Long-Term Chronic Intermittent Hypobaric Hypoxia in Rats Causes an Imbalance in the Asymmetric Dimethylarginine/Nitric Oxide Pathway and ROS Activity: A Possible Synergistic Mechanism for Altitude Pulmonary Hypertension?

Abstract

Chronic intermittent hypoxia (CIH) and chronic hypoxia (CH) are associated with high-altitude pulmonary hypertension (HAPH). Asymmetric dimethylarginine (ADMA), a NO synthase (NOS) inhibitor, may contribute to HAPH. This study assessed changes in the ADMA/NO pathway and the underlying mechanisms in rat lungs following exposure to CIH or CH simulated in a hypobaric chamber at 428 Torr. Twenty-four adult Wistar rats were randomly assigned to three groups: CIH2x2 (2 days of hypoxia/2 days of normoxia), CH, and NX (permanent normoxia), for 30 days. All analyses were performed in whole lung tissue. L-Arginine and ADMA were analyzed using LC-MS/MS. Under both hypoxic conditions right ventricular hypertrophy was observed (p < 0.01) and endothelial NOS mRNA increased (p < 0.001), but the phosphorylated/nonphosphorylated vasodilator-stimulated phosphoprotein (VASP) ratio was unchanged. ADMA increased (p < 0.001), whereas dimethylarginine dimethylaminohydrolase (DDAH) activity decreased only under CH (p < 0.05). Although arginase activity increased (p < 0.001) and L-arginine exhibited no changes, the L-arginine/ADMA ratio decreased significantly (p < 0.001). Moreover, NOX4 expression increased only under CH (p < 0.01), but malondialdehyde (MDA) increased (up to 2-fold) equally in CIH2x2 and CH (p < 0.001). Our results suggest that ADMA and oxidative stress likely reduce NO bioavailability under altitude hypoxia, which implies greater pulmonary vascular reactivity and tone, despite the more subdued effects observed under CIH.

Chronic Intermittent Hypobaric Hypoxia (4600 M) Attenuates Pulmonary Vasodilation Induced by Acetylcholine or Sodium Nitroprusside

“…However, nicootinamide adenine dinucleotide phosphatase oxidase plays a crucial role in eNOS uncoupling since it produces reactive oxygen species (Cuzzocrea et al, 2004;Norton et al, 2013). Intolerant rats exposed to CIHH displayed increased O 2 and decreased NO (Siqués et al, 2014;Brito et al, 2015;Lüneburg et al, 2016). In agreement with these studies, our intolerant rats have low NOx levels and a low vascular response to Ach.…”

“…NO production can be regulated by L-arginine plasma levels as a result of the competition between NOS and arginase (Pernow and Jung, 2013). We previously reported that exposure to CIHH upregulates arginase II (Lopez et al, 2009) and increases arginase activity (Lüneburg et al, 2016). Therefore, we propose that our intolerant rats present increased arginase II expression, leading to decreased bioavailability for NOS and decreased NO; inducing the development of pulmonary AHT in rats exposed to CIHH.…”

“…Therefore, decreased L-arginine availability can contribute to NO deficiency. However, the L-arginine plasma concentration is not altered in rats exposed to CIHH (Lüneburg et al, 2016). One explanation is that arginase converts L-arginine into ornithine and urea, reducing Larginine availability and NO production.…”

“…This suggests that in the CIHH pathway NO/sGC/cGMP/PKG/vasodilation is reduced. The results from studies performed in rats exposed to CIHH show that they develop a right ventricular hypertrophy (Lüneburg et al, 2016) as a result of reduced NO availability rather than reduced sGC activity. However, studies performed in newborns exposed to chronic hypoxia showed increased phosphodiesterase type V (PDE5) activity and/or decreased pulmonary artery sensitivity to cGMP ( Jernigan and Resta, 2002).…”

Reactive Oxygen Species at High Altitude (Hypobaric Hypoxia) on the Cardiovascular System

“…In the nitric oxide (NO) pathway, studies have reported that intermittent hypobaric hypoxia exposure reduces the bioavailability of NO in lung parenchyma and vasculature [27,38]. NO is an endogenous vasodilator that activates cyclic GMP, which in turn activates protein kinase G (PKG) and ultimately causes reuptake of Ca 2+ and the opening of calcium-activated potassium channels, leading to the relaxation of vascular smooth muscle cells (VSMCs).…”