Conjugated dopamine: peripheral origin, distribution, and response to acute stress in the dog

Abstract

Conjugated catecholamines in the circulation and in peripheral tissues were measured together with free catecholamines in an attempt to investigate whether there are in vivo correlates to a possible biological role of dopamine sulfate suggested by an in vitro finding of direct conversion of dopamine sulfate to free norepinephrine by dopamine β-hydroxylase.Following the strong sympathoadrenergic stimulus of surgical stress accompanied by an increase in blood pressure and heart rate, conjugated dopamine showed a twofold rise in arterial plasma (p < 0.005) together with increases of all free catecholamines (0.005 < p < 0.02), while conjugates of noreprinephrine and epinephrine decreased in the circulation (0.01 < p < 0.05). Measurements of arteriovenous differences have shown that release of conjugated dopamine occurred from the adrenal gland during operation along with free catecholamines. However, the venous outflow of conjugated dopamine from liver and kidney did not exceed its arterial influx. Conjugated dopamine, in contrast with other conjugates, is present in adrenals, liver, small intestine, and kidney with higher concentrations than free dopamine in the adrenals (p < 0.01). After ultracentrifugation, the chromaffin granule fraction of the adrenal medulla (site of dopamine β-hydroxylase) contains large amounts of conjugated dopamine (apparently sulfate) suggesting a selective accumulation of dopamine sulfate as a readily available free norepinephrine precursor during stress.These findings establish major in vivo differences between peripheral conjugated dopamine and conjugates of norepinephrine and epinephrine. They suggest that there may be biological roles for conjugated dopamine beyond that of a dopamine metabolite.