1. Agonist regulation of platelet α2-adrenoceptors was examined in human volunteers after acute elevations of adrenoceptor agonist and during chronic elevation of plasma catecholamines in two patients with phaeochromocytoma.
2. Platelet α2-adrenoceptor number was measured by radioligand binding ([3H]yohimbine) and α2-adrenoceptor function measured by turbidimetric platelet aggregation.
3. Short term infusion of adrenoceptor agonists with α2 activity caused reductions in the platelet response to adrenaline in vitro; conversely an increase in activity was observed postoperatively in two patients after removal of phaeochromocytoma.
4. The changes in platelet response were not accompanied by changes in α2-adrenoceptor number.
5. It is proposed that a process of receptor inactivation occurs during desensitization and this is responsible for the dynamic regulation of platelet responses.