Chronic hypoxia attenuates nitric oxide-dependent hemodynamic responses to acute hypoxia

Effects of β-adrenoceptor activation on haemodynamics during hypoxic stress in rats

NEW FINDINGS

What is the central question of this study? The acute hypoxic compensatory reaction is based on haemodynamic changes, and β-adrenoceptors are involved in haemodynamic regulation. What is the role of β-adrenoceptors in haemodynamics during hypoxic exposure? What is the main finding and its importance? Activation of β₂ -adrenoceptors attenuates the increase in pulmonary artery pressure during hypoxic exposure. This compensatory reaction activated by β₂ -adrenoceptors during hypoxic stress is very important to maintain the activities of normal life.

ABSTRACT

The acute hypoxic compensatory reaction is accompanied by haemodynamic changes. We monitored the haemodynamic changes in rats undergoing acute hypoxic stress and applied antagonists of β-adrenoceptor (β-ARs) subtypes to reveal the regulatory role of β-ARs on haemodynamics. Sprague-Dawley rats were randomly divided into control, atenolol (β₁ -AR antagonist), ICI 118,551 (β₂ -AR antagonist) and propranolol (non-selective β-AR antagonist) groups. Rats were continuously recorded for changes in haemodynamic indexes for 10 min after administration. Then, a hypoxic ventilation experiment [15% O₂ , 2200 m a.sl., 582 mmHg (0.765 Pa), P O 2 87.3 mmHg; Xining, China] was conducted, and the indexes were monitored for 5 min after induction of hypoxia. Plasma catecholamine concentrations were also measured. We found that, during normoxia, the mean arterial pressure, heart rate, ascending aortic blood flow and pulmonary artery pressure were reduced in the propranolol and atenolol groups. Catecholamine concentrations were increased significantly in the atenolol group compared with the control group. During hypoxia, mean arterial pressure and total peripheral resistance were decreased in the control, propranolol and ICI 118,551 groups. Pulmonary arterial pressure and pulmonary vascular resistance were increased in the propranolol and ICI 118,551 groups. During hypoxia, catecholamine concentrations were increased significantly in the control group, but decreased in β-AR antagonist groups. In conclusion, the β₂ -AR is involved in regulation of pulmonary haemodynamics in the acute hypoxic compensatory reaction, and the activation of β₂ -ARs attenuates the increase in pulmonary arterial pressure during hypoxic stress. This compensatory reaction activated by β₂ -ARs during hypoxic stress is very important to maintain activities of normal life.

Role of nitric oxide-containing factors in the ventilatory and cardiovascular responses elicited by hypoxic challenge in isoflurane-anesthetized rats

Exposure to hypoxia elicits changes in mean arterial blood pressure (MAP), heart rate, and frequency of breathing (fR). The objective of this study was to determine the role of nitric oxide (NO) in the cardiovascular and ventilatory responses elicited by brief exposures to hypoxia in isoflurane-anesthetized rats. The rats were instrumented to record MAP, heart rate, and fR and then exposed to 90 s episodes of hypoxia (10% O2, 90% N2) before and after injection of vehicle, the NO synthase inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), or the inactive enantiomer D-NAME (both at 50 μmol/kg iv). Each episode of hypoxia elicited a decrease in MAP, bidirectional changes in heart rate (initial increase and then a decrease), and an increase in fR. These responses were similar before and after injection of vehicle or D-NAME. In contrast, the hypoxia-induced decreases in MAP were attenuated after administration of L-NAME. The initial increases in heart rate during hypoxia were amplified whereas the subsequent decreases in heart rate were attenuated in L-NAME-treated rats. Finally, the hypoxia-induced increases in fR were virtually identical before and after administration of L-NAME. These findings suggest that NO factors play a vital role in the expression of the cardiovascular but not the ventilatory responses elicited by brief episodes of hypoxia in isoflurane-anesthetized rats. Based on existing evidence that NO factors play a vital role in carotid body and central responses to hypoxia in conscious rats, our findings raise the novel possibility that isoflurane blunts this NO-dependent signaling.

Long-acting phosphodiesterase 5 inhibitor, tadalafil, and superoxide dismutase mimetic, tempol, protect against acute hypoxia-induced pulmonary hypertension in rats

Long-acting phosphodiesterase 5 (PDE5) inhibitor, tadalafil, was recently approved for the treatment of pulmonary hypertension. Apart from being a PDE5 inhibitor, tadalafil also possesses antioxidant activity. The aim of this study was to probe whether tadalafil has any beneficial effect over tempol owing to its antioxidant action in addition to PDE5 inhibitory activity. Albino Wistar rats were pretreated with tadalafil (10 mg/kg) or vehicle 2 h before hypoxic exposure, whereas tempol (20 mg/kg) was given 5 min before induction of hypoxia. Right ventricular systolic pressure (RVSP), mean arterial pressure (MAP), heart rate (HR), right ventricular contractility (RVdP/dtmax) and cardiac output (CO) were recorded while subjecting rats to acute hypoxia for 30 min. Lipid peroxidation and reduced glutathione were estimated in serum before and after hypoxia exposure. Tadalafil as well as tempol significantly prevented hypoxia-induced rise in RVSP (p < 0.001) and RVdP/dtmax (p < 0.05). Both tadalafil and tempol pretreatment partially prevented (p < 0.01) the rise in CO due to hypoxia. Tadalafil did not produce any significant change in MAP, whereas tempol led to a significant fall (p < 0.01) in MAP. Acute hypoxia increased the oxidative stress levels. Tadalafil pretreatment partially prevented hypoxia-induced oxidative stress, while tempol pretreatment completely prevented hypoxia-induced oxidative stress. Results suggest that tadalafil because of its antioxidant action in addition to PDE5 inhibitory activity is more appropriate for the prevention of hypoxic pulmonary hypertension than tempol. Tempol also produced undesirable systemic hypotension as side effect, which was not seen with tadalafil because of its pulmonary selective action.

Exogenous ghrelin attenuates the progression of chronic hypoxia-induced pulmonary hypertension in conscious rats

Chronic exposure to hypoxia, a common adverse consequence of most pulmonary disorders, can lead to a sustained increase in pulmonary arterial pressure (PAP), right ventricular hypertrophy, and is, therefore, closely associated with heart failure and increased mortality. Ghrelin, originally identified as an endogenous GH secretagogue, has recently been shown to possess potent vasodilator properties, likely involving modulation of the vascular endothelium and its associated vasoactive peptides. In this study we hypothesized that ghrelin would impede the pathogenesis of pulmonary arterial hypertension during chronic hypoxia (CH). PAP was continuously measured using radiotelemetry, in conscious male Sprague Dawley rats, in normoxia and during 2-wk CH (10% O(2)). During this hypoxic period, rats received a daily sc injection of either saline or ghrelin (150 microg/kg). Subsequently, heart and lung samples were collected for morphological, histological, and molecular analyses. CH significantly elevated PAP in saline-treated rats, increased wall thickness of peripheral pulmonary arteries, and, consequently, induced right ventricular hypertrophy. In these rats, CH also led to the overexpression of endothelial nitric oxide synthase mRNA and protein, as well as endothelin-1 mRNA within the lung. Exogenous ghrelin administration attenuated the CH-induced overexpression of endothelial nitric oxide synthase mRNA and protein, as well as endothelin-1 mRNA. Consequently, ghrelin significantly attenuated the development of pulmonary arterial hypertension, pulmonary vascular remodeling, and right ventricular hypertrophy. These results demonstrate the therapeutic benefits of ghrelin for impeding the pathogenesis of pulmonary hypertension and right ventricular hypertrophy, particularly in subjects prone to CH (e.g. pulmonary disorders).

Changes in macrovessel pulmonary blood flow distribution following chronic hypoxia: assessed using synchrotron radiation microangiography

Structural and functional changes of the pulmonary circulation, particularly during the pathogenesis of pulmonary arterial hypertension (PAH), remain to be fully elucidated. In this study, we utilized monochromatic synchrotron radiation (SR) microangiography to assess changes in pulmonary arteriole blood flow in the intact-chest rat after 4 wk of chronic hypoxia. Sprague-Dawley rats were exposed to normoxia (N-rats) or chronic hypoxia (10% O2; CH-rats) for 28 days. Rats were anesthetized, and microangiography was performed on the left lung to assess 1) the branching distribution of pulmonary arteriole blood flow (internal diameter >80 μm) and 2) dynamic changes in vessel lumen diameter during acute hypoxic (8% O2 for 4 min) pulmonary vasoconstriction (HPV) before and after β-adrenoceptor blockade (2 mg/kg iv propranolol). Using SR angiography, we observed that the number of opaque third- and fourth-generation vessels (100–300 μm) for CH-rats was significantly fewer than the number for N-rats. The magnitude of HPV was not different between CH-rats and N-rats. β-Adrenoceptor blockade accentuated the HPV in 200- to 300-μm vessels for CH-rats, but even more so in N-rats. However, in CH-rats, β-adrenoceptor blockade also accentuated the HPV in 100- to 200-μm vessels. In summary, we utilized SR to assess gross blood flow changes and functional changes (i.e., HPV) of the pulmonary circulation in PAH. These results highlight the benefits of SR for assessing pulmonary circulatory pathology. Of particular importance, future use of SR will provide an effective method for assessing potential therapeutic treatments for PAH.

Does central nitric oxide elicit pulmonary hypertension in conscious rats?

The mechanisms involved in the pathogenesis of pulmonary arterial hypertension (PAH) remain unclear. Nitric oxide (NO) centrally attenuates sympathetic outflow and, therefore, may chronically modulate pulmonary arterial pressure (PAP), especially during the development of chronic hypoxia-induced PAH. To test this hypothesis, 20 male Sprague-Dawley rats (B.W. approximately 200-320 g) were chronically implanted with a telemetric transmitter for the continuous measurement of PAP, and an osmotic pump for intracerebroventricular (i.c.v.) administration of either aCSF (control), L-NAME (150 microg/kg/day) or the NO-donor, SIN-1 (100 microg/kg/day). Rats spent 7 days in normoxia, and then 14 days in hypoxia (CH=12% O2). In normoxia, exogenous NO elevated PAP by approximately 64%, although this increase in PAP could be prevented by isoproterenol (200 mug/kg). PAH occurred in all rats following 14 days of hypoxia. L-NAME did not alter baseline MPAP or the physiological responses to hypoxia. Our results suggest that central NO increases MPAP, although the mechanisms involved remain to be fully elucidated.

Long-term monitoring of pulmonary arterial pressure in conscious, unrestrained mice

INTRODUCTION

The ability to genetically engineer specific gene 'knock-out' mice has provided a powerful tool for investigating the various mechanisms that contribute to the pathogenesis of pulmonary arterial hypertension (PAH). Yet, so far there have been no reports describing the measurement of pulmonary arterial pressure (PAP) in the conscious wild type mouse-an essential requirement for monitoring dynamic changes associated with the pathogenesis of PAH. Therefore, in this study we describe a new technique for long-term measurement of PAP in conscious unrestrained mice using telemetry.

METHODS

In five male C57BL/6 mice (B.W. 25-30 g), the sensing catheter of a telemetric transmitter was inserted into the right ventricle and advanced into the pulmonary artery. The transmitter body was positioned either within the abdominal cavity or subcutaneously on the back. During recovery from surgery, mean PAP was recorded daily for 1 week. Subsequently, the PAP responses to acute hypoxia (8% O2 for 10 min) and L-NAME (50 mg/kg, s.c.) were tested in three mice.

RESULTS

By 1-week post surgery, all mice had fully recovered from surgery and baseline MPAP was stable at 14.9+/-0.7 mm Hg. Additionally, the pulmonary vascular stimulants acute hypoxia and L-NAME provoked a 63% and 86% increase MPAP, respectively.

DISCUSSION

In summary, this study has demonstrated the ability to accurately measure PAP by telemetry in conscious, unrestrained mice. One important application of this technique for future studies is the possibility to assess the relative contribution of specific genes (using 'knock-out' mice) during the chronic development of pulmonary pathological conditions.

Exogenous nitric oxide centrally enhances pulmonary reactivity in the normal and hypertensive rat

1. Chronic hypoxia causes sustained pulmonary hypertension and, although impairment of the pulmonary endothelial nitric oxide (NO) pathway has been implicated, no study has described the central role of NO in modulating pulmonary vascular tone and reactivity. Centrally, NO inhibits sympathetic outflow, so we hypothesised that central NO would modulate pulmonary vascular tone and its reactivity to acute hypoxia, especially in the hypertensive state. 2. Male adult Sprague-Dawley rats were exposed to normoxia (N) or chronic hypoxia (CH; 12% O2) for 14 days. Mean pulmonary arterial pressure (MPAP), systemic mean arterial blood pressure (MABP), cardiac output and heart rate were then measured in pentobarbitone-anaesthetized, artificially ventilated rats. The N and CH rats were exposed to acute hypoxia (10% O2 for 4 min) after the intracerebroventricular (i.c.v.) administration of artificial cerebrospinal fluid (control) and then again after either i.c.v. NG-nitro-L-arginine methyl ester (L-NAME; 150 microg in 10 microL) or 3-morpholino-sydnonimine hydrochloride (SIN-1; 100 microg in 10 microL). 3. Chronic hypoxia caused pulmonary hypertension (MPAP 20+/-1 vs 30+/-1 mmHg in N and CH rats, respectively) and attenuated acute hypoxic pulmonary vasoconstriction (HPV). Central inhibition of NO synthesis (by l-NAME) did not alter baseline MPAP or the acute HPV in either N or CH rats, but it did elevate MABP. The NO donor SIN-1 did not alter baseline MPAP, but it did enhance (N rats) or restore (CH rats) the HPV and decreased MABP. 4. The results of the present study indicate that central NO has a limited role in the tonic modulation of MPAP during normoxia and after chronic hypoxia. However, the acute HPV seems to be enhanced by exogenous NO.

Endogenous nitric oxide and pulmonary circulation response to hypoxia in high-altitude adapted Tibetan sheep

Nitric oxide (NO) is important for the pulmonary circulation response to acute and chronic hypoxia. We examined effects of endogenous nitric oxide synthase (NOS) inhibition on pulmonary vascular tone in response to hypoxia in Tibetan sheep dwelling at 3,000 m above sea level using a pressure chamber. Unanaesthetized male sheep living at 2,300 m above sea level ( n=7) were prepared for vascular monitoring. Pulmonary artery ( P(pa)), pulmonary artery wedge ( P(cwp)) and systemic artery pressures together with cardiac output (CO) were measured, and pulmonary vascular resistance (PVR) was calculated as ( P(pa)- P(cwp))/CO. A non-selective NOS inhibitor, N(omega)-nitro- l-arginine (NLA; 20 mg kg(-1)), and a selective NOS inhibitor, ONO-1714 (0.1 mg kg(-1)), were used and measurements were made at 0 m, 2,300 m, and 4,500 m, with and without the NOS inhibitors. After NLA, P(pa) increased slightly and CO decreased in animals at baseline (2,300 m). The increased PVR after NLA at 4,500 m was greater than that at 2,300 m ( P<0.05). Selective NOS inhibition increased PVR at baseline, but not at 4,500 m. The enhanced pulmonary vasoconstriction after NO inhibition at basal and hypoxic conditions suggests a modulating role of NOS bioactivity in the pulmonary circulation and that augmented endothelial NOS plays a counterregulatory role in the pulmonary vasoconstrictor response to acute hypoxia in high-altitude adapted Tibetan sheep.

Decreased total nitric oxide production in patients with duchenne muscular dystrophy

Plasma nitric oxide (NO) levels in Duchenne muscular dystrophy (DMD) patients were significantly lower than those observed in both healthy controls and in patients with other neuromuscular disorders. The correlation between NO level and ejection fraction was significant (r = -0.384, p = 0.0391) in the DMD group. Disruption of NO systems may contribute to the development of muscular dystrophy and have implications for therapeutic strategies.

Thalidomide reduces lipopolysaccharide/zymosan-induced acute lung injury in rats

Pharmacological therapies targeting fulminant lung inflammation in acute lung injury (ALI) need to be improved. We evaluated the effect of thalidomide, a chemical modulating both acute and chronic inflammation, on ALI induced by intravenous administration of lipopolysaccharide (LPS) and zymosan in male Sprague-Dawley rats. Injection of LPS and zymosan induced significant lung inflammation, as evidenced by increased neutrophil sequestration in lung tissue as well as enhanced nitric oxide metabolite (NO(x)(-)) production in the serum and bronchoalveolar lavage (BAL) fluid. Lactate dehydrogenase (LDH) activity and protein concentration in BAL fluid were significantly increased after administration of LPS and zymosan. Pulmonary microvascular permeability was determined using the Evans blue retention method, which showed a significant increase in microvascular permeability after LPS and zymosan administration, indicating the development of ALI. Animals that received thalidomide (100 mg/kg) 2 h prior to LPS injection had significantly reduced pulmonary NO(x)(-) production, pulmonary microvascular permeability, and LDH activity and protein concentration in BAL fluid. We therefore conclude that thalidomide ameliorates lung inflammation and reduces ALI induced by combined LPS and zymosan administration in rats.