eNOS and prostanoid enzymes in lungs of newborn piglets with chronic aortopulmonary shunts

Downregulation of PGI2 pathway in Pulmonary Hypertension Group-III patients

Pulmonary hypertension (PH) is a progressive and life-threating lung disorder characterized by elevated pulmonary artery pressure and vascular remodeling. PH is classified into five groups, and one of the most common and lethal forms, PH Group-III is defined as PH due to lung diseases and/or hypoxia. Due to the lack of studies in this group, PH-specific drug therapies including prostacyclin (PGI₂) analogues have not been approved or recommended for use in these patients. PGI₂ is synthesized by the PGI₂ synthase (PGIS) enzyme, and its production is determined by measuring its stable metabolite, 6-keto-PGF_1α. An impaired PGI₂ pathway has been observed in PH animal models and in PH Group-I patients; however, there are contradictory results. The aim of this study is to determine whether PH Group-III is associated with altered expression of PGIS and production of PGI₂ in humans. To explore this hypothesis, we measured PGIS expression (by western blot) and PGI₂ production (by ELISA) in a large variety of preparations from the pulmonary circulation including human pulmonary artery, pulmonary vein, distal lung tissue, pulmonary artery smooth muscle cells (hPASMC), and bronchi in PH Group-III (n = 35) and control patients (n = 32). Our results showed decreased PGIS expression and/or 6-keto-PGF_1α levels in human pulmonary artery, hPASMC, and distal lung tissue derived from PH Group-III patients. Moreover, the production of 6-keto-PGF_1α from hPASMC positively correlated with PGIS expression and was inversely correlated with mean pulmonary artery pressure. On the other hand, PH Group-III pulmonary veins and bronchi did not show altered PGI₂ production compared to controls. The deficit in PGIS expression and/or PGI₂ production observed in pulmonary artery and distal lung tissue in PH Group-III patients may have important implications in the pathogenesis and treatment of PH Group-III.

Regression of flow-induced pulmonary arterial vasculopathy after flow correction in piglets

OBJECTIVES

Chronic thromboembolic pulmonary hypertension is due to partial obstruction of the pulmonary arterial bed and may resolve after pulmonary thromboendarterectomy. Persistent pulmonary hypertension, the main complication after pulmonary thromboendarterectomy, may reflect vessel alterations induced by high flow in unobstructed lung territories. The aim of this study was to determine whether correcting high flow led to reversal of the vasculopathy in piglets.

METHODS

The effects of high pulmonary blood flow were investigated 5 weeks after creation of an aortopulmonary shunt (n = 10), and reversibility of vessel disease was evaluated at 1 week (n = 10) and 5 weeks after shunt closure (n = 10), compared to sham-operated animals (n = 10). Hemodynamic variables, pulmonary artery reactivity, and morphometry were recorded. We also investigated the endothelin, angiopoietin, and nitric oxide synthase pathways.

RESULTS

High flow increased medial thickness in distal pulmonary arteries (55.6% +/- 1.2% vs 35.9% +/- 0.8%; P < .0001) owing to an increase of smooth muscle cell proliferation (proliferating cell nuclear antigen labeling). The endothelium-dependent relaxation was altered (P < .05). This phenomenon was associated to an overexpression of endothelin-1, endothelin-A, angiopoietin 1, angiopoietin 2, and Tie-2 (P < .05). After 1 week of shunt closure, all overexpressed genes returned to control values, the proliferation of smooth muscle cells stopped, and smooth muscle cell apoptosis increased (terminal deoxynucleotidyl transferase-mediated dUTP nick end labeling), preceding the normalization of the wall thickness hypertrophy and the pulmonary artery vasoreactivity observed at 5 weeks after shunt closure.

CONCLUSION

These results demonstrate that endothelin-1 and angiopoietin pathways are involved in vasculopathy development and may be important therapeutic targets for preventing persistent pulmonary hypertension after pulmonary thromboendarterectomy.

Atrasentan treatment of pulmonary vascular disease in piglets with increased pulmonary blood flow

We studied the effect of chronic endothelin A receptor blockade by atrasentan on the pulmonary endothelin-1 system and vascular endothelial growth factor (VEGF) expression in piglets with high pulmonary blood flow. Twenty-five 4-week-old piglets with high pulmonary blood flow were randomized to three groups: sham operated (n = 8), placebo (water) (n = 7), or treatment with atrasentan (2 mg/kg per day) (n = 10). After 3 months, mean pulmonary arterial pressure (PAP) was higher in the placebo group than in the sham group [18 +/- 2 mm Hg versus 14 +/- 1 mm Hg; P < 0.05 (ANOVA)]. Atrasentan treatment was associated with lower cardiac output, PAP (14 +/- 1 mm Hg), and medial wall thickness of pulmonary arteries (diameter: 50-150 microM) compared with placebo [13.6 +/- 3.0% versus 18.1 +/- 4.2%; P < 0.05 (ANOVA)]. Quantitative real-time polymerase chain reaction for endothelin-1, endothelin B receptor, and endothelin-converting enzyme-1 mRNA in lung tissue did not differ. However, immunostaining as well as mRNA for VEGF were lower in atrasentan-treated animals (relative gene expression: atrasentan versus placebo: 0.8 +/- 0.3 versus 1.5 +/- 0.3; P = 0.009). Atrasentan treatment effectively reduces medial hypertrophy in piglets with chronic pulmonary hyperperfusion. Chronic endothelin A receptor blockade by atrasentan may interfere with the expression of VEGF.

Pulmonary vascular changes in piglets with increased pulmonary blood flow and pressure

In this model of pulmonary vascular disease, high pulmonary blood flow was created by an anastomosis between the left subclavian artery and the main pulmonary artery [Blalock-Taussig (BT) shunt] in 4-week-old piglets (n = 6). Additional ligation of the left pulmonary artery (LPA) was used to increase pulmonary artery pressure (n = 6). Seven piglets were sham-operated. After 3 months, mean pulmonary artery pressure was higher in animals with BT shunt and LPA ligation (22 +/- 5; mean+/-SD) compared to sham-operated animals (15 +/- 2). In addition, thickening of the medial coat (20.1 +/- 2.8% versus 13.6 +/- 3.1% wall thickness) and increased immunostaining for vascular endothelial growth factor A (VEGF-A) were observed. Relative gene expression for endothelin-converting enzyme-1 (ECE-1) mRNA was 1.8 times higher, and VEGF-A mRNA was 2.5 times higher in pigs with BT shunt and LPA ligation compared with sham-operated animals. VEGF receptor-1 and VEGF receptor-2 mRNA was lower in shunted animals and in animals with additional ligation of LPA. Upregulation of ECE-1 and VEGF-A, as well as changes in VEGFR expression in the pulmonary hypertensive lung, may contribute to pulmonary vascular changes.

Pathophysiologic mechanisms of persistent pulmonary hypertension of the newborn

Persistent pulmonary hypertension of the newborn (PPHN), among the most rapidly progressive and potentially fatal of vasculopathies, is a disorder of vascular transition from fetal to neonatal circulation, manifesting as hypoxemic respiratory failure. PPHN represents a common pathway of vascular injury activated by numerous perinatal stresses: hypoxia, hypoglycemia, cold stress, sepsis, and direct lung injury. As with other multifactorial diseases, a single inciting event may be augmented by multiple concurrent/subsequent phenomena that result in differing courses of disease progression. I review the various mechanisms of vascular injury involved in neonatal pulmonary hypertension: endothelial dysfunction, inflammation, hypoxia, and mechanical strain, in the context of downstream effects on pulmonary vascular endothelial-myocyte interactions and myocyte phenotypic plasticity.

Arachidonic acid metabolites and an early stage of pulmonary hypertension in chronically hypoxic newborn pigs

Our purpose was to determine whether production of arachidonic acid metabolites, particularly cyclooxygenase (COX) metabolites, is altered in 100-400-microm-diameter pulmonary arteries of piglets at an early stage of pulmonary hypertension. Piglets were raised in either room air (control) or hypoxia for 3 days. A cannulated artery technique was used to measure responses of 100-400-microm-diameter pulmonary arteries to arachidonic acid, a prostacyclin analog, or the thromboxane mimetic. Radioimmunoassay was used to determine pulmonary artery production of thromboxane B(2) (TxB(2)) and 6-keto-prostaglandin F(1alpha) (6-keto-PGF(1alpha)), the stable metabolites of thromboxane and prostacyclin, respectively. Assessment of abundances of COX pathway enzymes in pulmonary arteries was determined by immunoblot technique. Arachidonic acid induced less dilation in pulmonary arteries from hypoxic than in pulmonary arteries from control piglets. Pulmonary artery responses to prostacyclin and were similar for both groups. 6-Keto-PGF(1alpha) production was reduced, whereas TxB(2) production was increased in pulmonary arteries from hypoxic piglets. Abundances of both COX-1 and prostacyclin synthase were reduced, whereas abundances of both COX-2 and thromboxane synthase were unaltered in pulmonary arteries from hypoxic piglets. At least partly due to altered abundances of COX pathway enzymes, a shift in production of arachidonic acid metabolites, away from dilators toward constrictors, may contribute to the early phase of chronic hypoxia-induced pulmonary hypertension in newborn piglets.