Effect of phentolamine on platelet aggregation in patients with pheochromocytoma

Abstract

Since exogeneous catecholamines potentiate ADP-induced aggregation in vitro, and this effect is blocked directly by phentolamine, it was assumed that platelets from patients with pheochromocytoma would be unusually sensitive to ADP and that this sensitivity would be reduced in the presence of phentolamine. Findings in four patients with pheochromocytoma were compared to results in 20 normals. Phentolamine had no immediate effect in either group. Pheochromocptoma platelets became more responsive to ADP after standing and this increase in responsiveness was inhibited by phentolamine. These results: 1) suggest that catecholamine concentrations in the plasma of patients with pheochromocytoma are not high enough to potentiate ADP aggregation and 2) may be explained by assuming that pheochromocytoma platelets are saturated with catecbolamines.

Sodium depletion increases platelet and plasma catecholamines in hypertensive men.

“…One may speculate that an increased platelet content of catecholamines can stimulate aggregation of platelets (i.e., explain the enhanced platelet aggregation to adenosine diphosphate in pheochromocytoma). 21 Plasma free dopamine concentration is lower than plasma epinephrine concentration and just above the detection limit for the radioenzymatic assay. 13 ' 15 However, platelet dopamine was two to three times higher than platelet epinephrine, and dopamine had the highest platelet to plasma concentration ratio of the three catecholamines.…”