Effects of PDE-5 Inhibition on the Cardiopulmonary System After 2 or 4 Weeks of Chronic Hypoxia

Abstract

Purpose: In pulmonary hypertension (PH), hypoxia represents both an outcome and a cause of exacerbation. We addressed the question whether hypoxia adaptation might affect the mechanisms underlying PH alleviation through phosphodiesterase-5 (PDE5) inhibition.

Methods: Eight-week-old male Sprague-Dawley rats were divided into two groups depending on treatment (placebo or sildenafil, a drug inhibiting PDE5) and were exposed to hypoxia (10% O₂) for 0 (t0, n = 9/10), 2 (t2, n = 5/5) or 4 (t4, n = 5/5) weeks. The rats were treated (0.3 mL i.p.) with either saline or sildenafil (1.4 mg/Kg per day).

Results: Two-week hypoxia changed the body weight (- 31% vs. - 27%, respectively, P = NS), blood hemoglobin (+ 25% vs. + 27%, P = NS) and nitrates+nitrites (+ 175% vs. + 261%, P = 0.007), right ventricle fibrosis (+ 814% vs. + 317%, P < 0.0001), right ventricle hypertrophy (+ 84% vs. + 49%, P = 0.007) and systolic pressure (+ 108% vs. + 41%, P = 0.001), pulmonary vessel density (+ 61% vs. + 46%, P = NS), and the frequency of small (< 50 µm wall thickness) vessels (+ 35% vs. + 13%, P = 0.0001). Most of these changes were maintained for 4-week hypoxia, except blood hemoglobin and right ventricle hypertrophy that continued increasing (+ 52% vs. + 42%, P = NS; and + 104% vs. + 83%, P = 0.04). To further assess these observations, small vessel frequency was found to be linearly related with the right ventricle-developed pressure independent of hypoxia duration.

Conclusions: Thus, although hypoxia adaptation is not yet accomplished after 4 weeks, PH alleviation by PDE5 inhibition might nevertheless provide an efficient strategy for the management of this disease.

Keywords: Adaptation; Chronic hypoxia; Phosphodiesterase-5 inhibition; Pulmonary angiogenesis; Right-ventricle hypertrophy.

Fig. 1

Effects of various durations of hypoxia in the absence (filled circles) and in the presence (empty squares) of sildenafil (1.4 mg/kg/day). The vertical bars represent the SEM. Sildenafil data points are slightly nudged to the right to enable distinguishing overlapping symbols and error bars. $,§P < 0.05 (ANOVA and Bonferroni tests) with respect to t0 and t2, respectively. *P < 0.05 (unpaired Student’s t test) between control and sildenafil groups for each time point

Fig. 2

Hemodynamic parameters. Effects of various durations of hypoxia in the absence (filled circles) and in the presence (empty squares) of sildenafil (1.4 mg/kg/day). The vertical bars represent the SEM. Sildenafil data points are slightly nudged to the right to enable distinguishing overlapping symbols and error bars. $,§P < 0.05 (ANOVA and Bonferroni tests) with respect to t0 and t2, respectively. *P < 0.05 (unpaired Student’s t test) between control and sildenafil groups for each time point

Fig. 3

Effects of various durations of hypoxia in the absence (filled circles) and in the presence (empty squares) of sildenafil (1.4 mg/kg/day). The vertical bars represent the SEM. Sildenafil data points are slightly nudged to the right to enable distinguishing overlapping symbols and error bars. $,§P < 0.05 (ANOVA and Bonferroni tests) with respect to t0 and t2, respectively. *P < 0.05 (unpaired Student’s t test) between control and sildenafil groups for each time point. The microphotographs show representative examples of the Masson’s staining to evaluate lung tissue fibrosis in the various cases considered in this study

Fig. 4

Effects of various durations of hypoxia in the absence (filled circles) and in the presence (empty squares) of sildenafil (1.4 mg/kg/day) on RV cardiac fibrosis. The vertical bars represent the SEM. Sildenafil data points are slightly nudged to the right to enable distinguishing overlapping symbols and error bars. $,§P < 0.05 (ANOVA and Bonferroni tests) with respect to t0 and t2, respectively. *P < 0.05 (unpaired Student’s t test) between control and sildenafil groups for each time point. The microphotographs show representative examples of the Masson’s staining to evaluate tissue fibrosis in the various cases considered in this study

Fig. 5

Effects of various durations of hypoxia in the absence (filled circles) and in the presence (empty squares) of sildenafil (1.4 mg/kg/day). The vertical bars represent the SEM. Sildenafil data points are slightly nudged to the right to enable distinguishing overlapping symbols and error bars. $,§P < 0.05 (ANOVA and Bonferroni tests) with respect to t0 and t2, respectively. *P < 0.05 (unpaired Student’s t test) between control and sildenafil groups for each time point. Representative Western blots are shown

Fig. 6

Relationship between the frequency of small diameter vessels (0–50 μm) and the pressure developed by the left (upper panel) and right (lower panel) ventricle for each experimental data point. Empty and filled symbols refer to rats treated with placebo and sildenafil, respectively. Triangles, squares, and circles represent data taken at t0, t2, and t4, respectively. The best-fit lines and the 95% confidence limits referring to all available data points are shown. The values of the slope are − 1.357 ± 0.9287 and + 2.352 ± 0.2563 for the left and right ventricle, respectively (P = NS and P < 0.0001 vs. zero)