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Chronic hypoxic pulmonary hypertension (HPH) is associated with large pulmonary artery (PA) stiffening, which is correlated with collagen accumulation. However, the mechanisms by which collagen contributes to PA stiffening remain largely unexplored. Moreover, HPH may alter mechanical properties other than stiffness, such as pulse damping capacity, which also affects ventricular workload but is rarely quantified. We hypothesized that collagen content and cross-linking differentially regulate the stiffness and damping capacity of large PAs during HPH progression. The hypothesis was tested with transgenic mice that synthesize collagen type I resistant to collagenase degradation (Col1a1(R/R)). These mice and littermate controls (Col1a1(+/+)) were exposed to hypoxia for 10 days; some were treated with β-aminopropionitrile (BAPN), which prevents new cross-link formation. Isolated PA dynamic mechanical tests were performed, and collagen content and cross-linking were measured. In Col1a1(+/+) mice, HPH increased both collagen content and cross-linking, and BAPN treatment prevented these increases. Similar trends were observed in Col1a1(R/R) mice except that collagen content further increased with BAPN treatment. Mechanical tests showed that in Col1a1(+/+) mice, HPH increased PA stiffness and damping capacity, and these increases were impeded by BAPN treatment. In Col1a1(R/R) mice, HPH led to a smaller but significant increase in PA stiffness and a decrease in damping capacity. These mechanical changes were not affected by BAPN treatment. Vessel-specific correlations for each strain showed that the stiffness and damping capacity were correlated with the total content rather than cross-linking of collagen. Our results suggest that collagen total content is critical to extralobar PA stiffening during HPH.

Role of collagen content and cross-linking in large pulmonary arterial stiffening after chronic hypoxia