Calpain I activation is not correlated with aggregation in human platelets.
Calpain-catalysed hydrolysis of platelet substrates such as cytoskeletal and calmodulin-binding proteins, and of protein kinase C, is assumed to contribute to platelet aggregation. We have measured calpain I activation by immunoblotting, and [Ca2+]i (cytoplasmic Ca2+ concn.) by fura-2 fluorescence, in parallel with measurement of aggregation, in stirred human platelets treated at different [Ca2+]ext (extend Ca2+ concns.) with A23187, leupeptin, phorbol ester and thrombin. Hydrolysis of actin-binding protein, and [3H]5-hydroxytryptamine release, were also measured in some cases. A rise in [Ca2+]i, platelet aggregation and calpain activation often occurred together. With some combinations of agonists and [Ca2+]ext, however, this correlation was clearly not maintained. It was shown: (a) that activation of calpain and its hydrolysis of platelet substrates were not strictly necessary conditions for platelet secretion and aggregation; (b) conversely, that calpain activation could occur without aggregation.
Elce
J S
JS
Department of Biochemistry, Queen's University, Kingston, Ontario, Canada.
Sigmund
L
L
Fox
M J
MJ
D017302:Annexins; D002135:Calcium-Binding Proteins / Q000097:blood / Q000502:physiology*; D006801:Humans; D008565:Membrane Proteins / Q000097:blood / Q000502:physiology; D010974:Platelet Aggregation
D017302:Annexins; D002135:Calcium-Binding Proteins; D008565:Membrane Proteins