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(Circulation. 1996;94:1941-1947.)
© 1996 American Heart Association, Inc.
Articles |
Inhibition of Tissue Angiotensin-Converting Enzyme With Quinapril Reduces Hypoxic Pulmonary Hypertension and Pulmonary Vascular Remodeling
the Center for Transgene Technology and Gene Therapy (Z.N., J.-M.S., L.M., D.C.), Flanders Interuniversity Institute for Biotechnology; and Division of Cardiology (S.J.), University Hospital Gasthuisberg, University of Leuven, Belgium.
Correspondence to Stefan Janssens, MD, PhD, Division of Cardiology and Center for Transgene Technology and Gene Therapy, University Hospital Gasthuisberg, 49 Herestraat, B-3000 Leuven, Belgium. E-mail stefan.janssens@med.kuleuven.ac.be.
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Abstract |
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Background Angiotensin II may contribute to hypoxic pulmonary hypertension via its vasoconstrictor and growth-stimulatory effects on vascular smooth muscle cells (VSMCs). Therefore, the use of ACE inhibitors might reduce hypoxic pulmonary hypertension by decreasing pulmonary vasomotor tone or vascular remodeling.
Methods and Results Pulmonary hemodynamics and vascular remodeling were compared in chronically hypoxic (FIO2=0.10) rats treated with 0, 1, and 10 mg·kg-1·d-1 quinapril, a potent tissue ACE inhibitor, both during and after the development of pulmonary hypertension. Quinapril reduced the development of pulmonary hypertension after 12 days of hypoxia from 26±1 to 19±1 mm Hg (P<.05). When started in established pulmonary hypertension, quinapril reduced pulmonary artery pressure and total pulmonary resistance index from 29±1 to 25±1 mm Hg and from 0.136±0.01 to 0.101±0.005 mm Hg·mL-1·min-1 per kg, respectively (P<.05). Chronically hypoxic rats showed a small pulmonary vasoconstrictor response that was not affected by quinapril. In contrast, percent medial thickness in alveolar duct blood vessels was reduced by quinapril treatment both in developing and in established pulmonary hypertension (10.0±0.2% versus 8.9±0.1% [P<.05] and 11.2±0.2% versus 9.1±0.2% [P<.05], respectively). 5'-Bromo-deoxyuridine–positive VSMCs were detected in 56±3% of hypoxic control pulmonary resistance vessels versus 41±3% of vessels after quinapril treatment (P<.05).
Conclusions Pulmonary ACE and angiotensin II contribute to the development and maintenance of hypoxic pulmonary hypertension in rats. ACE inhibition with quinapril reduces the development of hypoxic pulmonary hypertension and in part reverses established pulmonary hypertension, most likely via inhibition of pulmonary VSMC proliferation and/or growth.
Key Words: pulmonary hypertension • vascular remodeling • angiotension-converting enzyme • hypoxia
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Introduction |
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Pulmonary hypertension is a manifestation of a wide variety of cardiac and pulmonary diseases, including congestive heart failure, mitral stenosis, congenital heart disease, primary pulmonary hypertension, and cor pulmonale. The increased pulmonary artery pressure may result from a rise in pulmonary vascular tone, hypertrophy, and hyperplasia of medial SMCs and intimal cell proliferation.1 2
Chronic hypoxia is a well-characterized experimental model of pulmonary hypertension, which is caused by polycythemia, vasoconstriction, and pulmonary vascular remodeling.3 4 Vascular remodeling is characterized by hyperplasia and hypertrophy of SMCs in small, muscularized pulmonary arteries and by the appearance of new SMCs in distal, previously nonmuscular pulmonary arteries.5 6 The mechanism governing these architectural changes is unknown, although angiotensin, among other mediators, may be involved.7 Angiotensin II may contribute to the development of chronic hypoxic pulmonary hypertension via its vasoconstrictor action or via effects on vascular SMC migration and growth.8 9 Indeed, ACE is present in very high concentrations in the lungs, and its activity is further increased by chronic hypoxia.10 11 12
The administration of early-generation ACE inhibitors, such as captopril, to patients with pulmonary hypertension has resulted in limited functional improvement and a transient vasodilatory effect on pulmonary vessels in some but not all patients.13 14 15 However, recent data demonstrated that the RAS is not only a circulating hormonal system but also a tissue system, widespread in cardiovascular organs, which has been implicated in vascular remodeling accompanying various cardiovascular diseases.16 More recently developed ACE inhibitors have a higher potency of binding to and neutralizing tissue ACE, which may confer a theoretical advantage in targeting remodeling processes. Among these ACE inhibitors, quinapril is particularly potent in vivo, with prolonged inhibition of tissue ACE despite clearing from the plasma.10 17 Quinapril has been extensively investigated in systemic hypertension and LV failure and has been shown to induce regression of medial hypertrophy in systemic blood vessels and reverse LV hypertrophy in systemic hypertension and congestive heart failure. The beneficial effect of quinapril on vessel wall structure and compliance results from preload and afterload reduction and from inhibition of ACE activity in the vessel wall.18 19 Inhibition of tissue ACE is the major site of action for quinapril and is more important than inhibition of plasma ACE in mediating the systemic antihypertensive effect of the drug.20 Similarly, quinapril inhibits neointima formation in rats after carotid artery balloon angioplasty by preventing local formation of angiotensin II, a potent smooth muscle mitogen.21 In vitro experiments in rat heart also indicate that quinapril acts directly on the early growth response of myocardial proteins.22
Therefore, in the present study we examined the effect of quinapril on the development of hypoxia-induced pulmonary hypertension and cardiovascular remodeling in rats (prevention protocol), as well as its ability to intervene with established pulmonary hypertension and ongoing remodeling (intervention protocol).
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Methods |
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Experimental Animals
All animal experiments were conducted according to institutional guidelines for experimental animal procedures. Pulmonary and systemic hemodynamic measurements were recorded in male Wistar rats (300 to 400 g) under three different conditions: (1) baseline normoxia, (2) acute hypoxia (inspiratory oxygen fraction [FIO2]=0.10 for 25 minutes), and (3) chronic hypoxia (FIO2=0.10 for 12 and 24 days with all pressure readings recorded 60 minutes after return to room air). For experiments in acute hypoxia, animals were anesthetized with an intraperitoneal injection of 50 mg/kg pentobarbital, orally intubated with a polyethylene cannula, and mechanically ventilated with a tidal volume of 7 mL/kg and respiratory rate of 60/min with 10% O2. In six additional animals (three normoxic and three chronically hypoxic rats), we cannulated the carotid artery for arterial blood gas and pH analyses (blood gas system 288, Ciba Corning) during these ventilatory settings. Chronically hypoxic rats were exposed to 10% O2 in a normobaric Lucite chamber. Oxygen concentration was monitored continuously and maintained at 10% by controlling the inflow rates through ventilation with room air (2.5 L/min) balanced with N2 (2.5 L/min) through the chamber (mixed with the use of a fan). The chamber was opened for 20 minutes daily to clean the cages, replenish food and water, and deliver the drugs via gastric intubation (1 mL/kg water or quinapril solution [1 or 10 mg/mL]). Normoxic control rats were kept under identical conditions in room air.
Experimental Protocol
Effect of Quinapril on Development of Hypoxic Pulmonary Hypertension (Prevention Protocol)
Rats were randomly assigned to one of two normoxic or hypoxic groups. The normoxic groups were sham treated (NC; n=6) or received quinapril 1 mg·kg-1·d-1 for 12 days (NQ; n=6). Similarly, hypoxic groups were either sham treated (HC; n=12) or received 1 mg·kg-1·d-1 quinapril for 12 days (HQ; n=12). After 12 days, animals were removed from hypoxia for hemodynamic evaluation and morphometric analysis as described below.
Effect of Quinapril on Established Hypoxic Pulmonary Hypertension (Intervention Protocol)
Rats were randomized after 12 days of hypoxia to HC (n=8) or HQ (n=8) and kept in hypoxia until day 24. Quinapril-treated rats received a dosage of 1 mg·kg-1·d-1 (n=8), which was subsequently increased to 10 mg·kg-1·d-1 (n=8) in a different group to ensure maximal pharmacological effect. After 24 days, hemodynamic and histological parameters were studied and compared with those of rats exposed to 12 days of hypoxia (n=4).
Effect of Quinapril on Acute Hypoxic Pulmonary Vasoconstriction
The vasodilatory action of quinapril on pulmonary resistance vessels during acute hypoxia was studied in normoxic and chronically hypoxic rats. Groups of four normoxic or hypoxic rats were sham treated or received 10 mg·kg-1·d-1 quinapril for 8 days. All animals were then subjected to a 25-minute acute hypoxic challenge, and pulmonary artery pressure and total pulmonary resistance were compared with values for respective control rats.
Effect of Quinapril on Pulmonary Vascular SMC Proliferation
The effect of quinapril on cell proliferation within the remodeling vessel wall was examined using BrdU labeling. To evaluate the effect of quinapril on SMC proliferation during chronic hypoxia, BrdU incorporation was studied at two different time points (4 and 8 days) during hypoxia in control and quinapril-treated rats. Rats were injected intraperitoneally with BrdU (50 mg/kg) at daily intervals starting 3 days before they were killed. The animals received a final injection at 2 hours before they were killed. These time periods were selected because previous studies during chronic hypoxia have shown that newly muscularized peripheral, intra-acinar arteries are first apparent at day 3 and that BrdU labeling index in these vessels is maximal after 1 week of hypoxia.23
Hemodynamic Measurements
After administration of anesthesia to the animals, pulmonary artery pressure was measured with a Silastic catheter (0.30 mm ID, 0.64 mm OD) introduced via the right jugular vein, right atrium, and RV into the pulmonary artery. Systemic arterial pressure was measured with a 2F catheter positioned in the right carotid artery. Both arterial catheters were connected to a pressure transducer positioned at the midthoracic level (model AA 016, Baxter) and linked to a display oscilloscope (Press Ampl 863, Siemens) and a thermal recorder (Mingograf 82, Siemens). The position of the pulmonary artery catheter was verified by the characteristic pressure tracing on the oscilloscope and was confirmed at autopsy. Electronically averaged pulmonary artery pressures and RV systolic pressures were recorded.
Cardiac output was measured according to the thermodilution technique as previously described.24 Briefly, a 1.5F thermodilution probe was inserted in the thoracic aorta via the right carotid artery and connected to a thermal dilution computer (model REF-1, Edwards) and a strip-chart recorder. For cardiac output measurements, 0.15 mL of saline was injected through the Silastic pulmonary artery catheter. All values were measured in triplicate and varied by <15%. CI (mL·min-1·kg-1) was defined as the ratio of cardiac output to body weight in kilograms. TPRI (mm Hg·min-1·mL-1 per kg) was computed by dividing MPAP by CI. After completion of hemodynamic measurements, a blood sample was collected by cardiac puncture and centrifuged in microcapillary tubes for 5 minutes to provide hematocrit readings.
Histological and Immmunohistochemical Examinations
The animals were killed with an overdose of sodium pentobarbital. The heart and lungs were excised, and the heart was separated from the lungs at the large vessels. The RV free wall was removed from the LV+S according to a modification of the procedure of Fulton et al,25 and both were weighed after being dried at 90°C for 72 hours. The ratios of the dry weight of the RV to the LV+S (RV/LV+S) and to FBW (RV/FBW) were calculated. Lungs were fixed in 4% buffered formaldehyde according to the tracheal insufflation technique after the pulmonary vessels were perfused with saline at 80 cm H2O pressure. Hematoxylin and eosin and elastin stains were prepared from paraffin-embedded 7-µm sections from the lower lobes of both lungs. Architectural changes were examined through two different methods. First, to assess peripheral pulmonary arteries alone and avoid pulmonary venules, only vessels that were landmarked to alveolar ducts and that contained distinct internal and external elastic laminae were considered. For measurements of medial wall thickness, all vessels adjacent to alveolar ducts with a diameter between 25 and 100 µm were considered. Percent medial WT was calculated as the ratio of WT to WDx100 (%WT/WD). The WD (distance between the external elastic laminae measured as the sum of the longest and the shortest distance divided by 2) and the medial WT (greatest distance between the internal and external elastic laminae) were measured with an ocular micrometer. Second, all small vessels with a definite elastic coat adjacent to alveolar ducts were counted, and the proportion of those with a double elastic lamina (TWPVs) was calculated and recorded as a percentage of the total peripheral vessels. A double elastic lamina was assumed when two laminae with a space between them were visible for at least half of the diameter in cross section.3 All slides were coded and read independently by two investigators, and the code was broken after all measurements had been obtained. Thirty to 40 alveolar duct vessels were measured per slide; to obtain percent TWPVs, 150 to 200 intra-acinar vessels were counted per slide.
For BrdU immunostaining, 2-mm transverse sections at the hilar level of the fixed lungs were taken, transferred to 20% sucrose in 0.07 mol/L sodium phosphate buffer at 4°C for 24 hours, and frozen on dry ice. Immunostaining using a mouse monoclonal antibody specific for BrdU was performed on 7-µm cryosections. Sections were exposed to normal rabbit serum to block nonspecific sites and incubated with a 1:200 dilution of the monoclonal antibody to BrdU (Sigma). Sections were then incubated with biotinylated rabbit antibodies to mouse immunoglobulin (dilution 1:100) followed by streptavidin-conjugated horseradish peroxidase (ABC kit, DAKO). Antibody binding was visualized with the use of diaminobenzidine. Sections were counterstained with hematoxylin and examined with the use of light microscopy. Immunopositive cells were quantified according to the method of Meyrick and Mcgavran for 3H-thymidine labeling.23 26 First, the number of vessels with a diameter of <100 µm with at least one BrdU-positive medial SMC was determined and expressed as percent of the total number of vessels (percentage positive vessels). For every experimental condition, eight sections were examined, and the number of vessels per section ranged from 100 to 160. Second, the number of positive cells in all alveolar duct vessels (50 to 80 per section) was determined, and the ratio of BrdU-positive cells per vessel was calculated.
Statistical Analysis
All values are given as mean±SEM. ANOVA and subsequent multiple comparison through the Fisher test were used to determine differences between the groups. Paired Student's t tests were used as appropriate. Significance in all cases was defined as two-sided P<.05.
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Results |
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Animal body weights were similar at the end of the 12- and 24-day experimental protocols in control and quinapril-treated animals, suggesting that the drug did not affect feeding or growth; hypoxic animals lost
20% body weight after 12 days, with no further change after 24 days of hypoxia (Table
s 1 and 2). Hematocrit values significantly increased in all hypoxic animals compared with normoxic controls but were not affected by quinapril (43±1% in NC and 43±2% in NQ to 64±1% in HC and 66±2% in HQ after 12 days, with no further rise after 24 days). Systolic blood pressure was not affected by 1 mg·kg-1·d-1 quinapril (164±3 mm Hg in NC, 162±3 in NQ, 163±4 in HC, and 162±4 in HQ) but significantly decreased after 10 mg·kg-1·d-1 quinapril (Table 2).
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Effect of Quinapril on Development of Hypoxic Pulmonary Hypertension (Prevention Protocol)
Baseline MPAP and CI measured in room air were similar in control and quinapril-treated rats (13±1 versus 12±1 mm Hg and 215±2 versus 203±4 mL·min-1·kg-1, respectively). After 12 days of hypoxia, pulmonary hypertension developed in both hypoxic groups, but the rise in pulmonary artery pressure was significantly lower in quinapril-treated rats (26±1 versus 19±1 mm Hg, P<.05, Fig 1). CI was higher in quinapril-treated than in control animals (246±9 versus 218±10 mL·min-1·kg-1, P=NS). Chronic hypoxia increased RV weight compared with LV+S weight or with FBW (Table 1
). The hypoxia-induced RV hypertrophy was significantly reduced by quinapril treatment as reflected by the reduction in RV weight, regardless of whether normalized to FBW. RV weight compared with LV+S weight was not significantly reduced after quinapril, possibly due to a lower LV+S weight in HQ (Table 1).
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Histological analysis of pulmonary alveolar duct vessels demonstrated a significant increase in medial WT and in percent TWPVs with chronic hypoxia. Quinapril had no effect on either parameter during normoxia (5.3±0.1% and 18±3% in NC versus 5.5±0.1% and 19±2% in NQ, P=NS), but it significantly reduced the increase in medial WT during chronic hypoxia (10.1±0.2% in HC versus 8.9±0.1% in HQ, P<.05), and there was also a trend toward a smaller percent muscularized pulmonary alveolar vessels (Table 1).
Effect of Quinapril on Established Pulmonary Hypertension (Intervention Protocol)
After 24 days of hypoxia, there was relatively little further increase in MPAP compared with values obtained after 12 days (26±1 mm Hg at 12 days versus 29±1 mm Hg at 24 days, P=NS; Fig 2A). The rise in MPAP during the second half of the chronic hypoxic exposure contributed 18% of the total increase over the 24 days of hypoxia. Total pulmonary artery resistance index further increased from 0.119±0.005 mm Hg·mL-1·min-1 per kg at 12 days of hypoxia to 0.136±0.010 mm Hg·mL-1·min-1 per kg at 24 days, P=NS (Fig 2B). RV weight, regardless of whether compared with LV+S weight, was significantly higher after 24 days of hypoxia than after 12 days of hypoxia. The rise in RV and RV/(LV+S) during the second half of the chronic hypoxic exposure was smaller than that during the first 12 days and contributed to 27% and 26% of the total increase, respectively. The percent WT of pulmonary resistance vessels further increased during the second 12 days of hypoxia from 10.1±0.2% to 11.2±0.2% at 24 days (P<.05).
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Quinapril treatment with 1 mg·kg-1·d-1 from day 13 to 24 did not affect pulmonary hemodynamics, RV hypertrophy, or pulmonary vascular remodeling. In contrast, treatment with the higher dose of 10 mg·kg-1·d-1 from day 13 to 24 reduced MPAP (29±1 mm Hg in HC to 25±1 mm Hg in HQ, P<.05; Fig 2A) and increased CI from 232±7 to 269±12 mL·min-1·kg-1 (P=NS). The combined effect resulted in a significant reduction of total pulmonary resistance index (0.136±0.01 in HC versus 0.101±0.005 mm Hg·mL-1·min-1 per kg in HQ, P<.05; Fig 2B). The percent medial WT and the percent muscularized pulmonary alveolar vessels were significantly lower in quinapril-treated rats than in HC rats (9.1±0.18% and 83±2% in HQ versus 11.2±0.2% and 94±2% in HC, P<.05, respectively), but RV hypertrophy was not lower (Table 2). The lower systemic blood pressure after 10 mg·kg-1·d-1 quinapril did not affect LV+S weight (Table 2).
Effect of Quinapril on Acute Hypoxic Vasoconstriction
MPAP was measured in room air before and after mechanical ventilation and 5, 15, and 25 minutes after ventilation with 10% O2. There was no difference in MPAP between NC and NQ. Hypoxic pulmonary vasoconstriction was measured as the highest MPAP recorded during the 25-minute acute hypoxic exposure and expressed as percent of baseline MPAP during mechanical ventilation. In animals maintained in room air, quinapril significantly reduced the degree of pulmonary artery pressure rise during acute hypoxia (from 54±3% to 31±5%), whereas in pulmonary vessels that had been remodeled after 8 days of hypoxia, the acute hypoxic vasoconstrictor response was virtually lost and no longer affected by quinapril (Fig 3). In both quinapril-treated and control rats, CI significantly and similarly decreased during acute hypoxia regardless of whether the pulmonary vessels were remodeled with prior hypoxic exposure (216±4 to 170±4 mL·min-1·kg-1 in NC, 207±6 to 164±5 mL·min-1·kg-1 in NQ, 215±3 to 178±7 mL·min-1·kg-1 in 8-day HC, and 220±6 to 182±3 mL·min-1·kg-1 in 8-day HQ). Blood gas values measured during mechanical ventilation with room air were normal both in normoxic rats and in rats previously exposed to chronic hypoxia (PO2, 96±3 mm Hg; PCO2, 39±2 mm Hg; pH, 7.40±0.02), excluding hypocapnia-induced changes in hypoxic vasoreactivity.
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mPAP, percent of baseline pressure) during acute hypoxic vasoconstriction in room air–maintained control rats and at the end of an 8-day (8D) hypoxic exposure. In normoxic rats, quinapril (10 mg·kg-1·d-1) reduced the degree of acute hypoxic vasoconstriction by 43% compared with control rats. After 8 days of chronic hypoxia, vasoreactivity was significantly reduced and was no longer affected by quinapril (10 mg·kg-1·d-1). *P<.05 vs normoxic control; **P<.05 vs normoxic control and normoxic quinapril-treated animals.
Immunohistochemical Analysis
In normoxic lungs, there was no difference between the number of BrdU-positive cells in control and quinapril-treated animals at the two time points studied (4 and 8 days). The results for both time points were therefore combined and averaged. Few BrdU-positive SMCs were observed in these vessels, which is consistent with the small number of proliferating cells in the media of the vessel wall during baseline conditions. Positive cells were seen in only 8±1% of the vessels, which amounted to 0.1±0.01 positive cell per vessel. After 4 days of hypoxia, BrdU-positive cells were detected in 56±3% of resistance vessels, with 2.5±0.1 positive cells per vessel, and this number was essentially unchanged after 8 days of hypoxia (55±3% BrdU-positive vessels, with 2.3±0.1 positive cells per vessel). A significant decrease in the number of BrdU-positive vessels was observed in quinapril-treated animals (Fig 4). The effect of quinapril was sustained, as reflected by the lower number of BrdU-positive vessels at both 4 and 8 days of hypoxia (41±3% of the vessels and 1.8±0.1 cells per vessel at 4 days versus 43±3% of the vessels and 1.5±0.1 cells per vessel at 8 days of hypoxia, P<.05 versus normoxia).
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Discussion |
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The present data demonstrate that administration of quinapril, an ACE inhibitor with potent inhibition of tissue ACE, not only decreases the development of chronic hypoxic pulmonary hypertension but also reduces the pulmonary artery pressure and the degree of vascular remodeling in established pulmonary hypertension. The beneficial effect of quinapril in chronic hypoxia is not accounted for by the vasodilatory action of the drug but results at least in part from an antiproliferative effect on medial vascular SMCs as indicated by the significantly reduced BrdU labeling index in hypoxic pulmonary vessels.
In chronic hypoxia, the rise in pulmonary artery pressure was most pronounced during the first week of hypoxia and reached a plateau after 12 days, with only a small further increase during prolonged hypoxia until 24 days. The pulmonary hypertension in chronic hypoxia is accounted for by vasoconstriction, vascular remodeling, and polycythemia with increased blood viscosity, an important determinant of the increased pulmonary vascular resistance.5 6 24 We found no differences in hematocrit between quinapril-treated and control rats, suggesting that quinapril most probably influenced hypoxic pulmonary hypertension through an effect on vasomotor tone or on vascular remodeling.
Previous studies on ACE inhibitors in chronic pulmonary hypertension have mainly focused on the inhibition of hypoxic vasoconstriction and have shown conflicting results depending on the experimental model and the pharmacological compound studied.27 28 29 30 31 Angiotensin II had no role in acute alveolar hypoxic vasoconstriction of the dog as administration of saralasin acetate, a competitive inhibitor of angiotensin II, was unable to diminish alveolar hypoxic vasoconstriction.31 In contrast, studies in rats have suggested that the RAS is involved in eliciting hypoxic vasoconstriction.29 We therefore wondered whether quinapril affected hypoxic pulmonary hypertension in rats via a vasodilatory action on pulmonary vessels, and we studied the hypoxic vasoconstrictor response in animals after the pulmonary vessels had been remodeled by 8 days of chronic hypoxia. Although quinapril reduced hypoxic vasoconstriction in rats kept in room air, it failed to do so in chronic hypoxic animals. Indeed, in the latter group with structurally remodeled pulmonary vessels, the acute hypoxic vasoconstrictor response was almost absent. It is, therefore, unlikely that quinapril affects hypoxic vascular remodeling through its vasodilatory properties. This is consistent with the findings of Stanbrook et al32 that complete inhibition of hypoxic pulmonary vasoconstriction by the calcium channel blocker nifedipine does not reduce pulmonary hypertension and vascular remodeling.
Elevated downstream pressures in the LV, left atrium, or pulmonary veins may theoretically have affected pulmonary artery pressure and pulmonary vascular resistance during hypoxia. The favorable effects of quinapril on the pulmonary circulation could then result from the unloading of the drug from the LV. This is also unlikely, however, because only moderate blood pressure changes occurred even with the higher quinapril dosage (10 mg·kg-1·d-1) and because end-diastolic LV pressures during hypoxia did not change (data not shown).
All pulmonary hemodynamic measurements were recorded after 1-hour equilibration in room air to eliminate confounding vasomotor changes. Therefore, the sustained elevation in pulmonary artery pressure in chronic hypoxia results, for the most part, from architectural changes in the pulmonary vascular bed as reflected by a significant increase in medial WT and in percent TWPV. The significant reduction in pulmonary artery pressure observed with quinapril, therefore, suggested an effect on one of the components of this remodeling process. In the first (prevention) protocol, hypoxic rats receiving quinapril had significantly less pulmonary arterial medial hypertrophy and RV hypertrophy than did hypoxic control animals. These structural findings are in agreement with previous pathological studies in rats exposed to hypobaric hypoxia, which showed prevention of pulmonary vascular changes by inhibition of ACE.33 Our study extends these earlier observations by including hemodynamic data and by exploring potential mechanisms of the effect of ACE inhibitors in this condition. Clozel et al34 recently observed complete prevention of hypoxia-induced pulmonary vascular remodeling using cilazapril, a potent ACE inhibitor with a long duration of action, although they reported higher pulmonary artery pressures with the ACE inhibitor. The reason for this difference is unclear.
In the second (intervention) protocol, we found that quinapril could successfully intervene with established pulmonary hypertension and ongoing vascular remodeling. Indeed, our data show that the chronically hypoxic rats treated with quinapril during the last 12 days of their 24-day hypoxic exposure had significantly reduced pulmonary hypertension and pulmonary arterial medial hypertrophy, yet a similar degree of RV hypertrophy. Either RV weight is not sufficiently sensitive enough to reflect moderate changes in hemodynamics after only a relatively short period of time or, perhaps, quinapril acts preferentially on SMCs and not on myocytes. Similar observations have been reported with cilazapril in established hypoxic pulmonary hypertension,35 although the mechanism was unknown. By including BrdU incorporation studies, the present study extends our understanding of the role of angiotensin II in the remodeling process.
Indeed, few quantitative data are available on the relative contribution of SMC hyperplasia and/or hypertrophy to hypoxia-induced pulmonary vascular remodeling. SMC proliferation results in medial wall thickening of already muscular vessels and in peripheral extension of SMCs in previously nonmuscular vessels.1 The appearance of new SMCs was clearly shown by increased mitotic activity of SMCs after 4 days of hypoxia. In hypoxic quinapril-treated rats, the drug significantly reduced the degree of hypoxia-induced SMC proliferation as reflected by the smaller number of BrdU-positive peripheral small vessels and by the smaller number of BrdU-labeled cells per vessel. These immunohistochemical data suggest that decreased muscularization of pulmonary arterioles and reduced medial wall thickness of pulmonary resistance vessels after quinapril are probably mediated through inhibition of SMC proliferation, although an effect on SMC hypertrophy cannot be excluded. Indeed, on a cellular level, quinapril may inhibit pulmonary vascular SMC growth via multiple pathways. Angiotensin II induces both hypertrophic and hyperplastic growth of vascular SMCs via increased expression of proto-oncogenes and autocrine growth factors.8 36 37 38 39 ACE inhibition with quinapril completely blocks conversion of angiotensin I to angiotensin II across all peripheral vascular beds40 and increases bradykinin levels, which may participate in the pulmonary vascular remodeling process.9 The mechanism by which bradykinin inhibits growth is less clear but may involve induction of vascular nitric oxide synthase and release of nitric oxide with its potent cGMP-dependent growth-inhibitory effects.41 42 Finally, quinapril directly affects cardiac myocyte growth through inhibition of protein synthesis via suppression of the early growth response-1 gene expression,22 and a similar direct effect on vascular SMC growth cannot be excluded.
The observed beneficial effects of quinapril in hypoxic rats warrant further studies in patients with pulmonary hypertension, for whom medical treatment is often still unsatisfactory.
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Acknowledgments |
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This work was supported in part by a research grant from the Belgian Society for Cardiology (Dr Janssens), the National Fund for Scientific Research (NFWO) from Belgium (Dr Janssens), and the Warner Lambert Corporation.
Received October 16, 1995; revision received April 25, 1996; accepted May 1, 1996.
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