Distribution of free and sulfate-conjugated catecholamines in human platelets

Sulfate conjugation represents a major pathway for the inactivation of catecholamines and is presumed to occur at sites, including platelets, where phenylsulfotransferase activity has been identified. This study examines the relative distribution of free and conjugated norepinephrine, epinephrine, and dopamine in the platelets and plasma of resting subjects. As active uptake of free catecholamines into platelets has been reported in vitro; the possibility of a similar mechanism, operating for sulfate conjugates, was also investigated. The presence of sulfated catecholamines in platelets is confirmed. The proportion of total catecholamines existing in the free form was 78 +/- 4% (mean +/- SE) for norepinephrine, 52 +/- 5% for epinephrine, and 50 +/- 10% for dopamine, which was considerably higher than for plasma. Increases in plasma free and conjugated catecholamines achieved by bicycle exercise and oral administration of amines, respectively, were not accompanied by corresponding increases in platelet levels. The origin of platelet conjugates would appear to be the result of intraplatelet sulfation of free amines rather than direct uptake from plasma. Because acute elevation of plasma free and conjugated amines are not reflected in increased platelet levels, it is unlikely that platelet uptake mechanisms play a significantly role in the disposal of circulating catecholamines.