An investigation into the release and mobilisation of sulphate-conjugated catecholamines by human platelets.
Collagen (5-160 microg/ml) induced release of free and sulphate conjugated noradrenaline (NA), adrenaline (Ad) and dopamine (DA) by platelets in platelet-rich plasma (PRP) was investigated in human subjects. Stimulated efflux of free NA, Ad and DA to the plasma (platelet-poor plasma; PPP) increased in a concentration-dependent manner, NA release being reflected by reduced platelet free NA contents. Collagen EC(50) values for free NA, Ad and DA release were similar, i.e. 3.8, 3.3 and 4.5 microg/ml collagen, respectively. Subtracting resting PPP free catecholamine (CA) concentrations from those obtained with 160 microg/ml collagen revealed that 52%, 31% and 33% of PPP free NA, Ad and DA, respectively, resulted from collagen stimulation. Under resting conditions sulphate conjugated NA, Ad and DA accounted for 74%, 78% and 99% of total (sum of free and sulphate conjugated) PPP concentrations. Collagen elicited concentration-dependent release of sulphate conjugated DA and, to a lesser extent, NA, but not Ad. Sulphate conjugated NA release was mirrored by decreases in platelet sulphate conjugated NA contents. Resting platelet sulphate conjugated NA and Ad represented 42% and 50% of total (free plus sulphate conjugated) NA and Ad concentrations. EC(50) values for sulphate conjugated DA and NA release were, respectively, 14.5 and 6.25 microg/ml collagen, which exceeded the values for free DA and NA release by 222% and 64%. With 160 microg/ml collagen 44% and 15% of PPP sulphate conjugated DA and NA, respectively, had resulted from platelet activation, and of the total (sum of free and sulphate conjugated) DA and NA released, 99.5% and 39.7% were sulphate conjugated. Total (sum of PPP and platelet) free and sulphate conjugated NA and Ad concentrations were unaltered by collagen stimulation indicating that platelet activation does not cause mobilisation (i.e. hydrolysis) of platelet sulphate conjugated CA. Thus, platelet activation causes the liberation of sulphate conjugated CA (particularly DA) as well as free CA. The differences with regard to the collagen EC(50) values for free and sulphate conjugated CA release indicate that these species arise from different subcellular compartments.
Smith
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Department of Medicine, University College London Medical School, Sir Jules Thorn Institute, The Middlesex Hospital, Mortimer Street, London W1N 8AA, UK.