Upregulation of arginase expression and activity in hypertensive rats exposed to chronic intermittent hypobaric hypoxia

Activation of arginase II by asymmetric dimethylarginine and homocysteine in hypertensive rats induced by hypoxia: a new model of nitric oxide synthesis regulation in hypertensive processes?

In recent years, the increase in blood pressure at high altitudes has become an interesting topic among high-altitude researchers. In our animal studies using Wistar rats, we observed the existence of two rat populations that exhibit differential physiological responses during hypoxic exposure. These rats were classified as hypoxia-induced hypertensive rats and nonhypertensive rats. A decrease in nitric oxide levels was reported in different hypertension models associated with increased concentrations of asymmetric dimethylarginine (ADMA) and homocysteine, and we recently described an increase in arginase type II expression under hypoxia. ADMA and homocysteine decrease nitric oxide (NO) bioavailability; however, whether ADMA and homocysteine have a regulatory effect on arginase activity and therefore regulate another NO synthesis pathway is unknown. Therefore, the aim of this study was to measure basal ADMA and homocysteine levels in hypoxia-induced hypertensive rats and evaluate their effect on arginase II activity. Our results indicate that hypoxia-induced hypertensive rats presented lower nitric oxide concentrations than nonhypertensive rats, associated with higher concentrations of homocysteine and ADMA. Hypoxia-induced hypertensive rats also presented lower dimethylarginine dimethylaminohydrolase-2 and cystathionine β-synthase levels, which could explain the high ADMA and homocysteine levels. In addition, we observed that both homocysteine and ADMA had a significant effect on arginase II activation in the hypertensive rats. Therefore, we suggest that ADMA and homocysteine have dual regulatory effects on NO synthesis. The former has an inhibitory effect on eNOS, and the latter has a secondary activating effect on arginase II. We propose that arginase II is activated by AMDA and homocysteine in hypoxia-induced hypertensive rats.

The role of arginase in the microcirculation in cardiovascular disease

In the microcirculation, the exchange of nutrients, water, gas, hormones, and waste takes place, and it is divided into the three main sections arterioles, capillaries, and venules. Disturbances in the microcirculation can be measured using surrogate parameters or be visualized either indirectly or directly.Arginase is a manganese metalloenzyme hydrolyzing L-arginine to urea and L-ornithine. It is located in different cell types, including vascular cells, but also in circulating cells such as red blood cells. A variety of pro-inflammatory factors, as well as interleukins, stimulate increased arginase expression. An increase in arginase activity consequently leads to a consumption of L-arginine needed for nitric oxide (NO) production by endothelial NO synthase. A vast body of evidence convincingly showed that increased arginase activity is associated with endothelial dysfunction in larger vessels of the vascular tree. Of note, arginase also influences the microcirculation. Arginase inhibition leads to an increase in the bioavailability of NO and reduces superoxide levels, resulting in improved endothelial function. Arginase inhibition might, therefore, be a potent treatment strategy in cardiovascular medicine. Recently, red blood cells emerged as an influential player in the development from increased arginase activity to endothelial dysfunction. As red blood cells directly interact with the microcirculation in gas exchange, this could constitute a potential link between arginase activity, endothelial dysfunction and microcirculatory disturbances.The aim of this review is to summarize recent findings revealing the role of arginase in regulating vascular function with particular emphasis on the microcirculation.

Plasmatic Concentrations of ADMA and Homocystein in Llama (Lama glama) and Regulation of Arginase Type II: An Animal Resistent to the Development of Pulmonary Hypertension Induced by Hypoxia

There are animal species that have adapted to life at high altitude and hypobaric hypoxia conditions in the Andean highlands. One such species is the llama (Lama glama), which seem to have developed efficient protective mechanisms to avoid maladaptation resulting from chronic hypoxia, such as a resistance to the development of hypoxia -induced pulmonary hypertension. On the other hand, it is widely known that different models of hypertension can arise as a result of changes in endothelial function. The respect, one of the common causes of deregulation in endothelial vasodilator function have been associated with down-regulation of the NO synthesis and an increase in plasma levels of asymmetric dimethylarginine (ADMA) and homocysteine. Additionally, it is also known that NO production can be regulated by plasma levels of L-arginine as a result of the competition between nitric oxide synthase (NOS) and arginase. The objective of this study, was to determine the baseline concentrations of ADMA and homocysteine in llama, and to evaluate their effect on the arginase pathway and their involvement in the resistance to the development of altitude-induced pulmonary hypertension.

METHOD

Lowland and highland newborn sheep and llama were investigated near sea level and at high altitude. Blood determinations of arterial blood gases, ADMA and homocysteíne are made and the effect of these on the arginase activity was evaluated.

RESULTS

The basal concentrations of ADMA and homocysteine were determined in llama, and they were found to be significantly lower than those found in other species and in addition, the exposure to hypoxia is unable to increase its concentration. On the other hand, it was observed that the llama exhibited 10 times less arginase II activity as compared to sheep, and the expression was not induced by hypoxia. Finally, ADMA y Hcy, has no effect on the type II arginase pathway.

CONCLUSION

Based on our results, we propose that low concentrations of ADMA and homocysteine found in llamas, the low expression of arginase type II, DDAH-2 and CBS, as well as its insensitivity to activation by homocysteine could constitute an adaptation mechanism of these animals to the hypoxia.

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

Moraga, Fernando A., Giselle Miranda, Vasthi López, Carmen Vallejos, and Daniel Silva. Chronic intermittent hypobaric hypoxia (4600 M) attenuates pulmonary vasodilation induced by acetylcholine or sodium nitroprusside. High Alt Med Biol. 19:149-155, 2018.

BACKGROUND

Previous studies performed in rats exposed to chronic intermittent hypobaric hypoxia (CIHH), at a simulated altitude of 4600 m, showed reduced nitric oxide (NO) production, increased arginase activity, and increased oxidative stress. However, studies on vascular function are scarce. Our aim was to measure plasma nitrate and nitrite (NOx) concentration and study pulmonary vascular function in rats exposed to CIHH in the presence of potassium chloride (KCl), acetylcholine (Ach), and sodium nitroprusside (SNP).

METHODS

Thirty male Wistar rats were divided into two groups: A control group (normoxia (N), n = 10) and a CIHH group (2N × 2H × 30 days, n = 20). CIHH exposure was performed in a hypobaric chamber at 428 Torr (4600 m). Noninvasive systolic blood pressure (SBP), heart rate, and body weight (BW) were measured. Blood samples were obtained to measure NOx levels and hematocrit (Hct). CIHH animals that gained BW and presented a Hct <20% and maintained SBP were classified as tolerant, and animals that lost >30% of their BW, increased Hct and SBP >20% were classified as intolerant. Animals were sacrificed and small pulmonary arteries (SPA) were obtained to perform concentration-response curves to KCl, Ach, and SNP.

RESULTS AND CONCLUSIONS

Intolerant rats (30%) had decreased NOx levels. SPA had a larger vasocontraction response to KCl and a lower dilation response to SNP in the SPA compared to tolerant and control animals. In addition, SPA had a lower dilatation response to Ach compared with the control. Together, these results show that CIHH alters endothelium-dependent vasodilation.

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?

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.

Protective effects of polyunsatutared fatty acids supplementation against testicular damage induced by intermittent hypobaric hypoxia in rats

BACKGROUND

Intermittent hypobaric hypoxia (IHH) induces changes in the redox status and structure in rat testis. These effects may be present in people at high altitudes, such as athletes and miners. Polyunsaturated fatty acids (PUFA) can be effective in counteracting these oxidative modifications due to their antioxidants properties. The aim of the work was to test whether PUFA supplementation attenuates oxidative damage in testis by reinforcing the antioxidant defense system. The animals were divided into four groups (7 rats per group): normobaric normoxia (~750 tor; pO2 156 mmHg; Nx); Nx + PUFA, supplemented with PUFA (DHA: EPA = 3:1; 0.3 g kg(-1) of body weight per day); hypoxic hypoxia (~428 tor; pO2 90 mmHg; Hx) and, Hx + PUFA. The hypoxic groups were exposed in 4 cycles to 96 h of HH followed by 96 h of normobaric normoxia for 32 days. Total antioxidant capacity (FRAP) and lipid peroxidation (malondialdehyde, MDA) in plasma and reduced (GSH)/oxidized glutathione (GSSG) ratio, tissue lipid peroxidation (TBARS) and antioxidant enzymes activity were assessed at the end of the study in testis. Also, SIRTUIN 1 and HIF-1 protein expression in testis were determined.

RESULTS

IHH increased lipid peroxidation in plasma and HIF-1 protein levels in testis. In addition, IHH reduced FRAP levels in plasma, antioxidant enzymes activities and SIRTUIN 1 protein levels in testis. PUFA supplementation attenuated these effects, inducing the increases in FRAP, in the antioxidant enzymes activity and HIF-1 levels.

CONCLUSIONS

These results suggest that the IHH model induces a prooxidant status in plasma and testis. The molecular protective effect of PUFA may involve the induction of an antioxidant mechanism.

Exogenous sphingosine-1-phosphate boosts acclimatization in rats exposed to acute hypobaric hypoxia: assessment of haematological and metabolic effects

Background

The physiological challenges posed by hypobaric hypoxia warrant exploration of pharmacological entities to improve acclimatization to hypoxia. The present study investigates the preclinical efficacy of sphingosine-1-phosphate (S1P) to improve acclimatization to simulated hypobaric hypoxia.

Experimental Approach

Efficacy of intravenously administered S1P in improving haematological and metabolic acclimatization was evaluated in rats exposed to simulated acute hypobaric hypoxia (7620m for 6 hours) following S1P pre-treatment for three days.

Major Findings

Altitude exposure of the control rats caused systemic hypoxia, hypocapnia (plausible sign of hyperventilation) and respiratory alkalosis due to suboptimal renal compensation indicated by an overt alkaline pH of the mixed venous blood. This was associated with pronounced energy deficit in the hepatic tissue along with systemic oxidative stress and inflammation. S1P pre-treatment improved blood oxygen-carrying-capacity by increasing haemoglobin, haematocrit, and RBC count, probably as an outcome of hypoxia inducible factor-1α mediated erythropoiesis and renal S1P receptor 1 mediated haemoconcentation. The improved partial pressure of oxygen in the blood could further restore aerobic respiration and increase ATP content in the hepatic tissue of S1P treated animals. S1P could also protect the animals from hypoxia mediated oxidative stress and inflammation.

Conclusion

The study findings highlight S1P’s merits as a preconditioning agent for improving acclimatization to acute hypobaric hypoxia exposure. The results may have long term clinical application for improving physiological acclimatization of subjects venturing into high altitude for occupational or recreational purposes.

Celiprolol reduces oxidative stress and attenuates left ventricular remodeling induced by hypoxic stress in mice

We have previously reported that intermittent hypoxic stress, which is relevant to sleep apnea syndrome (SAS), increases oxidative stress and induces left ventricular (LV) remodeling. Celiprolol, a β1-selective adrenoreceptor blocker, is known to have not only an antihypertensive effect but also an antioxidant effect through releasing nitric oxide. The aim of this study was to examine the hypothesis that celiprolol might ameliorate the LV remodeling induced by intermittent hypoxia through its antioxidant effect. Male C57BL/6J mice (8 weeks old) were exposed to intermittent hypoxia (30 s of 5% oxygen followed by 30 s of 21% oxygen) for 8 h day(-1) during the daytime for 10 consecutive days or were maintained under normoxic conditions. Animals were treated with either celiprolol (100 mg kg(-1) day(-1) by gavage) or vehicle. Hypoxic stress caused fluctuations in blood pressure (BP), an increase in the mean cardiomyocyte diameter, perivascular fibrosis and a decrease in endothelial nitric oxide synthase (eNOS) expression. These changes were associated with increased levels of 4-hydroxy-2-nonenal protein, superoxide, tumor necrosis factor-α mRNA and brain natriuretic peptide mRNA in the LV myocardium. Celiprolol significantly suppressed BP fluctuation, restored eNOS expression and reduced oxidative stress and superoxide production, thus ameliorating hypoxia-induced LV remodeling in mice. These findings suggest that treatment with celiprolol might prevent cardiovascular events in borderline hypertensive patients with SAS.

L-arginine pathway in neonates with meconium-stained amniotic fluid

OBJECTIVE

To study the arginase, nitric oxide synthase and nitric oxide pathways associated with passage of meconium.

STUDY DESIGN

Cord blood samples were collected from 20 newborns with meconium-stained amniotic fluid (MSAF) and from 23 newborns with clear amniotic fluid. Cord blood pH, arginase, nitric oxide synthase and nitric oxide levels were compared between the groups.

RESULT

The differences between the arginase and nitric oxide measurements of the newborns with MSAF and those with clear amniotic fluid were significant. In the MSAF group arginase levels were significantly lower (p=0.007) and nitric oxide levels were significantly higher (p=0.032) than the clear amniotic fluid group.

CONCLUSION

Hypoxia may be involved in the pathogenesis of meconium passage due to decreased arginase and increased nitric oxide levels.

Aluminum bone toxicity in immature rats exposed to simulated high altitude

Aluminum (Al) is an element to which humans are widely exposed. Chronic administration induces a negative effect on bone tissue, affecting collagen synthesis and matrix mineralization. Its toxic effects are cumulative. Hypobaric hypoxia induces stress erythropoiesis, leading to hypertrophy of the erythropoietic marrow affecting the bone. This study was designed to evaluate the risk of Al bone toxicity among immature rats maintained at simulated high altitude (SHA) by mechanical assessment of stiffness and strength, calculation of some indicators of bone material and geometrical properties, as well as blood determinations. Forty growing rats were divided into control and experimental groups whether injected with vehicle or Al, as Al(OH)(3), three times a week for 3 months. Half of each group was exposed to hypobaric conditions (HX) by placing the animals in a SHA chamber. Both treatments negatively affected structural properties of bones, decreasing the maximum capacity to withstand load, the limit elastic load and the capacity of absorbing energy in elastic conditions. Al administration significantly depressed mandible structural stiffness, although diaphyseal stiffness was not modified. Indicators of bone material intrinsic properties, elastic modulus and stress, were significantly reduced by Al or HX. Treatments increased the diaphyseal sectional bending moment of inertia, suggesting that femur, but not mandible, compensates for the decline in the material properties with an adaptation of its architecture to maintain structural properties. The different biomechanical behaviors between the two kinds of bone are probably due to their different embryological origin and specific functions, as mandible is a bone that adjusts its strength to biting forces, whereas femur is designed to support load.