The in vitro pulmonary vascular effects of FK409 (nitric oxide donor): a study in normotensive and pulmonary hypertensive rats

Abstract

1 Vasorelaxant responses to the nitric oxide (NO) donor, FK409 ((+)-(E)-4-ethyl-2-[(E)-hydroxyimino]-5-nitro-3-hexenamide), were evaluated on precontracted isolated ring preparations of main pulmonary artery and intralobar pulmonary artery from rats. 2 On main pulmonary artery FK409 fully reversed the precontractions. Responses were inhibited by methylene blue but were independent of the endothelium. The potency (7log EC 50 ) of FK409 was the same on preparations contracted with noradrenaline (7.62) or the thromboxane-mimetic, U44619 (7.63). 3 On intralobar pulmonary artery FK409 caused only 80% reversal of the precontraction and was 2 fold less potent than on main pulmonary artery. These di erences in maximum response and potency between main and intralobar arteries are in keeping with previous ®ndings with other NO donors. 4 Pulmonary hypertension was induced in rats by chronic exposure to hypoxia (10% O 2 ) for 1 or 4 weeks. Main pulmonary arteries from 1 week hypoxic rats had inherent tone and showed spontaneous contractile activity. In these arteries FK409 reversed not only the precontraction induced by noradrenaline but also the inherent tone. However, FK409 was 17 fold less potent than in control arteries, re¯ecting previous ®ndings with other NO donors. Main pulmonary arteries from 4 week hypoxic rats had minimal inherent tone and were quiescent and FK409 was 4.5 fold less potent than in control arteries. In intralobar pulmonary arteries from 4 week hypoxic rats FK409 was 12 fold less potent than in controls. 5 Treatment of arteries with either (a) in vitro hypoxic conditions (PO 2 of solution in organ bath 510 mmHg) or (b) superoxide dismutase (SOD; 150 u ml 71 ) together with catalase (1200 u ml 71 ) signi®cantly increased the potency of FK409 in preparations from hypoxic rats but had no e ect on the potency in control preparations. Neither SOD nor catalase, alone, nor the nitric oxide synthase inhibitor, N G -nitro-L-arginine methyl ester, had any e ect on the potency of FK409 in preparations from control or hypoxic rats. 6 It is concluded that the reduction in potency of FK409 seen in pulmonary arteries from rats with chronic hypoxic pulmonary hypertension may be due in part to the presence of one or more reactive oxygen species (either hydroxyl or superoxide plus hydrogen peroxide).

Functional effects of 4-aminopyridine (4-AP) on pulmonary and systemic vessels from normoxic control and hypoxic pulmonary hypertensive rats

“…The main and intralobar pulmonary artery resting forces are based on previously published data (Wanstall et al 1997), and take into account the different in vivo pulmonary artery pressures in normoxic and hypoxic rats.…”

“…Some of the rats were housed in a hypoxic chamber, as described previously (Wanstall et al 1997), for a period of 1 week, whereas normoxic rats were housed in room air (21% O 2 ). The hypoxic chamber was flushed continuously with a mixture of N 2 and compressed air at a flow rate of 0.94 l min -1 .…”

Contribution of oxygen radicals to altered NO-dependent pulmonary vasodilation in acute and chronic hypoxia

“…Either SOD (150 U/ml) plus catalase (1,200 U/ml) or tiron (10 mM) was added to the recirculating reservoir (40 ml) immediately after lung isolation and was present throughout the experiment. These doses of SOD/catalase (5,40) and tiron (36) have been previously employed by other investigators to scavenge oxygen radicals in this preparation. Responses to SNAP (0.5, 1.0, and 10 M) and ET-1 (1 nM) were then determined in each group during both NV and HV as described above.…”

“…Indeed, even before endothelium-derived relaxing factor (EDRF) was identified as NO, Rubanyi and Vanhoutte (34) demonstrated that EDRF could be inactivated by superoxide and stabilized by superoxide dismutase (SOD). More recent studies have confirmed the ability of antioxidants to improve vasodilatory responses following acute (5) and chronic hypoxia (40), suggesting that reoxygenation following hypoxia may play an important role in ROS production. However, it is equally probable that hypoxia, per se, elevates ROS production (13,16,17).…”

“…Although this could potentially be explained by a decrease in smooth muscle sensitivity to NO, this seems unlikely since we have recently shown that the ability to stimulate the NOdependent vasodilatory signaling pathway via activation of cGMP (11) and PKG (12) is not diminished following CH, but rather, it appears to be enhanced. Although the mechanism(s) for this inconsistency between exogenous and endogenous NO-dependent vasodilatory responsiveness following CH is unclear, some investigators suggest the generation of reactive oxygen species (ROS) during hypoxia-reoxygenation impairs NO formation as well as directly inactivating NO (5,8,40). The present study examines the mechanisms responsible for the differential effects of acute and chronic hypoxic exposure on vasodilation to endogenous vs. exogenous NO and the role of ROS on NO-dependent vasoreactivity.…”