Functional relationships between platelet alpha2-adrenoceptors and sympathetic nerve activity in clinical hypertensive states

Abstract

Tritiated yohimbine binding was used to estimate platelet membrane alpha 2-adrenoceptor characteristics in normal subjects (n = 12) and patients with essential hypertension (n = 30), primary hyperaldosteronism (n = 10) and phaeochromocytoma (n = 10). There was a 20-fold increase in mean levels of resting norepinephrine and epinephrine in the phaeochromocytoma group. Total binding sites (Bmax) and dissociation constant (Kd) for 3H-yohimbine did not differ significantly among the four groups. Following curative surgery for phaeochromocytoma, plasma catecholamine levels were normalized but the Bmax remained unchanged. Following surgery for primary hyperaldosteronism, levels of plasma norepinephrine rose but Bmax was not changed. In all but the phaeochromocytoma patients, Bmax values correlated positively (r = 0.437, n = 48, P less than 0.01) with resting plasma norepinephrine. A significant negative correlation was observed between the change in plasma norepinephrine in response to postural change and resting Bmax. Treatment for 2-4 weeks with guanabenz or bethanidine induced a parallel fall in both Bmax and plasma norepinephrine. Treatment with reserpine was followed by a comparable fall in plasma norepinephrine, but Bmax values were unchanged. The findings support the view that platelet alpha 2-adrenoceptor density is functionally regulated in parallel with sympathetic nerve activity rather than circulating catecholamine levels, although it is not known which neuronal substance(s) may participate in this regulation.