Role for endogenous dopamine in modulating sympathetic-adrenal activity in humans.

Dopamine (DA) is synthesized and secreted in central as well as peripheral nervous system and in the adrenal medulla. Neuronal DA receptors, which have been characterized as D2 receptors, mediate an inhibition of adenylate cyclase and are located prejunctionally on sympathetic nerve endings and on chromaffin cells. Their pharmacological activation causes an inhibition of in vitro and in vivo norepinephrine (NE) release from sympathetic nerve terminals and an inhibition of in vitro epinephrine (E) release from the adrenal medulla. Endogenous DA, co-secreted with the other catecholamines (CA), modulates sympathetic-adrenal discharge only during high sympathetic stimulation through an autocrine mechanism, limiting excessive sympathetic adrenal discharge. Also pheochromocytoma cells synthesize and express D2 receptors. In patients with pheochromocytoma D2 antagonists cause hypertensive crises but the mechanism mediating this effect is still unknown as well as whether endogenous DA might modulate tumoral secretion.