Generation of oxidative stress contributes to the development of pulmonary hypertension induced by hypoxia

Abstract

Chronic hypoxia causes pulmonary hypertension and right ventricular hypertrophy associated with pulmonary vascular remodeling. Because hypoxia might promote generation of oxidative stress in vivo, we hypothesized that oxidative stress may play a role in the hypoxia-induced cardiopulmonary changes and examined the effect of treatment with the antioxidant N-acetylcysteine (NAC) in rats. NAC reduced hypoxia-induced cardiopulmonary alterations at 3 wk of hypoxia. Lung phosphatidylcholine hydroperoxide (PCOOH) increased at days 1 and 7 of the hypoxic exposure, and NAC attenuated the increase in lung PCOOH. Lung xanthine oxidase (XO) activity was elevated from day 1 through day 21, especially during the initial 3 days of the hypoxic exposure. The XO inhibitor allopurinol significantly inhibited the hypoxia-induced increase in lung PCOOH and pulmonary hypertension, and allopurinol treatment only for the initial 3 days also reduced the hypoxia-induced right ventricular hypertrophy and pulmonary vascular thickening. These results suggest that oxidative stress produced by activated XO in the induction phase of hypoxic exposure contributes to the development of chronic hypoxic pulmonary hypertension.

Contribution of xanthine oxidase-derived superoxide to chronic hypoxic pulmonary hypertension in neonatal rats

“…Our observation of an increase in expression of XOR in the hypoxia-exposed lung, which was attenuated by Allopurinol, is in keeping with reports by other groups demonstrating hypoxia-induced upregulation of XO activity and XOR expression in vascular endothelium in vitro (34,62,63) and in the adult lung in vivo (25). Conversion of XOR to the oxidase form is believed to occur through two mechanisms: direct exposure to hypoxia through an adenosine-dependent process (34) and endothelial binding of circulating XO (26) secreted by other organs, such as the gut (5) and the liver (66).…”

“…Pups received either Allopurinol (5 l/g body wt of 10 mg/ml suspended in 0.9% saline vehicle ϭ 50 mg/kg), U74389G [5 l/g body wt of 2 mg/ml solution in CS4 vehicle (20 mM citric acid monohydrate, 3.2 mM sodium citrate dihydrate, 77 mM NaCl, pH 3.0) ϭ 10 mg/kg], Tempol (5 l/g body wt of 20 mg/ml solution in 0.9% saline vehicle ϭ 100 mg/kg), or vehicle alone by daily intraperitoneal injection. The doses of the above compounds used were similar to those previously reported to be effective in vivo by our group and others (16,25,30). At the end of each exposure period, pups were either killed by pentobarbital overdose or exsanguinated after anesthesia.…”

“…A major pathological role for increased generation of reactive oxygen species (ROS) in the pathogenesis of experimental PHT is evidenced by antioxidant intervention studies performed in fetal lambs (24,39), in hyperoxia-exposed neonatal rats (30), and in hypoxia- (25,38,41) or monocrotalineexposed (9) adult rodents. A putative role for oxidative stress is also supported by observational studies in adult humans with idiopathic PHT (7,10).…”

“…Recent studies in experimental models of PHT have indicated that pulmonary vascular-derived ROS may predominantly originate from enzymatic sources. Major enzymatic sources described in adult rats and mice chronically exposed to hypoxia have included xanthine oxidase (XO) (25) and reduced nicotinamide adenine dinucleotide phosphate [NAD(P)H] oxidase (42), respectively. Furthermore, both NAD(P)H oxidase and uncoupled endothelial nitric oxide synthase (eNOS) have been described as major vascular sources of ROS in fetal lambs with ductal ligation-induced PHT (24,36).…”

“…Under conditions of tissue hypoxia (53), ATP is broken down to the purine hypoxanthine, a substrate for XOR, and reversible oxidation or irreversible proteolytic cleavage (17,54), which converts XOR to an oxidase form that can produce large quantities of ROS, including O 2 •Ϫ and its dismutation product, hydrogen peroxide (H 2 O 2 ). In adults, increased vascular O 2 •Ϫ production during chronic exposure to hypoxia has been attributed in major part to XO, which has been found to adversely impact endothelial function by impairing nitric oxide (NO) signaling (26) and to directly contribute to experimental pulmonary vascular remodeling (25).…”