The influence of age, sex, and the use of oral contraceptives on the inhibitory effects of endothelial cells and PGI2 (prostacyclin) on platelet function

Platelet cGMP inversely correlates with age in healthy subjects

Biochemical modifications associated with the increase in platelet activity with age are not well defined. Furthermore it is well known that the nitric oxide/cyclic 3', 5'-guanosine monophosphate (cGMP) pathway regulates platelet aggregation. The aim of the present study was to examine the relationship between platelet content of cGMP and age. 120 normal subjects, evaluating the cGMP platelet concentration, 17betaE2, IGF-I, dehydroepiandrosterone sulphate (DHEAS), insulin, plasma glucose, lipid pattern, homocysteine and PAI-I antigen, were studied. The multivariate analysis in a model with cGMP as dependent variable and with age, LDL, apolipoprotein B (ApoB), DHEAS, E2 and insulin-like growth factor (IGF)-I as independent variables shows a negative correlation between cGMP and age (p<0.01, beta=-0.388). In conclusion our data suggest that the reduced efficiency of the system constitutive nitric oxide synthase (cNOS)/guanylate cyclase represents at least one of the reasons of the increased platelet aggregability with age.

Occlusive vascular diseases in oral contraceptive users. Epidemiology, pathology and mechanisms

Despite being an unprecedented departure from normal physiology, the combined oral contraceptive is not only highly effective, but it also has a remarkably good safety record. Concerns over safety persist, though, particularly with regard to venous thromboembolism (VTE), stroke and myocardial infarction (MI). Epidemiological studies consistently show an increase in risk of VTE, but the results are more contentious with regard to arterial diseases. Despite 40 years of research, the mechanisms behind these adverse effects are not understood. In this review, we integrate information from published studies of the epidemiology and pathology of the occlusive vascular diseases and their risk factors to identify likely explanations for pathogenesis in oral contraceptive users. Oral contraceptives induce both prothrombotic and fibrinolytic changes in haemostatic factors and an imbalance in haemostasis is likely to be important in oral contraceptive-induced VTE. The complexity of the changes involved and the difficulty of ascribing clinical significance has meant that uncertainty persists. A seriously under-researched area concerns vascular changes in oral contraceptive users. Histologically, endothelial and intimal proliferation have been identified in women exposed to high plasma estrogen concentrations and these lesions are associated with thrombotic occlusion. Other structural changes may result in increased vascular permeability, loss of vascular tone and venous stasis. With regard to arterial disease risk, epidemiological information relating to dose effects and joint effects with other risk factors, and studies of pathology and changes in risk factors, suggests that oral contraceptive use per se does not cause arterial disease. It can, nevertheless, synergise very powerfully with subclinical endothelial damage to promote arterial occlusion. Accordingly, the prothrombotic effects of the oral contraceptive estrogen intervene in a cycle of endothelial damage and repair which would otherwise remain clinically silent or would ultimately progress - in, for example, the presence of cigarette smoking or hypertension - to atherosclerosis. Future work in this area should focus on modification of the effects of established risk factors by oral contraceptive use rather than modification of the supposed risk of oral contraceptive use by established risk factors. Attempts to understand vascular occlusion in oral contraceptive users in terms of the general features of VTE or with reference to atherosclerosis may be limiting, and future work needs to acknowledge that such occlusions may have unique features. Unequivocal identification of the mechanisms involved would contribute considerably to the alleviation of fears over vascular disease and to the development of even safer formulations.

Hemostatic concomitants of syndrome X

The elevated risk factors classically associated with Syndrome X do not adequately explain the increased cardiovascular risk linked to insulin resistance. Considerable indirect evidence and more limited direct evidence suggests that increases in certain prognostically significant hemostatic variables--notably plasma fibrinogen, factor VII coagulant activity, and plasminogen activator inhibitor-1 levels--may be concomitants of Syndrome X. Increased platelet aggregability, and a reduced capacity of vascular endothelium for prostacyclin generation, may also be associated with insulin resistance. Large clinical studies, involving quantification of insulin sensitivity and measurement of hemostatic variables, are required to test this hypothesis. It is unlikely that hyperinsulinemia mediates the connections between insulin resistance and hemostatic variables (or indeed other risk factors in Syndrome X). In light of the remarkably broad impact of insulin resistance on cardiovascular risk factors, promotion of optimal insulin sensitivity should assume a central role in preventive cardiology.

Oxidative status and oral contraceptive. Its relevance to platelet abnormalities and cardiovascular risk

Oral contraceptive (OC) use is a risk for thrombogenic events. This paper reviews effects of OC on oxidative status, coagulation, and platelet activity. Complicating effects of cardiovascular risk factors such as smoking, diabetes, hyperpidemia, and hypertension, are discussed. From these data we conclude that: 1. OC use modifies slightly but significantly the oxidative status in women and in animals by decreasing in plasma and blood cells the antioxidant defenses (vitamins and enzymes). 2. The changes in the oxidative status are related to an increase in plasma lipid peroxides apparently responsible for the hyperaggregability and possibly the imbalance in clotting factors associated with the OC-induced prethrombotic state. 3. These effects of OC appear to be increased by a high intake of polyunsaturated fat and counteracted by supplements of vitamin E. 4. The risk factors acting synergistically with OC, have all been shown to increase platelet reactivity. In addition, smoking, diabetes, and, to some extent, dyslipidemia are associated with an increased level of lipid peroxides and concomitant changes in the antioxidant defenses that can be additive to those induced by OC. Thus, free radicals and lipid peroxidation could be the underlying mechanism in the predisposition to thrombosis induced by most risk factors in OC users. 5. Results of epidemiologic and experimental studies in this field will be concordant only when diet and natural antioxidants will be systematically taken into consideration.

Low molecular weight heparin as an anticoagulant for in vitro platelet function studies

We evaluated the suitability of low molecular weight (LMW) heparin as an anticoagulant for in vitro platelet function tests in 11 normal volunteers. Results with citrated platelets were considered as the standard. Spontaneous platelet aggregation and the aggregation responses to ADP, epinephrine, collagen, ristocetin and thrombin were measured turbidimetrically in an aggregometer. Dose-response and dose-rate curves were constructed for ADP- and epinephrine-induced aggregation. The maximum aggregation response (EDmax) and rate (EDRmax), and the estimated dose of agonist to induce 50% of the maximum response (ED50) and rate (EDR50) were calculated from these curves. The inhibition of ADP-induced aggregation with PGI2 was expressed as per cent inhibition. The release of ATP and TxA2, from platelets aggregated with collagen was measured. No spontaneous aggregation occurred with either anticoagulant. The ED50 and the EDR50 for heparinized platelets were significantly lower for ADP induced aggregation (0.8 +/- 0.3 microM vs 2.1 +/- 1.0 microM [p = 0.001] and 0.4 +/- 0.1 microM vs 0.8 +/- 0.3 microM [p = 0.003]). The EDRmax with ADP was significantly higher (p = 0.004) for heparinized platelets (64.6 +/- 17.0 units/ml vs 50.4 +/- 7.6 units/ml). The heparinized platelets aggregated slightly, but significantly, less in response to ristocetin than the citrated platelets. The response of washed heparinized and citrated platelets to thrombin was not significantly different. The per cent inhibition of ADP aggregation with PGI2, was significantly lower with heparinized platelets. The release of TxA2 and ATP was similar for both anticoagulants. These results indicate that LMW heparin is a satisfactory anticoagulant for platelet aggregation tests.

Technical considerations for platelet aggregation and related problems

Platelet aggregation generally is ordered by the physician to evaluate platelet function in hemorrhagic or thrombotic disorders. Malfunction of the platelet may be the result of an intrinsic congenital defect or an acquired problem induced by drugs or certain circulating plasma factors. It is necessary to obtain information from the patient with respect to family history, drug ingestion, physical or mental stress. In addition, other laboratory studies should be obtained to rule out general coagulation disorders affecting the plasma factors. A bleeding time will be helpful in establishing the severity of any platelet dysfunction. Technical considerations with regard to the preparation of the samples are of primary importance in determining platelet aggregation. Aggregating studies require the use of a variety of binding agents. (Studies on shape change, adhesion of platelets, release of platelet granule substance, and or lysis with extrusion of cytoplasmic constituents may be helpful in certain cases.) Instrumentation for platelet aggregation presently is available in many hospitals. The technical factors to be considered for routine aggregation studies include the type and strength of anticoagulant, centrifugation technique used in preparing the platelet-rich and platelet-poor plasma, platelet concentration, time of storage of the sample after venipuncture and after centrifugation, temperature, and the mixing of the sample. In general, critical concentrations of each reagent should be employed to improve the discrimination capability of the assay. Small differences in response may be obliterated by using excessive concentrations of a given reagent. Comparison in response to the test platelets with control platelets is best done at the same time by performing the aggregation in a dual instrument so that handling procedures will be identical and artifactual differences eliminated.

Oral contraceptives and blood coagulation: a critical review

Based on epidemiologic data, women who take oral contraceptives seem to have an increased risk of developing thromboembolic disease. In order to explain this association, some studies have been undertaken to find abnormalities in the hemostasis system. Many conflicting test results have been reported, probably reflecting in part the technical difficulties in use of reliable, reproducible, and specific assay systems. None of the data obtained seems to be specific for users of oral contraceptive drugs, and none was found uniformly in all takers of the drugs. Many findings are probably unimportant from the overall point of view of the physiology of hemostasis. Unfortunately, there seems to be no test procedure or even combination of tests that will reliably predict which patient might develop a thromboembolic episode in association with oral contraceptives or in association with any other condition recognized as being a risk factor. Even in most patients who do have a thrombosis, the test procedures fail to signal this event with reliability and predictability. At this time, it is not known whether oral contraceptives do or do not trigger the hemostasis system to develop a thrombosis or by which mechanism they could trigger the event. Fortunately, only a very small number of women develop thrombosis in association with oral contraceptives.

Catechol estrogens and thrombosis: lack of a direct effect of 2-hydroxyestradiol on platelets

Women who use the oral contraceptive pill are at risk for a variety of thromboembolic complications, but the precise mechanism is unclear. Since 2-hydroxyestradiol is a major metabolite of estradiol and possesses some of the structural and functional properties of catecholamines, we examined whether this compound might influence thrombogenesis, in particular whether it would exert an epinephrine-like effect on the platelet. In vitro, 2-hydroxyestradiol does not cause or potentiate aggregation or thromboxane release, nor does it prevent the rise in cyclic AMP caused by anti-aggregatory prostaglandins. While 2-hydroxyestradiol does not exert a direct effect on the platelet in vitro, it may play a role in thrombosis in women who take oral contraceptives through another mechanism.

Prostaglandins and ischemic heart disease

There is an abundance of information suggesting that prostaglandins are involved in the development and clinical expression of atherosclerosis. Many studies demonstrate a relationship between prostaglandins and the risk factors for peripheral and coronary artery disease. Thus, part of the mechanism by which hyperlipidemia, diabetes mellitus, smoking, hypertension, sex hormones, age, heredity, emotional stress and diet contribute to the development and progression of atherosclerosis may be through an imbalance between thromboxane A2 and prostaglandin I2. Recent studies show a temporal relationship between acute ischemic events (specifically, unstable angina) and a transcardiac increase in thromboxane B2, while others demonstrate a salutary effect of disaggregatory and vasodilatory prostaglandins in such patients. If prostaglandins and thromboxane prove important in ischemic vascular disease, attention will be directed at the correction of their pathologic imbalance. This may be accomplished by dietary manipulation as well as by the development of prostaglandin receptor antagonists or inhibitors of specific prostaglandin pathways.