Altered platelet o 2 adrenoreceptor in acute myocardial infarction and its relation to plasma catecholamine concentrations KEIJI

Changes in platelet a2 adrenoreceptors and their relation to plasma catecholamine concentrations were studied in 11 patients with acute transmural myocardial infarction. A radiolabelled a2 adrenoreceptor antagonist, [3H]-yohimbine, was used to assay x2 adrenoreceptors on platelet membranes, and plasma catecholamine concentrations were measured by high performance liquid chromatography. The number of platelet a2 adrenoreceptors, the dissociation constant, and plasma noradrenaline and adrenaline concentrations were studied 6-6 (3 3) (mean (SD)) hours after the onset of acute myocardial infarction and one month later. The mean (SD) number of adrenoreceptors increased significantly from 94'5 (50'5) fmol/mg protein immediately after infarction to 157-0 (65 7) fmol/mg protein one month later. The dissociation constant, however, did not change significantly (4 33 (1-40) nmol/l vs 437 (1-22) nmol/l). Raised noradrenaline (5-60 (4 37) nmol/l) and adrenaline (0-28 (0 14) nmol/l) concentrations had fallen significantly to normal values (1 21 (0 67) and 0 09 (0 05) nmol/l respectively) a month after infarction. The decrease in the number of a2 adrenoreceptors soon after infarction may be beneficial because such a change will reduce the strength of various reactions to catecholamines, such as vasoconstriction.