Actin Polymerization Contributes to Augmented Basal Pulmonary Arterial Tone and Endothelin‐1‐Induced Vasoconstriction Following Chronic Hypoxia

Chronic hypoxia increases pulmonary vascular resistance and promotes pulmonary hypertension through mechanisms that stimulate vasoconstriction and vascular remodeling. We have previously shown that chronic hypoxia augments pulmonary vasoconstrictor reactivity to endothelin‐1 (ET‐1) through reactive oxygen species generation and Rho kinase (ROCK)‐dependent Ca 2+ sensitization. Because ROCK promotes actin polymerization and the actin cytoskeleton is important for tension generation in smooth muscle, we hypothesized that actin polymerization is required for both basal pulmonary arterial tone and enhanced ET‐1 dependent vasoconstriction following chronic hypoxia. To test this hypothesis, we applied the actin polymerization inhibitors cytochalasin B (10 −6 M) and latrunculin A (5 × 10 −7 M) to isolated, endothelium‐disrupted pulmonary arteries from control and chronically hypoxic (4 wk at 0.5 atm) rats that were exposed to increasing intraluminal pressure, and we monitored the development of tone. Arteries from each group were also exposed to increasing concentrations of ET‐1 (10 −10 – 10 −7 M) in the presence and absence of cytochalasin B. Consistent with our hypothesis, inhibition of actin polymerization reduced pressure‐dependent tone and attenuated ET‐1‐induced vasoconstriction in arteries from chronically hypoxic animals, while having no effect on arteries from control rats. To test whether chronic hypoxia directly alters the actin profile in pulmonary arteries, we measured filamentous (F) actin to globular (G) actin ratios by two methods: fluorescent labeling of F‐actin and G‐actin in fixed pulmonary artery segments, and actin sedimentation assays on homogenized pulmonary artery lysates. Results from both sets of experiments indicated that there is no significant difference in actin polymerization between pulmonary arteries from control and chronically hypoxic rats under baseline conditions, but that ET‐1 (10 −8 M) stimulation significantly enhances actin polymerization in pulmonary arteries from rats exposed to chronic hypoxia. We conclude that actin polymerization contributes to pulmonary vasoreactivity following chronic hypoxia, under both pressure‐induced and ET‐1 stimulated conditions, but enhanced levels of actin polymerization following chronic hypoxia are only detected after stimulation with ET‐1. Support or Funding Information NIH R01 HL132883 AHA 16GRNT27700010 K12 GM088021 AHA 15GRNT25090039