Platelets, carotids, and coronaries. Critique on antithrombotic role of antiplatelet agents, exercise, and certain diets

Cerebral protection against ischemia by locomotor activity in gerbils. Underlying mechanisms

BACKGROUND AND PURPOSE

A previous communication of this laboratory demonstrated reduced mortality and neuronal damage by spontaneous locomotor activity preceding forebrain ischemia in Mongolian gerbils. The present experiments seek to elucidate potential mechanisms of protection by measurement of cerebral blood flow, cerebral tissue conductance as an indicator of ischemic cell swelling, and the cerebral release of eicosanoids.

METHODS

Gerbils were maintained either in conventional cages (nonrunners) or with free access to running wheels (runners) for 2 weeks preceding 15 minutes of forebrain ischemia. During ischemia and 2.5 hours of reperfusion, cerebral tissue conductance was determined with a two-electrode system. Simultaneously, prostaglandin D2, prostaglandin F2 alpha, and thromboxane B2 were measured in ventriculocisternal perfusate. In additional animals cerebral blood flow was assessed by hydrogen clearance.

RESULTS

Decreases in tissue conductance during ischemia were similar in nonrunners (56 +/- 3%) and runners (62 +/- 3%) but normalized more rapidly in runners during reperfusion. In both groups reperfusion was accompanied by marked increases of perfusate prostaglandin D2, prostaglandin F2 alpha, and thromboxane B2. In nonrunners, however, thromboxane B2 was already elevated during ischemia (147 +/- 9%, P < .01) and remained elevated longer during recirculation (P < .05). Postischemic perfusion maxima were higher in runners (70.8 +/- 7.4 versus 47.0 +/- 5.0 mL/100 g per minute, P < .05) and were observed sooner (27.4 +/- 6.9 versus 62.2 +/- 12.3 minutes, P < .05). Both groups displayed delayed hypoperfusion of a similar magnitude (runners, 29.0 +/- 2.4 mL/100 g per minute; nonrunners, 30.1 +/- 2.4 mL/100 g per minute).

CONCLUSIONS

Protection by preischemic locomotor activity may involve enhanced postischemic reperfusion, leading to more rapid normalization of conductance and thus of cell volume. Enhanced reperfusion may be the consequence of attenuated thromboxane liberation during and after ischemia.

Reduced mortality and brain damage after locomotor activity in gerbil forebrain ischemia

BACKGROUND AND PURPOSE

Preischemic spontaneous locomotor activity was distinguished in this laboratory as a factor influencing outcome after 15 and 20 minutes of forebrain ischemia in gerbils. Histological investigations were carried out to analyze potential relations between postischemic survival and a reduction of cerebral damage by spontaneous locomotor activity.

METHODS

Male Mongolian gerbils were divided into two groups, one with access to running wheels ("runners") and one kept in conventional cages ("nonrunners") for 2 weeks preceding forebrain ischemia of 15 or 20 minutes. A total of 99 gerbils were divided in subgroups and were allowed to recover for 2 weeks for assessment of survival. Other subgroups (n = 7 to 9) were killed at day 4 for quantitative histology of selectively vulnerable areas such as hippocampus, cortex, striatum, and thalamus.

RESULTS

Two weeks after 15-minute ischemia, 44% of non-runners had survived compared with 90% of runners (P < .01). With 20-minute ischemia all runners survived compared with 21% of nonrunners. Quantitative histology (15-minute ischemia) revealed selective nerve cell injury in various cerebral regions in both groups. In runners, however, with the exception of the CA1 sector, damage was attenuated in cortex, striatum, and hippocampus. Furthermore, the extent of thalamic infarction was reduced (P < .05).

CONCLUSIONS

Locomotor activity before global cerebral ischemia is highly efficient in protecting the brain as demonstrated by enhanced survival and a reduction of tissue damage in Mongolian gerbils. The mechanisms underlying this protection are currently unclear. However, further understanding of this intriguing phenomenon should enhance the understanding of ischemia pathophysiology and lead to the development of new treatment strategies.

Modification of experimental and clinical atherosclerosis by dietary fish oil

A number of experimental studies have reported that dietary fish oil can attenuate the development of atherosclerosis in cholesterol-fed rats, quails, rabbits, pigs, and monkeys. Epidemiologic studies suggest that dietary fish oil can reduce the development of cardiovascular disease in humans. Data are limited but suggest that laboratory animals, normal volunteers, and patients with hyperlipidemia show similar responses to the consumption of fish oil. The major effect of dietary fish oil on serum lipoproteins is a reduction in plasma triglyceride levels, with inconsistent effects on plasma cholesterol and HDL-cholesterol. Dietary fish oil induces a significant reduction of platelet aggregation associated with a prolonged bleeding time. This antithrombotic effect may be partially related to a decreased thromboxane A2 and to an increased prostacyclin level. Dietary fish oil may also have anti-inflammatory and anti-immunologic effects through an elevation of prostaglandins and a reduction in the level of leukotriene B4. Recent experimental data suggest that either fish oil or verapamil can bring on a regression in atherosclerosis in cholesterol-fed rabbits put on a normal diet; however, there was no additive effect of the combination of these agents. Overall, data suggest that fish oil may have a role in attenuating the development of atherosclerosis.

Mechanisms of vascular graft thrombosis: role of altered canine platelet sensitivity to thromboxane

Using the standard turbidimetric method of platelet aggregation and quantitation of platelet secretion with 14C-Serotonin, we have examined the responsiveness of the platelets of mongrel dogs to arachidonic acid (AA), and the thromboxane agonist U46619 in the presence and absence of a subthreshold concentration of epinephrine. In response to stimulation with 750 microM AA, the platelets of 18 dogs produced irreversible aggregation (Group I), the platelets of 22 dogs showed, at most, reversible aggregation (Group II), while the platelets of 8 dogs demonstrated no aggregatory response (Group III). In the presence of AA and a subthreshold concentration of epinephrine (0.5 microM), the platelets of all three groups demonstrated enhanced aggregatory and secretory responses although the extent of 14C-Serotonin secretion differed significantly between all three groups. These in vitro differences in platelet aggregation correlate with the in vivo deposition of platelets onto synthetic vascular grafts and the maintenance of graft patency. When stimulated with 0.5 microM U46619 and a subthreshold concentration of epinephrine, the platelets of 97% Group I dogs and 75% of Group II dogs exhibited irreversible aggregation, while the platelets of all Group III dogs showed only reversible aggregation. In addition, significant differences in the extent of 14C-Serotonin secretion to this combination of agonists were observed between groups. Further examination of the specific effects of U46619 on canine platelets revealed that although the aggregatory and secretory responses to U46619 vary between the different canine platelet populations, the threshold concentration of U46619 required to produce platelet shape change is identical among all groups. Quantitation of the stable metabolite of AA produced via the cyclooxygenase pathway, thromboxane B2 (TxB2), revealed no significant differences in the production of TxB2 by the platelets of these different populations upon stimulation with AA. Our results suggest that the mechanisms underlying the differences in responsiveness of canine platelets to AA, are likely due to differences in sensitivity of canine platelets to TxA2, and may be localized to the mechanism responsible for mediating platelet aggregation and secretion in response to TxA2.

Reactivity of equine palmar digital arteries and veins to vasodilating agents

Palmar digital arteries and veins removed surgically from healthy horses under general anesthesia were cut into 4 mm vascular rings, suspended in tissue baths, and attached to force displacement transducers for continuous measurement of vascular tension. In vitro vascular responses were determined for acetylcholine, acepromazine, isoxsuprine hydrochloride (isoxsuprine), prostaglandin E2 (PGE2), and prostaglandin I2 (prostacyclin). After preconstriction with norepinephrine hydrochloride (norepinephrine), or prostaglandin F2 alpha (PGF2 alpha), the concentrations needed to produce 50% maximum relaxation (EC50) and the maximum percentage of relaxation were determined for each drug. Acetylcholine was the most potent arterial vasodilator (smallest EC50 value) and PGE2 was the least potent. Prostacyclin was the least potent venodilator (highest EC50 value); there were no differences between acetylcholine, acepromazine, isoxsuprine, and PGE2. Isoxsuprine produced greater arterial relaxation than all other agents. Isoxsuprine and acepromazine produced significantly greater venous relaxation than did acetylcholine and PGE2. Prostacyclin produced minimal vasodilation of arteries or veins. Acepromazine and isoxsuprine relaxed the veins significantly more than the arteries. When PGF2 alpha was used instead of norepinephrine to preconstrict the arteries and veins, the potency and effectiveness of acepromazine and isoxsuprine to produce vasodilation were significantly decreased. Results indicate that acepromazine and isoxsuprine can relax the equine digital vasculature but their effectiveness varies depending on the origin of the constriction.

Fish-oil dietary supplementation in patients with Raynaud's phenomenon: a double-blind, controlled, prospective study

PURPOSE

The ingestion of omega-3 fatty acids could benefit patients with Raynaud's phenomenon because, among other effects, these fatty acids induce a favorable vascular response to ischemia. The aim of our study was to investigate, in a double-blind, placebo-controlled manner, the effects of fish-oil fatty-acid dietary therapy in patients with rheumatic disease.

PATIENTS AND METHODS

Thirty-two patients with primary or secondary Raynaud's phenomenon were randomly assigned to olive-oil placebo or fish-oil groups. Patients ingested 12 fish-oil capsules daily containing a total of 3.96 g eicosapentaenoic acid and 2.64 g docosahexaenoic acid or 12 olive-oil capsules and were evaluated at baseline and after six, 12, and 17 weeks. All patients ingested olive oil between Weeks 12 to 17. Digital systolic blood pressures and blood flow were measured at room air and water baths of 40 degrees C, 25 degrees C, 15 degrees C, and 10 degrees C using strain gauge plethysmography. Onset of Raynaud's phenomenon was timed with a stop watch and defined as plethysmographic evidence of cessation of blood flow and blood pressure in the study finger.

RESULTS

In the fish-oil group, the median time interval before the onset of Raynaud's phenomenon increased from 31.3 +/- 1.3 minutes baseline to 46.5 +/- 2.1 minutes at six weeks (p = 0.04). Patients with primary Raynaud's phenomenon ingesting fish oil had the greatest increase in the time interval before the onset of the condition. Five of 11 patients (45.5 percent) with primary Raynaud's phenomenon ingesting fish oil in whom the phenomenon was induced at baseline could not be induced to develop Raynaud's at the six- or 12-week visit compared with one of nine patients (11 percent) with primary Raynaud's ingesting olive oil (p = 0.05). The mean digital systolic pressures were higher in the patients with primary Raynaud's phenomenon ingesting fish oil than in patients with primary Raynaud's ingesting olive oil in the 10 degrees C water bath (+32 mm Hg, p = 0.02).

CONCLUSION

We conclude that the ingestion of fish oil improves tolerance to cold exposure and delays the onset of vasospasm in patients with primary, but not secondary, Raynaud's phenomenon. These improvements are associated with significantly increased digital systolic blood pressures in cold temperatures.

Inhibition of atherosclerosis by fish oil in cholesterol-fed rabbits

To evaluate the effects of dietary fish oil on cholesterol-induced atherosclerosis, 36 New Zealand rabbits in four groups were fed a 0.3% cholesterol diet for 10 weeks. One group served as control, whereas groups I, II and III received 1, 2 and 3 ml/day, respectively, of fish oil (Protochol, eicosapentaenoic acid, 180 mg, and docosahexaenoic acid [DHA], 120 mg/ml). The percent of aortic and pulmonary atherosclerosis was measured by planimetry of sudanophilic lesions. The percent of aortic lesions in the control group was 59 +/- 22%. The two higher dose fish oil groups showed a significant reduction in aortic lesions: group I (40 +/- 26%, p = NS), group II (18 +/- 11%, p less than 0.01) and group III (36 +/- 22%, p less than 0.05). Area of pulmonary artery lesions was significantly higher in the control group (48 +/- 22%) as compared with group I (15 +/- 13%, p less than 0.01), group II (4 +/- 3%, p less than 0.01) and group III (8 +/- 9%, p less than 0.01). The high cholesterol diet in the control group decreased bleeding time from 82 +/- 17 to 59 +/- 22 s (p less than 0.05). Groups II and III showed an increased bleeding time (62 +/- 15 to 84 +/- 17 s and 66 +/- 22 to 95 +/- 27 s; p less than 0.05, respectively). Fish oil did not significantly alter total serum cholesterol and high density lipoprotein (HDL) cholesterol. In group II triglyceride decreased from 128 +/- 22 to 64 +/- 25 mg/dl (p less than 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)

Platelets and antiplatelet therapy in ischemic heart disease

Blood platelets have been shown to play an important role not only in thrombosis, but also in the pathogenesis of coronary artery disease and its complications. Drugs that affect platelets have been shown to reduce mortality in survivors of acute myocardial infarction, to reduce the risk of myocardial infarction in patients with unstable angina, and to preserve the potency of saphenous venous grafts used to bypass obstructed coronary arteries. The drugs may also play a role in the primary prevention of arteriosclerosis and in preventing thrombotic complications following coronary angioplasty.

Fatty acid pattern of red blood cell membranes and risk of ischemic brain infarction: a case-control study

The fatty acid composition of red blood cell membranes (which reflects dietary fat intake) was studied in 28 male patients with recent (less than 3 days) ischemic stroke and 56 matched controls. Fifteen fatty acids were measured by means of chromatographic analysis. Percentages of linoleic, 22:5, and 22:6 acids were significantly lower in red blood cell membranes of stroke patients than in those of matched controls. The results suggest that a low unsaturated fatty acid diet could be an independent risk factor for ischemic brain infarction.

High-dose acetylsalicylic acid after cerebral infarction. A Swedish Cooperative Study

Within 3 weeks of the event, 505 patients with cerebral infarction, minor or major stroke, were randomly assigned to treatment with acetylsalicylic acid (ASA) 1.5 g/day or placebo in a double-blind clinical trial with a follow-up of 2 years in all patients. Primary events were considered to be recurrent stroke or death; secondary events, myocardial infarction and transient ischemic attack. There was no difference in stroke recurrence rate in the ASA and placebo groups (12 and 13%, respectively), nor was there any significant difference in the rate of recurrent stroke or death, first event counted (23% in the ASA and 22% in the placebo group). The risk of transient ischemic attack and myocardial infarction was not reduced in the ASA group. In the present study there was no prophylactic effect of high-dose ASA after cerebral infarction. A compilation of the major trials of ASA after transient ischemic attack and cerebral infarction is presented.

Coagulability and Rheology: Hematologic Benefits From Exercise, Fish, and Aspirin. Implications for Athletes and Nonathletes

In brief: Physical activity makes the blood more fluid and less likely to clot. The healthy hematologic adaptations to exercise (enhanced fibrinolysis, expanded plasma volume, decreased hematocrit, increased red cell deformability, and decreased blood viscosity) seem to enhance the delivery of oxygen and decrease the risk of thrombosis. Regular exercise, then, by changing the blood, may offer the elite athlete enhanced performance and the general population reduced risk of heart attack. Increased amounts of fish in the diet and-for selected persons-low-dose aspirin, may be useful antithrombotic adjuncts to exercise.

Rectal bleeding induced by Dipyridamole

Nineteen patients treated continuously with Dipyridamole were evaluated for rectal bleeding. Thirteen suffered from overt rectal bleeding and six served as controls. Hemorrhoids were found in all patients. Contact bleeding was found in 16. The bleeding continued despite rubber band ligation, and stopped only on withdrawal of the drug.

Thromboembolism in patients with aortic porcine bioprostheses

Thromboembolism was prospectively studied in 215 patients who survived aortic valve replacement with porcine bioprostheses. All patients were anticoagulated with warfarin sodium during the first 3 postoperative months. Thereafter, 80 patients received aspirin (325 or 650 mg per day) and 135 received no antiplatelet or anticoagulant drugs. The two groups of patients were similar. Thromboembolic complications were carefully searched for during the follow-up interviews. Patients were removed from the study after a thromboembolic event unless there was no change in their management. The follow-up ranged from 6 to 80 months (mean, 36 months). The linearized thromboembolic rate in patients on a regimen of aspirin was 1.3% per patient-year and in patients not taking aspirin, 5.2% per patient-year (p less than 0.02). Replacement of the ascending aorta and patch enlargement of the aortic annulus with a Teflon graft were identified as significant risk factors for thromboembolism in patients with aortic porcine bioprostheses. These findings indicate that patients with aortic porcine bioprostheses should receive aspirin, especially if they also had replacement of the ascending aorta or patch enlargement of the aortic annulus with a Teflon graft.

Runner's macrocytosis: a clue to footstrike hemolysis. Runner's anemia as a benefit versus runner's hemolysis as a detriment

This report of a runner in whom progressive macrocytosis developed with increasing mileage deals with the hematologic adaptations to exercise, introduces the concept of "runner's macrocytosis" as a compensated hemolysis of older red cells, and makes a case for "runner's anemia" as a benefit versus "runner's hemolysis" as a detriment. It presents the characteristic hematologic profile of footstrike hemolysis and explores the influence of racing, different levels of training, and different shoes. It shows that runner's hemolysis can be reduced by reducing mileage but not necessarily by changing shoes, and it suggests that runner's hemolysis can impair race performance by preventing the attainment of an optimal red cell mass and, in time, by evolving into iron-deficiency anemia.