Effects of NitricOxide-Containing Compounds on Increases in Cytosolic Ionized Ca2+ and on Aggregation of Human Platelets

Abstract

The present study was undertaken to determine the modulatory effects of nitric oxide (NO)-releasing compounds on increases in cytosolic ionized calcium ((Ca2+)i) and on aggregation of gel-filtered and human platelets induced via diverse agonists. We used various sydnonimines and organic nitrates as donors of NO. Gel-filtered and fura-2-loaded platelets were stimulated with ADP, collagen, or thrombin, respectively. Half-maximal inhibiting effects of sydno- nimines on agonist-evoked increases in (Ca2+)i were observed between 30 and 1000 nM, while half-maximal inhibiting effects of the compounds on aggregation were between 3 and 500 nM. The compound C 87-3754, which is the bioactive metabolite of pirsidomine, was a much stronger inhibitor of increases in (Ca2+)i than of platelet aggregation. This was due to an enhanced NO release from this compound exposed to ultraviolet light during Ca2+ measurement. The organic nitrates isosorbide 5-mononitrate and nicorandil inhibited both aggregation and increase of cytosolic ionized calcium in stimulated platelets at half-maximal concentrations of approximately 200 nM. The present results also suggest that some of the inhibitory effects of NO-releasing compounds correspond rather to the presence of the A forms (NO-containing intermediates) than to the presence of free NO.