Exocytosis plays an important role in catecholamine secretion from human pheochromocytoma.

Plasma dopamine beta-hydroxylase (DBY) activity, urinary catecholamine (CA) and vanillylmandelic acid (VMA) were studied pre- and postoperatively in 20 patients with pheochromocytoma. Preoperative plasma DBH activities were distributed widely, ranging from normal to very high; the mean value of DBH as well as urinary CA and VMA was significantly greater than that of normal subjects. Postoperative falls of plasma DBH activity, of urinary CA and of VMA were by 76%, 96% and 83% of the preoperative level, respectively. Significant correlations were observed between postoperative falls of individual DBH activities, urinary VMA and weight of the tumors, whereas there was correlation neither between preoperative DBH activity and urinary CA, nor between the rate of postoperative falls of these respective values. These findings suggest that pheochromocytoma is capable of secreting DBH, and wide variations in the rate of DBH output from these tumors may account for varying capacity of CA biosynthesis in the tumor. Further, it may be concluded that the process of exocytosis plays an important role in the mode of CA secretion by these tumors, although secretion through diffusion may be operating as well.