Nitric oxide scavenging by hemoglobin regulates hypoxic pulmonary vasoconstriction

Although the importance of red blood cells in augmenting hypoxic pulmonary vasoconstriction has been recognized for decades, only recently has it become clear that this occurs primarily because of the inactivation of nitric oxide (NO) by hemoglobin. This interaction between red blood cells, NO, and the pulmonary circulation is critical in understanding the effects of anemia and polycythemia on pulmonary blood flow distribution, gas exchange, and global O2 delivery and in understanding the development of hemoglobin-based oxygen carriers. This review will discuss the proposed mechanisms for initiation of hypoxic pulmonary vasoconstriction and regulation of hypoxic pulmonary vasoconstriction by red blood cells with an emphasis on hemoglobin-NO interactions. In addition, the review will discuss how biologic (S-nitrosation) or pharmacologic (cross-linking) modification of hemoglobin may affect pulmonary circulatory-hemoglobin interactions.     

Attenuation of acute hypoxic pulmonary vasoconstriction and hypoxic pulmonary hypertension in mice by inhibition of Rho-kinase

RhoA GTPase mediates a variety of cellular responses, including activation of the contractile apparatus, growth, and gene expression. Acute hypoxia activates RhoA and, in turn, its downstream effector, Rho-kinase, and previous studies in rats have suggested a role for Rho/Rho-kinase signaling in both acute and chronically hypoxic pulmonary vasoconstriction. We therefore hypothesized that activation of Rho/Rho-kinase in the pulmonary circulation of mice contributes to acute hypoxic pulmonary vasoconstriction and chronic hypoxia-induced pulmonary hypertension and vascular remodeling. In isolated, salt solution-perfused mouse lungs, acute administration of the Rho-kinase inhibitor Y-27632 (1 x 10(-5) M) attenuated hypoxic vasoconstriction as well as that due to angiotensin II and KCl. Chronic treatment with Y-27632 (30 mg x kg(-1) x day(-1)) via subcutaneous osmotic pump decreased right ventricular systolic pressure, right ventricular hypertrophy, and neomuscularization of the distal pulmonary vasculature in mice exposed to hypobaric hypoxia for 14 days. Analysis of a small number of proximal pulmonary arteries suggested that Y-27632 treatment reduced the level of phospho-CPI-17, a Rho-kinase target, in hypoxic lungs. We also found that endothelial nitric oxide synthase protein in hypoxic lungs was augmented by Y-27632, suggesting that enhanced nitric oxide production might have played a role in the Y-27632-induced attenuation of chronically hypoxic pulmonary hypertension. In conclusion, Rho/Rho-kinase activation is important in the effects of both acute and chronic hypoxia on the pulmonary circulation of mice, possibly by contributing to both vasoconstriction and vascular remodeling.     

Nebulization of the acidified sodium nitrite formulation attenuates acute hypoxic pulmonary vasoconstriction

Background

Generalized hypoxic pulmonary vasoconstriction (HPV) occurring during exposure to hypoxia is a detrimental process resulting in an increase in lung vascular resistance. Nebulization of sodium nitrite has been shown to inhibit HPV. The aim of this project was to investigate and compare the effects of nebulization of nitrite and different formulations of acidified sodium nitrite on acute HPV.

Methods

Ex vivo isolated rabbit lungs perfused with erythrocytes in Krebs-Henseleit buffer (adjusted to 10% hematocrit) and in vivo anesthetized catheterized rabbits were challenged with periods of hypoxic ventilation alternating with periods of normoxic ventilation. After baseline hypoxic challenges, vehicle, sodium nitrite or acidified sodium nitrite was delivered via nebulization. In the ex vivo model, pulmonary arterial pressure and nitric oxide concentrations in exhaled gas were monitored. Nitrite and nitrite/nitrate were measured in samples of perfusion buffer. Pulmonary arterial pressure, systemic arterial pressure, cardiac output and blood gases were monitored in the in vivo model.

Results

In the ex vivo model, nitrite nebulization attenuated HPV and increased nitric oxide concentrations in exhaled gas and nitrite concentrations in the perfusate. The acidified forms of sodium nitrite induced higher levels of nitric oxide in exhaled gas and had longer vasodilating effects compared to nitrite alone. All nitrite formulations increased concentrations of circulating nitrite to the same degree. In the in vivo model, inhaled nitrite inhibited HPV, while pulmonary arterial pressure, cardiac output and blood gases were not affected. All nitrite formulations had similar potency to inhibit HPV. The tested concentration of appeared tolerable.

Conclusion

Nitrite alone and in acidified forms effectively and similarly attenuates HPV. However, acidified nitrite formulations induce a more pronounced increase in nitric oxide exhalation.     

缺氧性肺血管收缩的细胞机制

缺氧直接作用于肺血管平滑肌细胞而使肺血管收缩。缺氧使细胞膜Ca~(2 )通透性增加,K~ 电导降低、膜电位下降,产生Ca~(2 )依赖性动作电位,导致肺血管张力增加和肺血管收缩。缺氧还能使平滑肌细胞内能量代谢发生改变,抑制氧化磷酸化和三羧酸循环作用,降低磷酸势能,引起肺血管收缩。缺氧减少细胞内氧自由基的产生而使细胞内氧化还原状态发生改变,GSH/GSSG和NADPH/NADP~ 比值增高,导致肺血管阻力增高。     

Effects of hypoxic pulmonary vasoconstriction on pulmonary gas exchange

Brimioulle, Serge, Philippe Lejeune, and Robert Naeije.Effects of hypoxic pulmonary vasoconstriction on pulmonary gasexchange. J. Appl. Physiol. 81(4):1535-1543, 1996.Several reports have suggested that hypoxicpulmonary vasoconstriction (HPV) might result in deterioration ofpulmonary gas exchange in severe hypoxia. We therefore investigated theeffects of HPV on gas exchange in normal and diseased lungs. Weincorporated a biphasic HPV stimulus-response curve observed in intactdogs (S. Brimioulle, P. Lejeune, J. L. Vachièry, M. Delcroix, R. Hallemans, and R. Naeije, J. Appl.Physiol. 77: 476-480, 1994) into a 50-compartment lung model (J. B. West, Respir.Physiol. 7: 88-110, 1969) to control the amount ofblood flow directed to each lung compartment according to the localhypoxic stimulus. The resulting model accurately reproduced the bloodgas modifications caused by HPV changes in dogs with acute lung injury.In single lung units, HPV had a moderate protective effect on alveolaroxygenation, which was maximal at near-normal alveolarPO₂ (75-80 Torr), mixed venousPO₂ (35 Torr), andPO₂ at which hemoglobin is 50%saturated (24 Torr). In simulated diseased lungs associated with40-60 Torr arterial PO₂,however, HPV increased arterial PO₂ by 15-20 Torr. We conclude that HPV can improve arterialoxygenation substantially in respiratory failure.

     

PEEP inhibits hypoxic pulmonary vasoconstriction in dogs

The effects of an increase in alveolar pressure on hypoxic pulmonary vasoconstriction (HPV) have been reported variably. We therefore studied the effects of positive end-expiratory pressure (PEEP) on pulmonary hemodynamics in 13 pentobarbital-anesthetized dogs ventilated alternately in hyperoxia [inspired O2 fraction (FIO2) 0.4] and in hypoxia (FIO2 0.1). In this intact animal model, HPV was defined as the gradient between hypoxic and hyperoxic transmural (tm) mean pulmonary arterial pressure [Ppa(tm)] at any level of cardiac index (Q). Ppa(tm)/Q plots were constructed with mean transmural left atrial pressure [Pla(tm)] kept constant at approximately 6 mmHg (n = 5 dogs), and Ppa(tm)/PEEP plots were constructed with Q kept constant approximately 2.8 l.min-1.m-2 and Pla(tm) kept constant approximately 8 mmHg (n = 8 dogs). Q was manipulated using a femoral arteriovenous bypass and a balloon catheter in the inferior vena cava. Pla(tm) was held constant by a balloon catheter placed by left thoracotomy in the left atrium. Increasing PEEP, from 0 to 12 Torr by 2-Torr increments, at constant Q and Pla(tm), increased Ppa(tm) from 14 +/- 1 (SE) to 19 +/- 1 mmHg in hyperoxia but did not affect Ppa(tm) (from 22 +/- 2 to 23 +/- 1 mmHg) in hypoxia. Both hypoxia and PEEP, at constant Pla(tm), increased Ppa(tm) over the whole range of Q studied, from 1 to 5 l/min, but more at the highest than at the lowest Q and without change in extrapolated pressure intercepts. Adding PEEP to hypoxia did not affect Ppa(tm) at all levels of Q.(ABSTRACT TRUNCATED AT 250 WORDS)     

Effects of acid-base status on acute hypoxic pulmonary vasoconstriction and gas exchange.

To investigate the relationship between hypoxic pulmonary vasoconstriction and respiratory and metabolic acidosis and respiratory alkalosis, the pulmonary gas exchange and pulmonary hemodynamic responses were measured in anesthetized, paralyzed, and mechanically ventilated dogs in two sets of experiments (series A, n = 6; series B, n = 10). The animals were treated with acute hypoxia, CO2 inhalation, hyperventilation, and dinitrophenol in various combinations. Multiple regression analysis indicated that mean pulmonary arterial pressure (Ppa) was significantly correlated with end-tidal PO2, mixed venous PO2, and the mean pulmonary capillary pH (average of arterial and mixed venous pH) as independent variables [series A: r = +0.999, standard error of estimate (SEE) = 0.4 mmHg; series B: r = +0.98, SEE = 1.4 mmHg]. Similar analyses of mean values published by other authors from an acute study on humans with exercise at sea level and simulated altitudes of 10,000 and 15,000 ft also indicated a good relationship (n = 14, r = +0.98, SEE = 2.1 mmHg). The mean data (n = 19) obtained in Operation Everest II at various exercise loads and simulated altitudes gave a correlation of r = +0.87, SEE = 6.1 mmHg. These empirical analyses suggest that variations in the rise of Ppa with hypoxia can be accounted for in vivo by the superimposed acid-base status. Furthermore, ventilation-perfusion inhomogeneity, as estimated in the dogs from end-tidal and arterial O2 and CO2 differences and assuming no true shunt or diffusion impairment, was highly correlated with Ppa and mean pulmonary capillary pH (r = +0.999 in series A, r = +0.77 in series B). The human data from the above studies also showed significant correlations between Ppa and directly measured ventilation-perfusion (standard deviation of perfusion obtained from inert gas measurements). These observations indicate that the beneficial effects of hyperventilation during hypoxia may be related to the marked alkalosis that serves to reduce Ppa and improve pulmonary gas exchange efficiency.     

Regional hypoxic pulmonary vasoconstriction in prone pigs.

Hypoxic pulmonary vasoconstriction (HPV) is known to affect regional pulmonary blood flow distribution. It is unknown whether lungs with well-matched ventilation (V)/perfusion (Q) have regional differences in the HPV response. Five prone pigs were anesthetized and mechanically ventilated (positive end-expiratory pressure = 2 cmH2O). Two hypoxic preconditions [inspired oxygen fraction (FI(O2)) = 0.13] were completed to stabilize the animal's hypoxic response. Regional pulmonary blood Q and V distribution was determined at various FI(O2) (0.21, 0.15, 0.13, 0.11, 0.09) using the fluorescent microsphere technique. Q and V in the lungs were quantified within 2-cm3 lung pieces. Pieces were grouped, or clustered, based on the changes in blood flow when subjected to increasing hypoxia. Unique patterns of Q response to hypoxia were seen within and across animals. The three main patterns (clusters) showed little initial difference in V/Q matching at room air where the mean V/Q range was 0.92-1.06. The clusters were spatially located in cranial, central, and caudal portions of the lung. With decreasing FI(O2), blood flow shifted from the cranial to caudal regions. We determined that pulmonary blood flow changes, caused by HPV, produced distinct response patterns that were seen in similar regions across our prone porcine model.     

Acetazolamide prevents hypoxic pulmonary vasoconstriction in conscious dogs.

Acute hypoxia increases pulmonary arterial pressure and vascular resistance. Previous studies in isolated smooth muscle and perfused lungs have shown that carbonic anhydrase (CA) inhibition reduces the speed and magnitude of hypoxic pulmonary vasoconstriction (HPV). We studied whether CA inhibition by acetazolamide (Acz) is able to prevent HPV in the unanesthetized animal. Ten chronically tracheotomized, conscious dogs were investigated in three protocols. In all protocols, the dogs breathed 21% O(2) for the first hour and then 8 or 10% O(2) for the next 4 h spontaneously via a ventilator circuit. The protocols were as follows: protocol 1: controls given no Acz, inspired O(2) fraction (Fi(O(2))) = 0.10; protocol 2: Acz infused intravenously (250-mg bolus, followed by 167 microg.kg(-1).min(-1) continuously), Fi(O(2)) = 0.10; protocol 3: Acz given as above, but with Fi(O(2)) reduced to 0.08 to match the arterial Po(2) (Pa(O(2))) observed during hypoxia in controls. Pa(O(2)) was 37 Torr during hypoxia in controls, mean pulmonary arterial pressure increased from 17 +/- 1 to 23 +/- 1 mmHg, and pulmonary vascular resistance increased from 464 +/- 26 to 679 +/- 40 dyn.s(-1).cm(-5) (P < 0.05). In both Acz groups, mean pulmonary arterial pressure was 15 +/- 1 mmHg, and pulmonary vascular resistance ranged between 420 and 440 dyn.s(-1).cm(-5). These values did not change during hypoxia. In dogs given Acz at 10% O(2), the arterial Pa(O(2)) was 50 Torr owing to hyperventilation, whereas in those breathing 8% O(2) the Pa(O(2)) was 37 Torr, equivalent to controls. In conclusion, Acz prevents HPV in conscious spontaneously breathing dogs. The effect is not due to Acz-induced hyperventilation and higher alveolar Po(2), nor to changes in plasma endothelin-1, angiotensin-II, or potassium, and HPV suppression occurs despite the systemic acidosis with CA inhibition.     

Reduction of hypoxic pulmonary vasoconstriction during trichloroethylene anesthesia.

A double-lumen tube was inserted into the trachea of dogs anesthetized with intravenous pentobarbital (30-40 mg/kg). Blood flow/unit lung volume in each lung was measured with 133Xe. Both lungs were initially ventilated with oxygen and measurements of pulmonary blood flow, CO2 output, cardiac output, and blood gases were made. When nitrogen was administered to one lung blood flow was diverted to the opposite lung. The diversion of flow was reduced by the inhalation of 1% trichloroethylene but returned after withdrawal of the anesthetic. There were no significant changes in cardiac output. Changes in CO2 output and arterial Po2 were compatible with the xenon results. It is concluded that trichloroethylene may increase arterial hypoxemia by reducing vasoconstriction in hypoxic areas of lung.