Assessment of lipid peroxidation by measuring malondialdehyde (MDA) and relatives in biological samples: Analytical and biological challenges

Endogenous modulators of platelet aggregability and vascular tone may play a part in coronary-artery disease. We therefore measured the release of prostaglandins and thromboxane into the coronary circulation in patients with various kinds of cardiac disease. Simultaneous coronary-sinus (CS) and ascending-aortic (AO) blood samples were obtained from 60 patients for measurement of 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha, a prostaglandin I2 metabolite) and of thromboxane B2 (TxB2). Samples from 45 of these patients were also tested for prostaglandin E2 (PGE2) and lactate. Patients with unstable angina pectoris who reported chest pain within 24 hours of study had higher TxB2 CS/AO ratios (5.8 +/- 2.8, mean +/- S.D.) than patients whose most recent anginal pain was more than 96 hours before study (1.3 +/- 0.6; P less than 0.05), than those with nonischemic chest pain (1.2 +/- 0.4; P less than 0.05), or with valvular or congenital nonischemic heart disease (1.2 +/- 0.6; P less than 0.05). Those whose most recent anginal pain occurred 24 to 96 hours before study were distributed bimodally: the majority had low TxB2 CS/AO ratios (range, 0.5 to 2.1) like the patients in the three aforementioned groups, whereas a few had markedly elevated values (range, 10.5 to 46.6). The 6-keto-PGF1 alpha and PGE2 CS/AO ratios and myocardial lactate extraction were not significantly different among the five groups. These data suggest that local thromboxane release is associated with recent episodes of angina in patients with unstable angina pectoris, but whether this release is a cause or an effect is not yet known.

Release of prostaglandins and thromboxane into the coronary circulation in patients with ischemic heart disease.

Nonsteroidal anti-inflammatory drugs and sulfinpyrazone compete dose-dependently with arachidonate for binding to platelet cyclooxygenase. Such a process closely follows systemic plasma drug concentrations and is reversible as a function of drug elimination. Peak inhibition and extent of its reversibility at 24 hr varies consistently with individual pharmacokinetic profile. Inhibition of platelet cyclooxygenase activity by these agents is associated with variable effects on prostaglandin (PG) synthesis in the gastric mucosa and the kidney. Aspirin acetylates platelet cyclooxygenase and permanently inhibits thromboxane (TX) A2 production in a dose-dependent fashion when single doses of 0.1 to 2.0 mg/kg are given. Acetylation of the enzyme by low-dose aspirin is cumulative on repeated dosing. The fractional dose of aspirin necessary to achieve a given level of acetylation by virtue of cumulative effects approximately equals the fractional daily platelet turnover. Serum TXB2 measurements obtained during long-term dosing with 0.11, 0.22, and 0.44 mg/kg aspirin in four healthy subjects could be fitted by a theoretical model assuming identical acetylation of platelet (irreversible) and megakaryocyte (reversible) cyclooxygenase. For a given dose within this range, both the rate at which cumulative acetylation occurs and its maximal extent largely depend upon the rate of platelet turnover. Continuous administration of low-dose aspirin (20 to 40 mg/day) has no statistically significant effect on urinary excretion of either 6-keto-PGF1 alpha or 2,3-dinor-6-keto-PGF1 alpha, i.e., indexes of renal and extrarenal PGI2 biosynthesis in vivo. Whether a selective sparing of extraplatelet cyclooxygenase activity by low-dose aspirin will result in increased antithrombotic efficacy, fewer toxic reactions, or both remains to be established in prospective clinical trials.

Clinical pharmacology of platelet cyclooxygenase inhibition.

This review discusses the role of three mediators, synthesized by vascular endothelial cells, that help to keep the surface of the normal endothelium nonthrombogenic. The first is prostacyclin, a product of arachidonic acid metabolism discovered in 1976. This labile prostanoid, with a half-life of approximately 3 minutes, relaxes vascular smooth muscle and inhibits the aggregation of blood platelets. Prostacyclin and its analogues are currently being tested clinically for use in cardiovascular diseases such as primary pulmonary hypertension. The second mediator discussed is endothelium-derived relaxing factor (EDRF), discovered in 1980, which also relaxes smooth muscle and inhibits the aggregation and adhesion of platelets. Substances that stimulate the release of EDRF include acetylcholine, bradykinin, and adenosine 5'-diphosphate. EDRF is even more labile than prostacyclin, with a half-life of about 6 seconds, and it has recently been identified as nitric oxide. Prostacyclin and EDRF are released together following stimulation of endothelial receptors and synergize to inhibit platelet aggregation. 13-Hydroxy-9,11-octadecadienoic acid, a third suggested mediator, is not released but acts from inside the cell to make the endothelial surface nonadhesive for circulating blood cells. It is proposed that these three mediators form the endothelial defense mechanism against blood-borne cells and chemicals and that breakdown of this barrier results in diseases such as hypertension and atherosclerosis.

Mediators produced by the endothelial cell.

Since the 1930s and the discovery by von Euler of a vasoactive, lipid-soluble substance that he erroneously assumed was generated by the prostate gland and therefore should be called "prostaglandin," the family of prostaglandins has grown to some 90 substances. These lipid mediators are derived from arachidonic acid in the "arachidonic acid cascade." In 1976, while looking for the enzyme that generates the unstable prostanoid thromboxane A2 from arachidonic acid, Moncada and Vane discovered prostaglandin I2 and renamed it "prostacyclin." Prostacyclin is the main product of arachidonic acid in all vascular tissues tested to date and strongly vasodilates all vascular beds studied. It is also the most potent endogenous inhibitor of platelet aggregation yet discovered, both inhibiting aggregation and dispersing existing aggregates. It acts through activation of adenylate cyclase, leading to increased levels of cyclic adenosine monophosphate. It also appears to have a "cytoprotective" activity, as yet not completely understood. Its effects are short-lasting, disappearing within 30 minutes of cessation of infusion. A stable, freeze-dried preparation of prostacyclin (epoprostenol) is available for administration to humans, and several analogs with therapeutically desirable characteristics are currently being clinically tested and should become commercially available soon. Clinical application of prostacyclin is bedeviled by 2 characteristics: it is pharmacologically unstable, so care must be taken in its use, and the correct dosage regimens have not yet been established.

Pharmacodynamic profile of prostacyclin

Circulatory and anti-platelet effects of intravenous prostacyclin in healthy men

Arachidonic acid (AA)-induced platelet aggregation was studied in platelet-rich plasma of 30 male patients who survived myocardial infarction and in 30 healthy men of similar age. Mean platelet aggregation thresholds to AA were 746 +/- 62 micrometer, and 869 +/- 57 micrometer, respectively. Only in 2 healthy subjects, but in 12 patients, irreversible platelet aggregation was induced consistently with low concentrations of AA, under 500 micrometer. The rate of conversion of AA to thromboxane A2 (TXA2) by platelets of these patients was augmented. Furthermore, less endogenous TXA2 was required to trigger aggregation of their platelets as compared to the controls. We have also shown that in platelet-poor plasma of these patients with "hyperreactive" platelets there exists a transferable factor which makes platelets of healthy subjects more prone to aggregatory action of AA. It is proposed that the assessment of platelet aggregability with AA provides a tool for identifying a subgroup of patients with coronary heart disease who might substantially benefit from the secondary preventive treatment with aspirin and with other antiplatelet drugs which inhibit the generation of TXA2 in platelets.

Musiał J

Papers

Circulatory and anti-platelet effects of intravenous prostacyclin in healthy men

Thromboxane generation and platelet aggregation in survivals of myocardial infarction.

Pulmonary and anti-platelet effects of intravenous and inhaled prostacyclin in man

Fibrinolytic activity of prostacyclin and iloprost in patients with peripheral arterial disease

Prostacyclin and the fibrinolytic system in ischemic vascular disease.