Inhibition of Platelet Aggregation by Roussin's Black Salt, Sodium Nitroprusside and Other Metal Nitrosyl Complexes

We have examined the action of a range of transition metal nitrosyl compounds in the inhibition of ADP-induced platelet aggregation. Inhibition results from the formation of the activated nitric oxide (NO) complex of guanylate cyclase, hence increasing platelet [cGMP]. Nitrosylation of guanylate cyclase may occur by release of NO from a nitrosyl complex, or, indirectly, by nitrosation of a thiol group followed by decomposition of the S-nitrosyl thiol to give NO. The latter process might be expected to be more efficient for compounds with a greater NO(+)character, and hence nitrosating ability, of the nitrosyl complex, but the results did not show a consistent relationship between NO character and the inhibitory potency on platelets. Inhibition of aggregation by Rousin's black salt, Na[Fe(4)S(3)(NO)(7)], was abolished by haemoglobin, and enhanced in the presence of M&B22948. These findings indicate that activation of guanylate cyclase is mediated by extracellular release of NO. For sodium nitroprusside, inhibition of platelet aggregation became progressively less sensitive to addition of haemoglobin, indicating that another process, such as release of cyanide, became significant as the incubation time was increased.