Characterization of furoxans as a new class of tolerance-resistant nitrovasodilators

The vasodilator effects of C92-4609 (4-hydroxymethyl-furoxan-3-carboxamide, CAS 1609), C92-4678(4-phenyl-furoxan-3-carboxylic acid (pyridyl-3-yl-methyl)-amide), C92-4679 (3-phenyl-furoxan-4-carboxylic acid (pyridyl-3-yl-methyl)-amide) and C93-4759 (3-hydroxymethyl-furoxan-4-carboxamide) were studied in the isolated rabbit femoral artery and jugular vein. All furoxans were potent vasodilators in the femoral artery (EC50 0.1-50 microM), while they were less potent in the jugular vein by at least one order of magnitude. Apart from C92-4679, the vasodilatory potency of the furoxans correlated well with their nitric oxide (NO)-releasing capacity which was estimated both by stimulation of purified soluble guanylyl cyclase activity and electron spin resonance spectroscopy with a trapping agent for NO. The hypothesis that furoxans stimulate soluble guanylyl cyclase in the smooth muscle by spontaneously releasing NO was supported by the marked attenuation of their vasodilator effect in the presence of oxyhaemoglobin (10 microM) or following treatment with methylene blue (30 microM). In contrast to earlier findings, NO release from these furoxans was not thiol-dependent, as demonstrated for C92-4609, the relaxant effect of which in the femoral artery was not altered in the presence of N-acetyl-L-cysteine (1 mM). Moreover the KCa+ channel inhibitor, tetrabutylammonium (3 mM), but not the KATP+ channel inhibitor, glibenclamide (3 microM), significantly attenuated the dilator response to C92-4679 in the femoral artery. Pretreatment of these segments with the cytochrome P450 inhibitor, SKF525a (30 microM), also reduced the C92-4679-induced relaxation in this vascular bed.