Stimulation of human platelets by collagen occurs by a Na+/H+ exchanger independent mechanism.
In stimulated human platelets dense-granule secretion in response to the 'weak agonists' ADP, adrenaline, platelet activating factor and low concentrations of thrombin as well as Ca2+ mobilisation in response to thrombin are enhanced by a Na+/H+ exchanger. In the present study the role of this antiport in collagen stimulated human platelets was examined. While stimulation of platelets loaded with the fluorescent intracellular pH-sensitive dye, bis-carboxyethyl-5-(6)-carboxyfluorescein (BCECF) with thrombin resulted in the activation of the Na+/H+ exchanger, activation of this antiport did not occur in collagen-stimulated platelets. The lack of antiport activity in response to collagen using BCECF-loaded platelets correlated with the lack of any functional role of the antiport in collagen stimulated platelets. In the presence of a Na+/H+ exchange inhibitor, ethylisopropylamiloride, neither collagen-induced platelet aggregation or dense-granule secretion was affected. Furthermore, while the removal of extracellular Na+ (Na+ext), a condition that also prevents activation of the antiport, inhibited dense-granule secretion in response to a low concentration of thrombin, collagen-induced secretion was potentiated. This potentiatory effect could not be attributed to changes in either the membrane potential or in collagen-induced phospholipase C or protein kinase C activity. The present results indicate that in contrast to the 'weak agonists' (1) collagen-induced platelet activation does not require activation of the Na+/H+ exchanger and (2) Na+ext per se is an inhibitor of collagen-induced secretion.
Joseph
S
S
Department of Haematology, University Hospital Utrecht, The Netherlands.
Siffert
W
W
Gorter
G
G
Akkerman
J W
JW
D001792:Blood Platelets / Q000187:drug effects* / Q000378:metabolism; D002118:Calcium / Q000378:metabolism; D002352:Carrier Proteins / Q000378:metabolism*; D002469:Cell Separation; D003094:Collagen / Q000494:pharmacology*; D003594:Cytoplasmic Granules / Q000378:metabolism; D005452:Fluoresceins; D006801:Humans; D006863:Hydrogen-Ion Concentration; D015539:Platelet Activation / Q000187:drug effects*; D010974:Platelet Aggregation; D012701:Serotonin / Q000378:metabolism; D017923:Sodium-Hydrogen Exchangers; D013917:Thrombin / Q000494:pharmacology
D002352:Carrier Proteins; D005452:Fluoresceins; D017923:Sodium-Hydrogen Exchangers; D012701:Serotonin; C043829:2',7'-bis(carboxyethyl)-5(6)-carboxyfluorescein; D003094:Collagen; D013917:Thrombin; D002118:Calcium