Variations in circulating catecholamines fail to alter human platelet alpha-2-adrenergic receptor number or affinity for [3H]yohimbine or [3H]dihydroergocryptine.

A series of studies were performed to determine the relationship between physiologic levels of circulating plasma norepinephrine and epinephrine and human platelet alpha-2 binding site number and the affinity (KD) of these sites for antagonist radioligands. In one study, alpha-2-adrenergic binding site number and affinity were compared using both [3H]yohimbine and [3H]dihydroergocryptine as radioligands. There was good absolute and relative comparison for binding site number, but only a relative relationship for KD. In 46 normal subjects, there was no significant relationship between site number or KD and age, plasma epinephrine, or plasma norepinephrine concentration. Even after plasma epinephrine was raised nearly 20-fold by means of an intravenous infusion for 4 h in seven normal subjects, neither sites (608 +/- 68 vs. 567 +/- 120 sites/platelet) nor KD (2.01 +/- 0.94 vs. 2.14 +/- 1.15 nM) were significantly changed. Similarly, neither sites (445 +/- 55 vs. 421 +/- 53 sites/platelet) nor KD (1.44 +/- 0.29 vs. 2.10 +/- 0.75 nM) were significantly changed in six normal subjects when plasma norepinephrine levels increased during oral administration of prazosin for 1 wk. Thus, in a cross-sectional analysis and after a change in plasma catecholamine concentrations, there was no relationship in normal subjects between platelet alpha-2 binding site number or affinity of these sites for antagonist radioligands and the circulating catecholamine levels to which the platelets were exposed. In a group (n = 7) of patients who lack epinephrine-induced platelet aggregation due to abnormal thrombopoiesis, binding site number was decreased (304 +/- 36 vs. 572 +/- 29 sites/platelet, P less than 0.001) and KD tended to be greater (8.69 +/- 2.44 vs. 5.40 +/- 0.31 nM, P = NS) than in normal subjects (n = 46), despite having similar plasma catecholamine levels. There was no difference in binding site number (491 +/- 116 sites/platelet) and KD (5.61 +/- 0.84 nM) in patients (n = 5) with autonomic insufficiency and low levels of upright plasma norepinephrine when compared with the normal subjects. Two patients were examined before and after the removal of a pheochromocytoma. Their binding site number and KD were normal before the operation and essentially unchanged after the tumor removal and fall of plasma catecholamines. Thus, this study demonstrates that within the physiologic and pathophysiologic range of plasma catecholamines (in men), there is no relationship between the circulating catecholamine concentration and either platelet alpha-2 adrenergic binding site number or the affinity of these sites for antagonist radioligands.

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