor-prostane-1,16-dioic acid), and 2,3-dinor-6-keto-prostaglandin F1 alpha, the main urinary metabolite of epoprostenol (prostacyclin, PGI2). In contrast, acetylsalicylic acid significantly decreased excretion rates of these prostanoids by 64, 59 and 61%, respectively. In both studies platelet aggregation and thromboxane formation induced by collagen, thrombin or arachidonic acid were not significantly affected by salicylate administration, whereas acetylsalicylic acid inhibited platelet aggregation induced by all three agents as well as thrombin- and arachidonic acid induced thromboxane formation.(ABSTRACT TRUNCATED AT 250 WORDS)