Sulfoconjugated
catecholamines have been regarded simply as metabolites of free
catecholamines. However, a conjugated form of the
catecholamine,
dopamine has recently attracted much attention because it is present at high levels in the plasma of humans and experimental animals. We carried out experimental and clinical studies to determine the physiological significance of this large amount of
dopamine conjugate in the plasma. Clinical studies showed that the plasma level of
dopamine sulfate decreased significantly during the acute phase of
heart failure, whereas that of free
dopamine increased. Moreover, the plasma level of conjugated
dopamine in patients with
essential hypertension was higher than that in control subjects, and being highest in patients with
renal hypertension. In experimental studies, we examined the activity for deconjugating DA
sulfate in homogenates of organs from dogs. The kidney and liver exhibited the highest activities, and in the heart, the activity was higher in the atrium than the ventricle. We also examined the effect of
dopamine sulfate on isolated perfused rat heart.
Dopamine sulfate was found to be converted to free
dopamine, which was responsible for the positive inotropic action, in atrial tissue. Moreover, deconjugation of DA
sulfate to the free form was accelerated by a high work lord on the heart. From these results, we conclude that the formation of
dopamine sulfate plays a role in regulating the level of plasma free
dopamine and that plasma
dopamine sulfate may be a storage or reserve form of
dopamine. Free (or active)
dopamine may be formed through a deconjugation reaction when necessary.