Platelet α2-Adrenergic Receptors in Treated and Untreated Essential Hypertension

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Abstract
One hypothesis to account for altered adrenergic response in hypertensive patients is alteration in adrenergic receptors on responsive cells. Radioligand binding methods were used to examine the .alpha.2-adrenergic receptors on platelets isolated from 17 normal men and from 19 men with essential hypertension. The .alpha.2-selective radioligand [3H]yohimbine was used to determine receptor number and affinity on intact platelets. The median number of receptors per platelet was 265 for the hypertensive patients vs. 246 for the controls. There was no difference between hypertensives and controls in Kd of the receptors for [3H]yohimbine or adrenaline [epinephrine]. Antihypertensive treatment with the .alpha.2-agonist guanabenz or the .beta.-antagonist propranolol did not change the number or affinity of platelet .alpha.2-receptors. In untreated hypertensives the receptor number did not correlate with age, blood pressure or 24 h urinary excretion of catecholamines or Na+. Apparently, neither hypertension nor anti-hypertensive treatment alters the number of .alpha.2-adrenergic receptors on human platelets. Because therapy with an .alpha.2-agonist does not alter the receptor number and because the receptor number did not correlate with urinary excretion of catecholamines (an index of sympathetic-nervous-system activity), down-regulation of human platelet .alpha.2-adrenergic receptors may not readily occur in vivo.