Chronic hypoxia does not cause wall thickening of intra-acinar pulmonary supernumerary arteries

Cellular Pathways Promoting Pulmonary Vascular Remodeling by Hypoxia

Exposure to hypoxia increases pulmonary vascular resistance, leading to elevated pulmonary arterial pressure and, potentially, right heart failure. Vascular remodeling is an important contributor to the increased pulmonary vascular resistance. Hyperproliferation of smooth muscle, endothelial cells, and fibroblasts, and deposition of extracellular matrix lead to increased wall thickness, extension of muscle into normally non-muscular arterioles, and vascular stiffening. This review highlights intrinsic and extrinsic modulators contributing to the remodeling process.

Aneurysm-type plexiform lesions form in supernumerary arteries in pulmonary arterial hypertension: potential therapeutic implications

Histological observations in human pulmonary arterial hypertension (PAH) suggest a link between plexiform lesions and pulmonary supernumerary arteries. Pulmonary microvascular endothelial cells are characterized as hyperproliferative and progenitor-like. This study investigates the hypothesis that aneurysm-type plexiform lesions form in pulmonary supernumerary arteries because of their anatomical properties and endothelial characteristics similar to pulmonary microvascular endothelial cells. To induce PAH, rats were injected with Sugen5416, and exposed to hypoxia (10% O₂) for 3 days (early stage) or 3 wk (mid-stage), or 3 wk of hypoxia with an additional 10 wk of normoxia (late-stage PAH). We examined morphology of pulmonary vasculature and vascular remodeling in lung serial sections from PAH and normal rats. Aneurysm-type plexiform lesions formed in small side branches of pulmonary arteries with morphological characteristics similar to supernumerary arteries. Over the course of PAH development, the number of Ki67-positive cells increased in small pulmonary arteries, including supernumerary arteries, whereas the number stayed consistently low in large pulmonary arteries. The increase in Ki67-positive cells was delayed in supernumerary arteries compared with small pulmonary arteries. In late-stage PAH, ~90% of small unconventional side branches that were likely to be supernumerary arteries were nearly closed. These results support our hypothesis that supernumerary arteries are the predominant site for aneurysm-type plexiform lesions in Sugen5416/hypoxia/normoxia-exposed PAH rats partly because of the combination of their unique anatomical properties and the hyperproliferative potential of endothelial cells. We propose that the delayed and extensive occlusive lesion formation in supernumerary arteries could be a preventive therapeutic target in patients with PAH.

Chronic hypoxia does not cause wall thickening of intra-acinar pulmonary supernumerary arteries

Chronic exposure to hypoxia causes pulmonary hypertension and pulmonary arterial remodeling. Although the exact mechanisms of this remodeling are unclear, there is evidence that it is dependent on hemodynamic stress, rather than on hypoxia alone. Pulmonary supernumerary arteries experience low hemodynamic stress as a consequence of reduced perfusion due to 90° branching angles, small diameters, and "valve-like" structures at their orifices. We investigated whether or not intra-acinar supernumerary arteries undergo structural remodeling during the moderate pulmonary hypertension induced by chronic hypoxia. Rats were exposed to either normoxia or hypoxia for 6 weeks. The chronically hypoxic rats developed pulmonary hypertension. For both groups, pulmonary arteries were selectively filled with barium-gelatin mixture, and the wall thickness of intra-acinar pulmonary arteries was measured in histological samples. Only thin-walled arteries were observed in normoxic lungs. In hypertensive lungs, we found both thin- and thick-walled pulmonary arteries with similar diameters. Disproportionate degrees of arterial wall thickening between parent and daughter branches were observed with supernumerary branching patterns. While parent arteries developed significant wall thickening, their supernumerary branches did not. Thus, chronic hypoxia-induced pulmonary hypertension did not cause wall thickening of intra-acinar pulmonary supernumerary arteries. These findings are consistent with the idea that hemodynamic stress, rather than hypoxia alone, is the cause of structural remodeling during chronic exposure to hypoxia.

Haemodynamic unloading reverses occlusive vascular lesions in severe pulmonary hypertension

AIMS

An important pathogenic mechanism in the development of idiopathic pulmonary arterial hypertension is hypothesized to be a cancer-like cellular proliferation independent of haemodynamics. However, because the vascular lesions are inseparably coupled with haemodynamic stress, the fate of the lesions is unknown when haemodynamic stress is eliminated.

METHODS AND RESULTS

We applied left pulmonary artery banding to a rat model with advanced pulmonary hypertension to investigate the effects of decreased haemodynamic stress on occlusive vascular lesions. Rats were given an injection of the VEGF blocker Sugen5416 and exposed to 3 weeks of hypoxia plus an additional 7 weeks of normoxia (total 10 weeks) (SU/Hx/Nx rats). The banding surgery to reduce haemodynamic stress to the left lung was done at 1 week prior to (preventive) or 5 weeks after (reversal) the SU5416 injection. All SU/Hx/Nx-exposed rats developed severe pulmonary hypertension and right ventricular hypertrophy. Histological analyses showed that the non-banded right lungs developed occlusive lesions including plexiform lesions with marked perivascular cell accumulation. In contrast, banding the left pulmonary artery not only prevented the development of but also reversed the established occlusive lesions as well as perivascular inflammation in the left lungs.

CONCLUSION

Our results indicate that haemodynamic stress is prerequisite to the development and progression of occlusive neointimal lesions in this rat model of severe pulmonary hypertension. We conclude that perivascular inflammation and occlusive neointimal arteriopathy are driven by haemodynamic stress.