Platelet lipid peroxides as determinant of platelet survival and sympathetic stimulation in patients with hypertension

The role of platelets in essential hypertension

Recent research is helping us understand the complex interactions that occur between platelets and their environment. The several intracellular events that occur during platelet activation are being identified as ar their effects on other platelets, the endothelium and coagulation factors. Heightened platelet activation is seen early in essential hypertension and probably plays an important role in the initiation and progression of atherosclerosis and the disorders associated with it. This review identifies some of the changes in platelet structure and function in essential hypertension and their role in the pathogenesis of hypertensive vascular disease.

Blood platelets in human essential hypertension

Blood platelets of patients with essential hypertension display signs of both increased sensitivity in vitro to aggregating stimuli believed to contribute to thrombosis and of activation in vivo possibly expressing the release of vasoactive products. The mean features of the modified platelet profile in hypertension include an increased alpha 2-adrenergic receptor density, an enhanced rate of adhesion/aggregation in particular in response to ADP and arachidonic acid, a greater sensitivity for thrombin and adrenaline to stimulate increases in cytoplasmic-free Ca2+, increased resting levels of cytoplasmatic-free Ca2+, a reduced content of serotonin often combined with a defective uptake mechanism, a facilitated efflux rate of noradrenaline, an exaggerated release reaction in vivo as indicated by the increased plasma levels of Beta-thromboglobulin and a shortened platelet life span. These changes occur to various extents in some, but not all, hypertensive patients and are not always strictly related to the degree of blood pressure increase. On the contrary, platelet cyclooxygenase and thromboxane synthetase activity are in the normal range.

Platelet release of beta-thromboglobulin within the coronary circulation during cold pressor stress

Cold stress by increasing circulating catecholamines may sensitize blood platelets to aggregate and release their constituents. This study investigates the effect of cold stress on the release of the platelet-specific protein beta-thromboglobulin into the coronary venous blood of 12 subjects with atherosclerotic coronary artery disease (CAD) and 7 subjects with angiographically normal coronary arteries (NCA). Cold pressor stress caused a greater increase in systolic arterial pressure in patients with CAD than in subjects with NCA (p less than 0.05). There was no significant difference between the platelet counts in the arterial or coronary venous blood either before or during cold stress. Arterial beta-thromboglobulin was higher in the group with CAD (77 +/- 18 ng/ml) than in subjects with NCA (49 +/- 12 ng/ml, p less than 0.01). Although there was no arteriovenous difference of beta-thromboglobulin at rest in either group, during cold stress, coronary venous beta-thromboglobulin increased in both the NCA (53 +/- 16 to 95 +/- 26 ng/ml, p less than 0.05) and CAD groups (76 +/- 13 to 117 +/- 53 ng/ml, p less than 0.025) despite no change in arterial beta-thromboglobulin. Release of beta-thromboglobulin, although not related to the presence of angiographic arterial disease, correlated with the systolic arterial pressure during cold stress (r = 0.66) and inversely with the platelet's ability to generate cyclic adenosine monophosphate (r = 0.69). The release of platelet constituents in the coronary circulation is provoked by cold stress and may play a role in stress-induced acute coronary occlusion in patients with atherosclerotic disease and in those with apparently normal vessels.