Fig. 22.1
Surgical procedure to place pulmonary artery banding. Pulmonary artery is dissected free from the aorta (a) white arrow and then a silk thread is positioned behind the pulmonary artery (b-c) white arrow marks pulmonary artery. An 18-gauge needle is placed alongside the pulmonary artery to set the degree of constriction (d) white arrow marks needle. The suture should be tied tightly around the needle, and the needle rapidly removed to produce a fixed constricted opening in the lumen equal to the diameter of the needle. Banding of the pulmonary artery can also be performed using a clip (f and g) white arrow marks pulmonary artery
Many researchers have utilized the PAB model to study the effects of mechanically induced pressure overload of RV. Olivetti and coworkers [16] performed PAB in male Wistar rats, reducing the pulmonary arterial luminal diameter by 35 % which caused a 70 % increase in the mass of the RV free wall (1.43 mm versus 0.8 in the sham-operated rats). The focus of their investigation was the RV capillary luminal volume and the number of RV mast cells. Their rationale for counting the mast cells is interesting. They were motivated by their knowledge that mast cells were implicated in tumor angiogenesis and they wondered whether the adaptive capillary response to RV pressure overload could be explained by mast cell-dependent release of heparin and subsequent heparin-mediated angiogenesis. They found a twofold increase in the RV mast cell number. Prior to these studies, the group of Anversa had characterized the changes in the RV myocardium 150 days after the banding procedure and part of the data are summarized in Table 22.1 [17]. Faber et al. [18] evaluated right and left ventricular function in PAB rats (see also Chap. 16) and found that 6 weeks of pressure overload resulted in enhanced baseline RV contractility while LV baseline contractility remained unaffected. These data are different from the data published by Piao et al. [19]. Piao, Archer, and collaborators showed that 4 weeks after PAB, the CO and treadmill distance were significantly reduced in the PAB versus control animals. Possible explanations for this discrepancy are the difference in pulmonary artery lumen reduction and the time allowed for RV adaptation, as it has been illustrated by the different degrees of RV expressed cytoplasmic proteins [20]. Bogaard et al. [14] revisited the rat PAB model and compared the measurements obtained with the SU5416/hypoxia model, a model of severe pulmonary hypertension and RVF (see below). The PAB rats were able to tolerate high RV systolic pressures for a remarkably long time and TAPSE (tricuspid annular plannar systolic excursion), a heart-rate-independent measurement of RV longitudinal contractility commonly used to evaluate RV function in patients with PAH, was similar to that of control rat RVs (3.25 mm in PAB versus 3.46 in controls). In order to explain why the RV of PAB rats was more “resilient,” microarray-based gene expression analysis of the compensated rat RVH was performed. This analysis allowed the characterization of a “RV failure transcriptional signature” [21]. Following PAB surgery, rats showed an increased expression of IGF-1 (insulin-like growth factor-1) mRNA, normal levels of phosphorylated Akt and VEGF protein levels, as well as and an increased amount of apelin (another pro-angiogenic factor) when compared to control RV tissues [21]. Another interesting feature of the expression of RV in PAB rats are the changes in the gene expression as they relate to cardiac metabolism. PAB induces an increase in the expression of genes encoding enzymes which control fatty acid oxidation and glycolysis [22]. In a recent study, it was found that the gene expression of acyl-coenzyme A dehydrogenases, a group of enzymes required for fatty acid β-oxidation, was increased in the RV from PAB versus control rats. This change in the expression of genes encoding enzymes required for fatty acid metabolism was associated with preserved citrate synthase activity and respiration of mitochondria isolated from the RV when tested for oxidative phosphorylation in vitro [23].
Table 22.1
Changes in right ventricular myocardium produced by pulmonary artery banding: absolute component volumes, length, and surface area
Parameter | SO rats | PAB rats | % difference | p< |
|---|---|---|---|---|
Volume, mm 3 | ||||
Ventricular wall | 281 ± 36 | 426 ± 82 | 52 | 0.001 |
Myocytes | 216 ± 30 | 338 ± 73 | 56 | 0.005 |
Cytoplasm | 213 ± 30 | 334 ± 73 | 57 | 0.005 |
Mitochondria | 73 ± 15 | 114 ± 27 | 56 | 0.005 |
Myofibrils | 119 ± 18 | 197 ± 45 | 66 | 0.001 |
Matrix | 22 ± 5 | 23 ± 10 | 5 | NS |
Interstitium | 64 ± 9 | 88 ± 14 | 38 | 0.005 |
Myocyte length (m) | 1,228 ± 296 | 1,252 ± 233 | 2 | NS |
Myocyte surface (mm2) | 73,743 ± 17,409 | 72,647 ± 22,870 | 1 | NS |
Sarcomere length (μm) | 2.15 ± 0.14 | 2.16 ± 0.09 | 1 | NS |
The differences between RV and LV subjected to pressure overload have also been addressed in rodents. The Stanford pediatric cardiology group performed PAB in mice and subjected RV and LV tissues to microarray gene expression analysis. Animals with moderate pulmonary stenosis had a 50 % survival of >50 days. Importantly, the right ventricular end-diastolic pressure increased 6 h postoperatively in mice with severe stenosis, but remained within the normal range in the animals with moderate outflow tract stenosis. Furthermore, the authors demonstrated a differential expression of genes between the pressure overloaded right ventricles when compared to the left counterparts. In particular, the expression of periostin was significantly different between the two models [24]. Altogether, the data suggest that PAB can be used as a model of RV pressure overload with associated adaptive hypertrophy.
Chronic Hypoxia Exposure
Similarly to PAB, chronic hypoxia alone induces significant RV hypertrophy in rats (and to a lesser degree in mice) but not RVF [2, 25]. As such, under physiological conditions, chronic hypoxia could be used to generate a model of adaptive RV hypertrophy, however if additional stressors are added or if essential genes required for RV adaptation are genetically ablated, chronic hypoxia might be sufficient to drive RV into failure. For example, Gautier et al. [26] demonstrated that inhaled carbon monoxide during hypoxic exposure is associated with fibrosis and necrosis of the RV, while carbon monoxide alone does not affect the RV tissue (Fig. 22.2). Few investigators have examined the effects of gene deletion on RV function in chronic PH. An early example was the study of heme oxygenase-1 (HO-1) knockout mice, which is, perhaps, the first study to show that there are genes which are required for the establishment of compensatory RV hypertrophy. HO-1 regulates the response to reactive oxygen species in the cell and, upon exposure to chronic hypoxia, HO-1−/− mice develop severe RV dilatation [27] (Fig. 22.3). More recently, Cruz et al. examined the effect of RV pressure overload in caveolin-1 KO mice. Caveolin-1 plays important roles in angiogenesis and cardiac hypertrophy. Similarly to the HO-1 KO mice, Cav-1−/− mice developed signs of RVF after 3 weeks of chronic hypoxia exposure [28]. The RV systolic pressure dropped (likely as the result of a reduction in cardiac output) and RV contractility was also decreased (Fig. 22.4). More importantly, the development of RVF was prevented by expressing an endothelial-specific Cav-1 transgene or by treating Cav-1−/− mice with a nitric oxide synthase inhibitor. These data suggest that, at least in Cav-1−/− mice, increased oxidative/nitrosative stress impairs the adaptive response of the RV to pressure overload, contributing to the transition from adaptive RV hypertrophy to RV failure.
Fig. 22.2
Carbon monoxide (50 ppm) alters the adaptive response of the RV to chronic hypoxia and induced ischemic lesions when carbon monoxide was applied during 3 weeks of hypoxia. Normal RV wall (a), RV necrosis indicated by the arrow (b), RV fibrosis (c), and RV tissue necrosis (d) occurring in the hypoxic plus carbon monoxide-treated group [reproduced from Gautier et al. with permission]
Fig. 22.3
Right ventricular dilatation and wall-adherent thrombus in hemeoxygenase-1 knockout (b) mice following 7 weeks of chronic hypoxia shown in comparison with the wild-type mouse RV (a). Chronic hypoxia generated a modest increase in the RV afterload in the mice. The degree of RV pressure increase was lower in the HO-1 knockout mice (c) [reproduced from Yet et al. with permission]
Fig. 22.4
The right ventricular pressure (RVSP) elevation is not maintained after 3 weeks of hypoxic exposure in caveolin-1 (Cav-1) knockout mice when compared to wild type (a) and caveolin-1 reconstituted mice. The cardiac output dropped during chronic hypoxia exposure in the caveolin knockout (Cav-1−/−) mice as well as the RV contractility index (b and c) [reproduced from Cruz et al. with permission]
Monocrotaline-Mediated Lung Injury
MCT is a macrocyclic pyrrolizidine alkaloid derived from the seeds of the Crotalaria spectabilis plant. The MCT alkaloid is metabolized in the liver to the active metabolite dehydromonocrotaline pyrrole (MCTP), a reaction that is highly dependent on CYP3A4 (cytochrome p450) [29, 30]. When administered in a dose of 60 mg/kg (usually subcutaneously), MCT induces a syndrome characterized—among other manifestations—by lethal pulmonary hypertension and RVH [31, 32]. Importantly, it has been recently reported that when administered in a dose of 40 mg/kg, which is frequently used to induce “moderate” pulmonary hypertension, the MCT syndrome is spontaneously reversible [30] MCT-induced pulmonary hypertension is one of the standard models of PAH, however, the model is problematic because liver damage and pulmonary fibrosis could contribute to the high animal mortality [29]. Indeed, rats treated with 60 mg/kg of MCT exhibit an unusually high mortality in relation to the function of the right ventricle, when compared to other models of PAH [29]. It has been demonstrated that, along with pathological vascular remodeling, MCT-treated rats exhibit a significant leukocyte infiltration of the myocardium in both left and right ventricles, a feature that is uncommon in the human pulmonary vascular diseases [29]. Furthermore, MCT-treated rats are highly prone to fatal arrhythmias [33, 34]. All together these findings have raised the question of whether the rats, in this particular model, die from pulmonary hypertension or with pulmonary hypertension. Other variations of the MCT model have been described. When combined with pneumonectomy, MCT (60 mg) causes severe PAH associated with dramatic obliteration of the pulmonary arterioles, a finding that is not normally observed with MCT alone. In this protocol rats develop substantial RVH (RV/LV + S: 0.08), but because the animals only survive for approximately 5 weeks, this short-survival limits the use of this model for preclinical studies. Furthermore, whether animals with pneumonectomy plus MCT (60 mg) develop RVF is unknown [35].
Several groups of investigators have made use of the MCT rat model of PH to assess RV function and a selection of these publications is listed in Table 22.2. However, many of the drugs that have been shown to prevent or reverse RV failure have also reduced RV afterload by affecting lung vascular remodeling. In this particular scenario, it is impossible to separate a potential direct myocardial drug effect from the effects secondary to afterload reduction. Recently researchers have demonstrated the beneficial effects of drugs on the RV without affecting the remodeled lung circulation. Bogaard et al. [36], as well as de Man et al. [37], have demonstrated that adrenergic receptor blockade with carvedilol or bisoprolol, respectively, improve RV function and delay the progression to RV failure.
Table 22.2
The investigation of right ventricular function in the monocrotaline rat model
RVH, CHF | Cardiac autonomic nerve abnormalities presynaptic vagal nerve degeneration | Sanyal et al. [44] |
RVH | ↑ plasma BNP and plasma norepinephrine ↑ RV tissue BNP and ANP | Usui et al. [45] |
RVH, RVF | ↓ TAPSE (1.3 mm) ↑ RV end-diastolic (6.4 mm) diameter | Hardziyenka et al. [46] |
RVH | ↑ wall stress | Borgdorff et al. [47] |
RVF | RV gene expression ↓ S1Rt1, PGC-1α, TFAM skeletal muscle mitochondrial dysfunction precedes RV impairment | Enache et al. [48] |
RVF | ↓ IN β1 adrenergic receptor density | Piao et al. [19] |
Treatment of RV dysfunction | ||
Simvastatin | No mortality, ↓ mPAP, ↓RVH | Nishimura et al. [49] |
Bisoprolol | Delay of time to RVF, no effect on RVSP | de Man et al. [37] |
Dehydroepiandrosterone [DHEA] | ↓ mPAP, ↓ RVH | Paulin et al. [50] |
Sildenafil | Prevention of PH and RV dysfunction | Jasinska-Stroschein et al. [51] |
Fasudil | Prevention of PH and RV dysfunction | |
Chloroquine | Prevents progression of PH | Long et al. [52] |
Exercise | Exercise ↑ RV capillarization, no change in RVSP when compared with “sedentary” MCT rats; unchanged pulmonary congestion
 Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
Get Clinical Tree app for offline access
| |