COLLAGEN-STIMULATED RELEASE OF SEROTONIN BY HUMAN PLATELETS INCLUDES A SULFATE CONJUGATED COMPONENT

Collagen (5-160 mu g/ml) induced release of free and sulphate conjugat ed serotonin (5-HT) from platelets in platelet-rich plasma (PRP) was e xamined in normal human subjects, Collagen stimulated release of free 5-HT to the plasma (platelet-poor; PPP) increased in a dose dependent manner (P <0.0001) and was mirrored by a decline in platelet free 5-HT content (P <0.0001). The half-maximum response occurred at a collagen concentration of 10.5 mu g/ml. Non-specific (resting) release of free 5-HT represented 2.5% of the total free 5-HT (sum of PPP and platelet concentrations). On stimulation with 5, 10, 20, 40, 80 and 160 mu g/m l collagen, PPP free 5-HT concentrations were 24.2, 45.5, 63.2, 74.9, 82 and 89.7% of the total respectively. Collagen also elicited dose de pendent release of sulphate conjugated 5-HT to PPP (P <0.05), which wa s reflected by a corresponding decline in platelet sulphate conjugated 5-HT content (P <0.0001). Half maximal release of sulphate conjugated 5-HT occurred with 6 mu g/ml collagen, Under resting conditions plate let and PPP sulphate conjugated concentrations were respectively 6 and 35.1% of total (sum of free and sulphate conjugated) 5-HT concentrati ons, whilst under collagen (160 mu g/ml) stimulated conditions these c oncentrations represented 12.8 and 7.7% of the total, Mobilisation (i. e. hydrolysis) of platelet sulphate conjugated 5-HT did not occur on c ollagen stimulation, as evidenced by the absence of alterations in tot al sulphate conjugated 5-HT concentrations (sum of PPP and platelet su lphate conjugated 5-HT concentrations). PPP concentrations of the 5-HT metabolite 5-hydroxyindoleacetic acid (5-HT) were unaltered by collag en stimulation indicating that it did not influence 5-HT metabolism, I t is concluded that release of platelet sulphate conjugated 5-HT, in a ddition to free 5-HT, occurs on platelet activation by collagen, altho ugh the sulphate conjugated fraction represents only a minor proportio n of the total (free and sulphate conjugated) 5-HT released, Further s tudies will establish the physiological significance of platelet sulph ate conjugated 5-HT.