NITRIC-OXIDE LIBERATING, SOLUBLE GUANYLATE-CYCLASE STIMULATING AND VASORELAXING PROPERTIES OF THE NEW NITRATE-COMPOUND SPM-3672

Development of tolerance as a consequence of organic nitrate therapy such as that which occurs with glyceryl trinitrate (GTN) appears to be associated with a depletion of free thiols in vascular smooth muscle. In this study, we investigated N-[3-nitratopivaloyl]-L-cysteinethylester (SPM 3672), a new compound containing a nitrate and a thiol moiety, in direct comparison with GTN. Liberation of nitric oxide (NO) from GTN and SPM 3672 measured in vitro was rather low and was markedly potentiated by addition of cysteine only in the case of GTN. Pronounced activation of a partially purified human soluble guanylate cyclase (sGC) by GTN was observed only after addition of cysteine, whereas a comparative activation by SPM 3672 occurred with and without addition of this thiol. In contrast, SPM 4946 (N-[3-hydroxypivaloyl]-L-cysteineethylester), a derivative of SPM 3672 lacking the nitrate-ester moiety, did not activate sGC. Activation of sGC by GTN and SPM 3672 was nearly abolished by oxyhemoglobin. Incubation of isolated porcine coronary artery rings with GTN or SPM 3672 resulted in a similar increase in vascular cyclic GMP levels. In rat aorta, GTN was a more potent vasorelaxant than SPM 3672 and produced a greater degree of tolerance. Vasorelaxation induced by GTN occurred with rapid onset and was brief, whereas SPM 3672 produced long-lasting relaxation with a more delayed onset. This kinetic pattern was confirmed in porcine coronary arteries, in which both nitrates exhibited marked relaxation, with GTN being slightly more potent than SPM 3672. Preincubation with 100 muM GTN produced a pronounced tolerance to GTN but not to SPM 3672, indicating the absence of cross-tolerance. SPM 3672, a nitrovasodilator with a long-lasting action and a comparably low tendency to induce tolerance may therefore serve as a therapeutic alternative to current nitrovasodilators in the future.