INHIBITION OF THE ACTION OF NITRIC-OXIDE PRODRUGS BY PYOCYANIN - MECHANISTIC STUDIES

Previously, we reported on the antagonism by pyocyanin (PYO) of the re laxant effects of nitrovasodilators such as glyceryl trinitrate, S-nit roso-N-acetylpenicillamine (SNAP), and 3-morpholinosydnonimine (SIN-1) . The purpose of the present study was to elucidate the mechanism of t his action of PYO by examining its effect on the steps considered to b e necessary for nitrovasodilator-induced relaxation of blood vessels. PYO (10 mu M) attenuated the accumulation of guanosine-3',5'-cyclic mo nophosphate (cGMP) in rabbit aorta induced by nitrovasodilators SIN-1, SNAP, and GTN, 65, 81, and 67%, respectively. Additionally, PYO (1 or 10 mu M) interfered with in vitro activation of soluble guanylyl cycl ase. PYO did not inhibit vascular relaxation induced by 8-bromo-cyclic guanosine monophosphate. PYO (10 mu M) also decreased the quantity of nitric oxide measured in the headspace above intact vascular tissue i ncubated with glyceryl trinitrate in the presence of oxygen. These obs ervations are consistent with the interpretation that PYO interfered w ith the nitrovasodilator action of glyceryl trinitrate by inactivation of NO or by inhibition of enzymatic biotransformation of GTN; this wo uld result in decreased guanylyl cyclase activation and thus lowering cellular levels of cGMP. NO chemiluminescence studies with SIN-1 (10 m u M) revealed that this NO donor produced NO in a time-dependent manne r and PYO (10 mu M) caused no inhibition of NO production, but in fact , potentiated NO release after 10 min of incubation (1395 +/- 179 pmol NO compared with 1088 +/- 154 pmol NO). NO production from 10 mu M SN AP was similarly potentiated by PYO after 0.5, 2, 5, and 10 min of inc ubation. Therefore. it is likely that PYO acts as an inhibitor of guan ylyl cyclase with respect to NO donors, SIN-1 and SNAP, but it also ap pears that PYO can exert additional inhibitory effects in the case of vascular relaxation by GTN. Such differences in relaxant effects may r eflect inhibition of enzymatic biotransformation that is unique to GTN or that PYO may complex with an alternative redox form of NO (perhaps NO+) that is generated by vascular metabolic activation of GTN.