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1. Plasma levels of beta-thromboglobulin, initial and total platelet aggregation (induced by adrenaline or adenosine diphosphate [ADP]) were determined in 26 normotensive subjects and 26 patients with untreated essential hypertension. Groups of 18 essential hypertensive patients and 18 age- and sex-matched normotensives were compared. 2. After 7 days of treatment with prazosin in a dose of 2-8 mg daily the above measures were repeated in 18 essential hypertensive patients. A significant increase in plasma levels of beta-thromboglobulin, initial and total adrenaline-induced as well as ADP-induced platelet aggregation was found in hypertensives. Prazosin restored the mean arterial blood pressure in hypertensives to normal, but it had no significant influence either on increased beta-thromboglobulin levels or on initial and total aggregability. 3. The results confirm increased platelet aggregation and in vivo platelet activation in patients with essential hypertension; however there is a discrepancy with previous reports about those results obtained after prazosin therapy. The results suggest that increased platelet aggregation and in vivo activation need not be restored to normal after effective antihypertensive therapy alone. They give reason for the combination of antihypertensive together with anti-aggregatory therapy in essential hypertension.

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