Plasma dopamine: source, regulation, and significance.

Plasma dopamine is present in free form in a concentration approximately equivalent to that of epinephrine and about 25% that of norepinephrine. It originates in a variety of tissues including sympathetic nerves and adrenal, and the percentage originating from the various sources remains unknown. Events that are associated with increases in sympathetic activity such as stress, exercise, standing, or hypovolemia are associated with increases in plasma dopamine concentration, although the responses may be of considerably smaller magnitude than those for plasma norepinephrine and epinephrine. The functional importance of dopamine in the periphery and of changes in circulating dopamine remain only poorly defined. These include possible roles in neurotransmission in sympathetic ganglia, altering renal vascular bed, modulation of aldosterone secretion and sodium excretion, mediation of certain stress responses, and others yet to be revealed.