This thesis examines the mechanisms underlying the activation of the store-operated calcium entry (SOCE) pathway in human platelets and how they relate to the Ca<sup>2+</sup> signals elicited in platelets stimulated with physiological agonists and phorbol esters. A novel role for an increase in the activity of the cysteine protease calpain in the activation of SOCE is demonstrated. Data is also presented dissociating the role of the actin cytoskeleton in the store-operated Ca<sup>2+</sup> entry from the store-operated Mn<sup>2+</sup> and Na<sup>+</sup> entries and demonstrate that the role of the cytoskeleton is not in the activation of the store-operated channel but in regulating Ca<sup>2+</sup> entry via the Na<sup>+</sup>/Ca<sup>2+</sup> exchanger (NCX). These data therefore provide strong evidence against the <i>de novo </i>conformational coupling hypothesis in human platelets. During further examination of the role of the NCX in SOCE in human platelets, it is demonstrated that SOCE elicits dense granule secretion in human platelets and inhibiting the platelets response to secreted autocrine messengers greatly reduces TG-evoked Ca<sup>2+</sup> entry, suggesting an important role of autocrine modulation of SOCEI. These results therefore question the specificity of using thapsigargin as a specific activator of this Ca<sup>2+</sup> entry pathway. The results also suggest the presence of two pharmacologically-distinct NCX subpopulations in human platelets. Results from experiments investigating the autocrine modulation of the Ca<sup>2+</sup> responses of platelets stimulated with physiological agonists such as ADP and thrombin are also presented. These experiments lead to the suggestion that a rise in [Na<sup>2+</sup>]<sub>i</sub> may play an important role in potentiating the activation of human platelets. Lastly data is presented suggesting the presence of the TRPV1 ion channel in human platelets and provides preliminary evidence towards a role for this ion channel in modulating the secretion of serotonin from these cells.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:603727 |
| Date | January 2007 |
| Creators | Harper, A. G. S. |
| Publisher | University of Cambridge |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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