Altered vasoreactivity in lungs isolated from rats exposed to nitric oxide gas

Pulmonary hypertension associated with cardiopulmonary bypass and cardiac surgery

BACKGROUND AND AIM

Pulmonary hypertension (PH) is frequently associated with cardiovascular surgery and is a common complication that has been observed after surgery utilizing cardiopulmonary bypass (CPB). The purpose of this review is to explain the characteristics of PH, the mechanisms of PH induced by cardiac surgery and CPB, treatments for postoperative PH, and future directions in treating PH induced by cardiac surgery and CPB using up-to-date findings.

METHODS

The PubMed database was utilized to find published articles.

RESULTS

There are many mechanisms that contribute to PH after cardiac surgery and CPB which involve pulmonary vasomotor dysfunction, cyclooxygenase, the thromboxane A2 and prostacyclin pathway, the nitric oxide pathway, inflammation, and oxidative stress. Furthermore, there are several effective treatments for postoperative PH within different types of cardiac surgery.

CONCLUSIONS

By possessing a deep understanding of the mechanisms that contribute to PH after cardiac surgery and CPB, researchers can develop treatments for clinicians to use which target the mechanisms of PH and ultimately reduce and/or eliminate postoperative PH. Additionally, learning about the most up-to-date studies regarding treatments can allow clinicians to choose the best treatments for patients who are undergoing cardiac surgery and CPB.

Inhibition of nitric oxide synthase unmasks vigorous vasoconstriction in established pulmonary arterial hypertension

It is widely accepted that impaired bioavailability of endothelial nitric oxide (NO) plays a critical role in the pathophysiology of pulmonary arterial hypertension (PAH). However, there are published data that show that relatively many PAH patients respond favorably to acetylcholine‐induced pulmonary vasodilation during their follow‐up period, when diverse stages of the disorder are included. We hypothesized that NO bioavailability varies depending on the progression of PAH. Adult rats were exposed to the VEGF receptor blocker Sugen5416 and 3 weeks of hypoxia followed by return to normoxia for various additional weeks. All rats developed increased right ventricular systolic pressure (RVSP) and occlusive lesion formation at 1, 3, 5, and 8 weeks after the Sugen5416 injection. Acute NO synthase blockade did not change the elevated RVSP at the 1‐week time point, while it further increased RVSP markedly at the 3‐, 5‐, and 8‐week time points, leading to death in all rats tested at 8 weeks. Acetylcholine caused significant reduction in RVSP at the 8‐week but not the 1‐week time point, whereas sodium nitroprusside decreased the pressure similarly at both time points. Increased NO‐mediated cGMP production was found in lungs from the 8‐week but not the 1‐week time point. In conclusion, despite its initial impairment, NO bioavailability is restored and endogenous NO plays a critical protective role by counteracting severe pulmonary vasoconstriction in established stages of PAH in the Sugen5416/hypoxia/normoxia‐exposed rats. Our results provide solid pharmacological evidence for a major contribution of a NO‐suppressed vasoconstrictor component in the pathophysiology of established PAH.

Involvement of Rho kinase in the pathogenesis of acute pulmonary embolism-induced polystyrene microspheres in rats

Acute pulmonary embolism (PE) is a life-threatening disease, and several vasoconstrictors, including endothelin-1 (ET-1), play a key role in vasoconstriction and hypoxemia during the development of PE. Rho kinase is activated by various vasoconstrictors resulting in vascular contraction and remodeling. Recent evidence has revealed an important role of Rho kinase in the pathogenesis of systemic and pulmonary vascular diseases. However, contribution of Rho kinase in PE remains unclear. We thus investigated the role of Rho kinase in the PE rat model induced by intrajugular administration of polystyrene microspheres (mean diameter, 26 microm). At 6 h following the administration of microspheres (1.5 ml/kg), right ventricular systolic pressure (RVSP) was higher in the PE than in the control rats (15.8 +/- 1.6 vs. 32.9 +/- 7.5 mmHg). Arterial oxygen tension was lower (92.3 +/- 12.5 vs. 66.0 +/- 17.7 Torr), and alveolar-arterial difference in oxygen partial pressure was higher (3.9 +/- 3.8 vs. 36.5 +/- 26.9 Torr) in the PE rats. Western blotting analysis revealed upregulation and downregulation in expression of vascular cell adhesion molecule-1 and endothelial nitric oxide synthase in lungs from the PE rats, respectively, and radioimmunoassay demonstrated an increase in plasma ET-1 levels. Lung Rho kinase alpha expression was greater in the PE rats. At 5 h following administration of microspheres (0.75 ml/kg), intravenous Rho kinase inhibitors HA1077 and Y27632 (3 mg/kg each) attenuated elevation of RVSP (22.0 +/- 3.7, 17.1 +/- 3.2, 14.3 +/- 2.6 mmHg, PE, PE+HA1077, PE+Y27632) and the severity of hypoxemia (66.3 +/- 16.2, 94.9 +/- 23.0, 89.1 +/- 8.5 Torr, PE, PE+HA1077, PE+Y27632) in the PE rats. These results suggest that pulmonary endothelial dysfunction and activation of Rho kinase may contribute to the potentiation of vasoconstriction and hypoxemia in the PE rats.

Role of cGMP-dependent protein kinase in development of tolerance to nitric oxide in pulmonary veins of newborn lambs

Continuous exposure to nitrovasodilators and nitric oxide induces tolerance to their vasodilator effects in vascular smooth muscle. This study was done to determine the role of cGMP-dependent protein kinase (PKG) in the development of tolerance to nitric oxide. Isolated fourth-generation pulmonary veins of newborn lambs were studied. Incubation of veins for 20 h with DETA NONOate (DETA NO; a stable nitric oxide donor) significantly reduced their relaxation response to the nitric oxide donor and to beta-phenyl-1,N2-etheno-8-bromo-cGMP (8-Br-PET-cGMP, a cell-permeable cGMP analog). Incubation with DETA NO significantly reduced PKG activity and protein and mRNA levels in the vessels. These effects were prevented by 1H-(1,2,4)oxadiazolo(4,3-a)quinoxalin-1-one (an inhibitor of soluble guanylyl cyclase) and Rp-8-Br-PET-cGMPS (an inhibitor of PKG). A decrease in PKG protein and mRNA levels was also observed after continuous exposure to cGMP analogs. The PKG inhibitor abrogated these effects. The decrease in cGMP-mediated relaxation and in PKG activity caused by continuous exposure to DETA NO was not affected by KT-5720, an inhibitor of cAMP-dependent protein kinase. Prolonged exposure to 8-Br-cAMP (a cell-permeable cAMP analog) did not affect PKG protein level in the veins. These results suggest that continuous exposure to nitric oxide or cGMP downregulates PKG by a PKG-dependent mechanism. Such a negative feedback mechanism may contribute to the development of tolerance to nitric oxide in pulmonary veins of newborn lambs.

Pulmonary toxicity associated with nitric oxide in term infants with severe respiratory failure

We prospectively analyzed airway specimens from 24 newborn infants. Inhaled nitric oxide (< or = 20 ppm for 1 to 4 days to 12 infants) did not affect the concentrations of the lipid peroxidation product, the surface activity, or the cytokines (interleukin-1, granulocyte-macrophage colony-stimulating factor, interleukin-1 receptor antagonist). Nitrotyrosine was detected after 10 days of life in the two infants requiring prolonged ventilation, suggesting toxicity of endogenous nitric oxide.