The effects of cyclic guanylate cyclase stimulation on right ventricular hypertrophy and failure alone and in combination with phosphodiesterase-5 inhibition

Physiology of the Right Ventricle Across the Lifespan

The most common cause of heart failure in the United States is ischemic left heart disease; accordingly, a vast amount of work has been done to elucidate the molecular mechanisms underlying pathologies of the left ventricle (LV) as a general model of heart failure. Until recently, little attention has been paid to the right ventricle (RV) and it has commonly been thought that the mechanical and biochemical properties of the RV are similar to those of the LV. However, therapies used to treat LV failure often fail to improve ventricular function in RV failure underscoring, the need to better understand the unique physiologic and pathophysiologic properties of the RV. Importantly, hemodynamic stresses (such as pressure overload) often underlie right heart failure further differentiating RV failure as unique from LV failure. There are significant structural, mechanical, and biochemical properties distinctive to the RV that influences its function and it is likely that adaptations of the RV occur uniquely across the lifespan. We have previously reviewed the adult RV compared to the LV but there is little known about differences in the pediatric or aged RV. Accordingly, in this mini-review, we will examine the subtle distinctions between the RV and LV that are maintained physiologically across the lifespan and will highlight significant knowledge gaps in our understanding of pediatric and aging RV. Consideration of how RV function is altered in different disease states in an age-specific manner may enable us to define RV function in health and importantly, in response to pathology.

Sex-Dependent Changes in Right Ventricular Gene Expression in Response to Pressure Overload in a Rat Model of Pulmonary Trunk Banding

Right ventricular hypertrophy (RVH) and subsequent failure are consequences of pulmonary arterial hypertension (PAH). While females are four times more likely to develop PAH, male patients have poorer survival even with treatment, suggesting a sex-dependent dimorphism in right ventricular (RV) hypertrophy/compensation. This may result from differential gene expression in the RV in male vs. female. To date, the sex dependent effect of pressure overload on RV function and changes in gene expression is still unclear. We hypothesize that pressure overload promotes gene expression changes in the RV that may contribute to a poorer outcome in males vs. females. To test this hypothesis, male and female Wistar rats underwent either a sham procedure (sham controls) or moderate pulmonary trunk banding (PTB) (a model of pressure overload induced compensated RV hypertrophy) surgery. Seven weeks post-surgery, RV function was assessed in vivo, and tissue samples were collected for gene expression using qPCR. Compared to sham controls, PTB induced significant increases in the right ventricular systolic pressure, the filling pressure and contractility, which were similar between male and female rats. PTB resulted in an increase in RVH indexes (RV weight, RV weight/tibia length and Fulton index) in both male and female groups. However, RVH indexes were significantly higher in male-PTB when compared to female-PTB rats. Whilst end of procedure body weight was greater in male rats, end of procedure pulmonary artery (PA) diameters were the same in both males and females. RV gene expression analysis revealed that the following genes were increased in PTB-male rats compared with the sham-operated controls: natriuretic peptide A (ANP) and B (BNP), as well as the markers of fibrosis; collagen type I and III. In females, only BNP was significantly increased in the RV when compared to the sham-operated female rats. Furthermore, ANP, BNP and collagen III were significantly higher in the RV from PTB-males when compared to RV from PTB-female rats. Our data suggest that pressure overload-mediated changes in gene expression in the RV from male rats may worsen RVH and increase the susceptibility of males to a poorer outcome when compared to females.

Effects of chronic treprostinil treatment on experimental right heart hypertrophy and failure

BACKGROUND

Right heart function is an important predictor of morbidity and mortality in pulmonary arterial hypertension and many CHD. We investigated whether treatment with the prostacyclin analogue treprostinil could prevent pressure overload-induced right ventricular hypertrophy and failure.

METHODS

Male Wistar rats were randomised to severe pulmonary trunk banding with a 0.5-mm banding clip (n=41), moderate pulmonary trunk banding with a 0.6-mm banding clip (n=36), or sham procedure (n=10). The banded rats were randomised to 6 weeks of treatment with a moderate dose of treprostinil (300 ng/kg/minute), a high dose of treprostinil (900 ng/kg/minute), or vehicle.

RESULTS

Pulmonary trunk banding effectively induced hypertrophy, dilatation, and decreased right ventricular function. The severely banded animals presented with decompensated heart failure with extracardial manifestations. Treatment with treprostinil neither reduced right ventricular hypertrophy nor improved right ventricular function.

CONCLUSIONS

In the pulmonary trunk banding model of pressure overload-induced right ventricular hypertrophy and failure, moderate- and high-dose treatment with treprostinil did not improve right ventricular function neither in compensated nor in decompensated right heart failure.

sGC-cGMP-PKG pathway stimulation protects the healthy but not the failing right ventricle of rats against ischemia and reperfusion injury

BACKGROUND

To investigate whether modulation of the sGC-cGMP-PKG pathway protects against ischemia and reperfusion injury in the healthy and the failing right ventricle (RV).

METHODS

Hearts from male Wistar rats with a healthy RV (n=39) or a hypertrophic and failing RV induced by pulmonary trunk banding (n=57) were isolated and perfused in a pressure-controlled modified Langendorff setup. The isolated hearts were randomized to control, ischemic preconditioning (IPC, 2×5min of global ischemia), a phosphodiesterase-5 (PDE5) inhibitor vardenafil (66nM) alone and in combination with a cGMP-dependent protein kinase (PKG) blocker KT 5823 (1μM). Failing hearts were exposed to the same protocols and to soluble guanylate cyclase stimulation/activation, and phosphodiesterase 9 inhibition. All interventions were followed by 40min of global ischemia and 120min of reperfusion. The effects on the RV were evaluated by measurement of the infarct size/area-at-risk ratio (IS/AAR).

RESULTS

In healthy hearts, IPC and pharmacological preconditioning with vardenafil reduced RV infarct size. PKG blockade by KT-5823 did not alter infarct size per se but abolished the cardioprotective effect of vardenafil. In the hypertrophic and failing hearts, none of the conditioning strategies altered RV infarct size.

CONCLUSION

PDE-5 inhibition by vardenafil protects the healthy right ventricle against ischemia and reperfusion injury by a PKG dependent mechanism. Neither ischemic preconditioning nor pharmacologic stimulation of the sGC-cGMP-PKG pathway induces cardioprotection in the hypertrophic and failing RV.

Right ventricular failure due to chronic pressure load: What have we learned in animal models since the NIH working group statement?

Right ventricular (RV) failure determines outcome in patients with pulmonary hypertension, congenital heart diseases and in left ventricular failure. In 2006, the Working Group on Cellular and Molecular Mechanisms of Right Heart Failure of the NIH advocated the development of preclinical models to study the pathophysiology and pathobiology of RV failure. In this review, we summarize the progress of research into the pathobiology of RV failure and potential therapeutic interventions. The picture emerging from this research is that RV adaptation to increased afterload is characterized by increased contractility, dilatation and hypertrophy. Clinical RV failure is associated with progressive diastolic deterioration and disturbed ventricular–arterial coupling in the presence of increased contractility. The pathobiology of the failing RV shows similarities with that of the LV and is marked by lack of adequate increase in capillary density leading to a hypoxic environment and oxidative stress and a metabolic switch from fatty acids to glucose utilization. However, RV failure also has characteristic features. So far, therapies aiming to specifically improve RV function have had limited success. The use of beta blockers and sildenafil may hold promise, but new therapies have to be developed. The use of recently developed animal models will aid in further understanding of the pathobiology of RV failure and development of new therapeutic strategies.

Inhibition of type 5 phosphodiesterase counteracts β2-adrenergic signalling in beating cardiomyocytes

AIMS

Compartmentalization of cAMP and PKA activity in cardiac muscle cells plays a key role in maintaining basal and enhanced contractility stimulated by sympathetic nerve activity. In cardiomyocytes, activation of adrenergic receptor increases cAMP production, which is countered by the hydrolytic activity of selective phosphodiesterases (PDEs). The intracellular regional dynamics of cAMP production and hydrolysis modulate downstream signals resulting in different biological responses. The interplay between beta receptors (βARs) signalling and phosphodiesterase 5 (PDE5) activity remains to be addressed.

METHODS AND RESULTS

Using combined strategies with pharmacological inhibitors and genetic deletion of PDEs and βAR isoforms, we revealed a specific pool of cAMP that is under dual regulation by PDE2 and, indirectly, PDE5 activity. Inhibition of PDE5 with sildenafil produces a cGMP-dependent activation of PDE2 that attenuates cAMP generation induced by βAR agonists, with concomitant modulation of stimulated contraction rate and calcium transients. PDE2 haploinsufficiency abolished the effects of sildenafil. The negative chronotropic effect of PDE5 inhibition through PDE2 activation was also observed in sinoatrial node tissue from adult mice. PDE5 inhibition selectively lowered contraction rate stimulated by β2AR, but not β1AR activation, supporting a compartmentalization of the cGMP-modulated pool of cAMP.

CONCLUSION

These data identify a new effect of PDE5 inhibitors on the modulation of cardiomyocyte response to adrenergic stimulation via PDE5-PDE2-mediated cross-talk.

Acute effects of levosimendan in experimental models of right ventricular hypertrophy and failure

Pulmonary arterial hypertension (PAH) is a fatal disease, and the ultimate cause of death is right ventricular (RV) failure. In this study, we investigated the acute hemodynamic effects of levosimendan in two rat models of RV hypertrophy and failure. Wistar rats were randomized to receive sham surgery (n = 8), pulmonary trunk banding (PTB; n = 8), or monocrotaline injection (MCT; n = 7). RV function was evaluated at baseline and after injection of placebo and two concentrations of levosimendan (12 and 60 μg/kg) using magnetic resonance imaging, echocardiography, and invasive pressure recordings. PTB and MCT injection caused hypertrophy, dilatation, and failure of the RV compared with sham surgery. Levosimendan increased RV end systolic pressure (sham surgery: 16.0% ± 3.8% [P = 0.0038]; MCT: 9.9% ± 3.1% [P = 0.018]; PTB: 24.5% ± 3.3% [P = 0.0001]; mean ± SEM) compared with placebo. Levosimendan markedly increased RV stroke volume (SV) in the MCT group (29.1% ± 8.3%; P = 0.012), did not change RV SV in the PTB group (0.4% ± 4.5%; P = 0.93), and decreased RV SV in the sham surgery group (-10.9% ± 3.7%; P = 0.020). Nitroprusside, which was used to mimic the systemic arterial vasodilator action of levosimendan, did not influence RV function. These data demonstrate that levosimendan acutely improves the failing right heart in a MCT model of PAH and that the mechanism involves a direct acute positive inotropic effect on the hypertrophic and failing RV of the rat.

New insights into the role of soluble guanylate cyclase in blood pressure regulation

PURPOSE OF REVIEW

Nitric oxide (NO)-soluble guanylate cyclase (sGC)-dependent signaling mechanisms have a profound effect on the regulation of blood pressure (BP). In this review, we will discuss recent findings in the field that support the importance of sGC in the development of hypertension.

RECENT FINDINGS

The importance of sGC in BP regulation was highlighted by studies using genetically modified animal models, chemical stimulators/activators and inhibitors of the NO/sGC signaling pathway, and genetic association studies in humans. Many studies further support the role of NO/sGC in vasodilation and vascular dysfunction, which is underscored by the early clinical success of synthetic sGC stimulators for the treatment of pulmonary hypertension. Recent work has uncovered more details about the structural basis of sGC activation, enabling the development of more potent and efficient modulators of sGC activity. Finally, the mechanisms involved in the modulation of sGC by signaling gases other than NO, as well as the influence of redox signaling on sGC, have been the subject of several interesting studies.

SUMMARY

sGC is fast becoming an interesting therapeutic target for the treatment of vascular dysfunction and hypertension, with novel sGC stimulating/activating compounds as promising clinical treatment options.