AJP - Endocrinology and Metabolism, Vol 253, Issue 3 E312-E316, Copyright © 1987 by American Physiological Society
Distribution of free and sulfate-conjugated catecholamines in human platelets
R. K. McCulloch, R. Vandongen, A. M. Tunney, L. J. Beilin and P. B. Rogers
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.
