A role for TRPV1 in agonist-evoked activation of human platelets

Platelets play a role in a number of inflammatory conditions including atherosclerosis; however, the mechanisms of platelet activation under these conditions are unclear.

To investigate the presence of the vanilloid receptor, TRPV1, which is stimulated by noxious stimuli and by inflammatory mediators, in human platelets.

Platelets loaded with fura-2 or sodium-binding benzofuran isophalate acetoxymethyl ester (SBFI) were used to monitor cytosolic calcium or sodium concentrations. 5-HT secretion was determined by fluorescence assay after conjugation with o-phthaldialdehyde. ATP secretion was determined using luciferin-luciferase.

TRPV1 was identified by Western blotting using a specific anti-hTRPV1 antibody. The TRPV1 agonist, capsaicin, evoked both Ca(2+) influx and Ca(2+) release from intracellular stores, responses that were blocked in a dose-dependent manner by the TRPV1 antagonists, 5'-Iodo-resiniferatoxin (5'-Iodo-RTX) and AMG 9810. Capsaicin also increased platelet cytosolic [Na(+)]. Capsaicin-evoked Ca(2+) release was abolished in the absence of extracellular Na(+) or by the 5-HT(2A) receptor antagonist, ketanserin. Capsaicin evoked 5-HT release from platelets, a response abolished in the absence of extracellular Na(+) or by 5'-Iodo-RTX. Thus capsaicin-evoked Ca(2+) release appeared to be mediated by Na(+)-dependent 5-HT release. TRPV1-dependent 5-HT release also contributed to ADP- and thrombin-evoked Ca(2+) entry and release. 5'-Iodo-RTX reduced ADP- and thrombin-evoked Ca(2+) signals, effects not additive with those of ketanserin, and 5'-Iodo-RTX inhibited agonist-evoked 5-HT and ATP release.

These results indicate that TRPV1 is present and functionally important in human platelets. The presence of this receptor may provide a link between inflammatory mediators and platelet activation in conditions such as atherosclerosis.