The effect of different organic nitrates on the activity of soluble guanylate cyclase prepared from rat liver was investigated. We found a close correlation between the number of nitrate ester groups and the potency of guanylate cyclase activation. For erythrityl tetranitrate (ETN, EC50 = 14.5 microM), glyceryl trinitrate (GTN, EC50 = 60 microM), isoidide dinitrate (IIDN, EC50 = 0.24 mM) and isosorbide-5-nitrate (IS-5-N, EC50 = 1 mM), we were able to set up an equation by which the EC50 could be calculated from the number of nitrate groups per molecule. Compared to these results, the effect of sterical factors and lipophilicity on organic nitrate-induced activation of guanylate cyclase was small. However, there were still significant differences in the EC50 values for the cyclic mononitrates. Isosorbide-2-nitrate (IS-2-N, EC50 = 0.75 mM) was found to be more potent than the stereoisomeric isosorbide-5-nitrate. Similarly, the cyclic dinitrates isomannide dinitrate (IMDN, EC50 = 0.20 mM), isoidide dinitrate and isosorbide dinitrate (ISDN, EC50 = 0.28 mM) exhibited small but significant differences in their guanylate cyclase stimulatory potency. Two lipophilic ester derivatives of isosorbide-5-nitrate showed a 2-fold potency difference for vasodilation but were equipotent for activation of guanylate cyclase (EC50 = 0.85 mM). Also, the increase in lipophilicity due to esterification of the free hydroxylic group had no major influence on guanylate cyclase activation by isosorbide-5-nitrate. These results demonstrate that in a cell-free system, the potency of organic nitrates for guanylate cyclase activation is mainly determined by the number of nitrate groups. Since organic nitrate-induced activation of guanylate cyclase may involve the formation of nitric oxide free radicals, it is conceivable that the differences in potency reflect a varying degree of nitric oxide release from each compound tested.