INFLUENCE OF AUTHENTIC NITRIC-OXIDE ON BASAL CYTOSOLIC [CA2+] AND CA2+ RELEASE FROM INTERNAL STORES IN HUMAN PLATELETS

1 Nitric oxide (NO) donors inhibit platelet function and Ca2+ mobiliza tion evoked by different agonists. This led us to investigate the dire ct effects of authentic NO on basal cytosolic Ca2+ concentration ([Ca2 +](i)) and on Ca2+ mobilization induced by thrombin or by two inhibito rs of intracellular Ca2+-ATPases, thapsigargin and 2,5-di-(t-butyl)-1, 4-benzohydroquinone (t-BuBHO). 2 Cytosolic Ca2+ concentration was eval uated with Fura-2, in the absence of Ca2+ influx. Addition of 5 mu M N O increased by 48% the basal cytosolic [Ca2+] of resting human platele ts whereas a lower concentration (0.1 mu M) did not induce significant modifications. This NO-induced Ca2+ increase was inversely correlated with the basal level of cytosolic [Ca2+]. 3 NO pretreatment for 30 to 120 s decreased by 42 to 57% the transient [Ca2+](i) peak evoked by 0 .10 u ml(-1) thrombin and strongly attenuated the initial rate of [Ca2 +](i) rise induced by 1 mu M thapsigargin or by 20 mu M t-BuBHQ. The t wo components of the thapsigargin response, the Ca2+ release due to in hibition of Ca2+ pumps and the thromboxane A(2)-dependent self-amplifi cation mechanism, were inhibited by NO. The observation that a subsequ ent stimulation was still capable of eliciting Ca2+ release suggests t he presence of NO-insensitive Ca2+ stores. 4 These findings indicate t hat nitric oxide can modulate basal cytosolic [Ca2+] in unstimulated h uman platelets and decrease the Ca2+ mobilization from NO-sensitive in ternal stores evoked by stimulation of receptors or by Ca2+-ATPase inh ibitors. This underlines the important role of nitric oxide in the mod ulation of platelet Ca2+ handling.