The pulmonary circulation is intimately coupled with RV function in health and disease. RV function is the most important determinant of survival in patients with PAH.14, 17
Despite the significant advances in our understanding and clinical practice in PAH over the past decades, RVF remains the common fatal pathway and consequence of PAH. However, our understanding of RVF is limited due to a paucity of research in this area.
In this study, we used an established, MCT-induced PAH model to study the structure-function relationship of T-tubules and intracellular Ca2+ in RVF. The major findings are as follows: 1) MCT-induced RVF manifested with marked RV hypertrophy, RV dilation and systolic dysfunction, blunted and dyssynchronous Ca2+ transients, severe T-tubule loss and disorganization, and alterations in levels of Ca2+ handling proteins; 2) sildenafil administered at an early stage prevented the development of PAH and RV hypertrophy and failure, preserved normal T-tubule ultrastructure, normal Ca2+ transients and expression of key Ca2+ handling proteins; 3) when given at a delayed stage (with established RV hypertrophy and failure), sildenafil reversed MCT-induced PAH but not RV hypertrophy, and partially restored RV contractile function, RV myocyte Ca2+ transients, T-tubule ultrastructure and Ca2+ handling proteins; 4) sildenafil treatment corrected MCT-induced impairment of LV filling; and 5) sildenafil did not acutely affect RV contractile function in live animals and intracellular Ca2+ handling in isolated RV myocytes of MCT-induced PAH-RVF rats. Our data provide evidence that T-tubule ultrastructural remodeling is reversible upon therapeutic treatment, independent of cardiac hypertrophy remodeling.
Our recent data suggested that (LV) T-tubule remodeling may represent a key mechanism underlying the transition from hypertrophy to (LV) heart failure.4
Severe T-tubule loss leads to a significant reduction or redistribution in Cav1.2 Ca2+
channel, increase in orphaned RyR2s, reduction in coupling efficacy between Ca2+
channels on T-tubule membrane and RyR2s on SR membrane, and therefore reduction in Ca2+
transient amplitude and synchrony.2, 3, 18
In this study, we proposed that RV myocyte T-tubule disruption during PAH-induced RV remodeling is an important factor in the development of RVF. On the other hand, improvement of the RV myocyte T-tubule ultrastructure was anticipated to increase RV contractile function. Consistently, our Ca2+
handling and echocardiographic results did support this notion. Future studies to further establish the temporal relationship between T-tubule disruption and changes in Ca2+
handling in MCT-injected rats, e.g., in the early stage, may provide a better mechanistic understanding of T-tubule remodeling and Ca2+
handling dysfunction in heart disease.
Sildenafil was recently approved for use in patients with PAH. The beneficial effects of sildenadil or other PDE5 inhibitors in PAH are thought to result from relatively selective vasodilatory and antiproliferative effects on the pulmonary vasculature.19
Our data reveal a potent effect of sildenafil on pulmonary vascular remodeling, consisted with the literature.20
A recent work from Michelakis group provided elegant evidence that PDE5 is increased in hypertrophied human RV myocardium, and acute inhibition of PDE5 improves RV myocyte contractility, suggesting a direct, acute effect of PDE5 inhibition on RV myocyte function in disease.19
However, our data showed that acute exposure of sildenafil did not increase contractile function of both ventricles (Figure S2 & S3
) nor did it enhance SR Ca2+
release function in RV myocytes (Figure S4
) from MCT-treated rats. Moreover, we showed that early treatment with sildenafil maintained normal pulmonary artery resistance and prevented afterload increase-induced RV structural (hypertrophy, wall thickness, chamber dimension) and ultrastructural (T-tubule) remodeling as well as functional alterations. In addition, our data from late treatment group (SilMCT/D23
) revealed a potential benefit of a PDE5 inhibitor on RV function: improving or reversing RV myocyte T-tubule remodeling and therefore Ca2+
handling in “advanced” disease. This represents a heretofore unappreciated mechanism of sildenafil as a therapeutic for PAH-RVF disease. Finally, we believe that benefit of PDE5 inhibitor therapy for PAH and RV dysfunction is mainly through its chronic effects on decreasing RV afterload and ensuing RV structural and functional improvement.
Sildenafil is effective in improving PAH-related RV dysfunction; however, it is still debating whether sildenafil exerts its therapeutic effects via its action on the pulmonary vasculature (e.g., afterload unloading) or by a direct anti-hypertrophic effect on RV remodeling. Kass and colleagues have published a number of studies suggesting that sildenafil prevents, arrests, and even reverses LV hypertrophy, fibrosis, and dilatation in mice subjected to LV pressure overload induced by transverse aortic constriction.21, 22
Recently, two independent groups 23, 24
examined whether sildenafil provides similar, direct protection against RV remodeling as shown in LV by the Kass group. Surprisingly, they both provided evidence that sildenafil does not prevent, but even exacerbates RV hypertrophy in a pre-established RV hypertrophy model induced by pulmonary trunk artery banding, indicating that sildenafil prevents myocardial remodeling in PAH mainly through an indirect action via RV unloading. The discrepant effects of sildenafil on pressure overload-induced LV vs. RV hypertrophy could possibly be due to different mechanisms involved in the development of hypertrophy in the RV and LV. It is being increasingly recognized that, in addition to important differences in gene expression, embryology, and physiology, the RV and LV may have divergent responses to stress, including activation of different signaling cascades.10, 25
The beneficial effects of sildenafil on T-tubule remodeling, particularly in the setting of un-improved RV hypertrophy, suggest that unloading of RV afterload (due to sildenafil-induced normalization of PAP) may play an important role. Increasing evidence demonstrates the occurrence of reversal remodeling in failing hearts after mechanical unloading (e.g., in patients after left ventricle assisted device (LVAD) surgery or lung transplantation). LVAD has been associated with improved cardiac function and a broad spectrum of reverse remodeling events in LV myocytes (e.g., changes in morphology and at the cellular, biochemical, molecular, and transcriptional levels).26–28
Our finding of reverse remodeling of the T-tubule ultrastructure reflects a new layer of cardiovascular benefits upon mechanical unloading in sildenafil-treated PAH-RVH rats. However, mechanical unloading is not always beneficial. Unwanted chronic mechanical unloading may lead to damage of the T-tubule system and defects in Ca2+
In patients with idiopathic PAH, the clinical course is determined by progressive loss of cross-sectional area in the pulmonary arterial tree, and by RVF resulting from increased afterload. Sildenafil was introduced into the treatment armamentarium for PAH by virtue of its known vasodilator properties. Here in this study, we elucidated the mechanisms of RVF in a PAH model and uncovered a new mechanism by which sildenafil provides therapeutic benefit in PAH. Specifically, RV myocytes rapidly develop maladaptive, ultrastructural remodeling and EC coupling dysfunction in response to an increase in pulmonary arterial resistance, leading to RV contractile failure during PAH. Early intervention with sildenafil prevents PAH and preserves RV structure and function at the (sub)cellular and whole-organ levels. Late intervention promotes partially reverse structural and functional remodeling in RV failing myocytes. The prevention and repair of maladaptive changes to RV T-tubules appears to be an important benefit of sildenafil in PAH. Although therapeutic effects were observed when sildenafil was introduced late in the disease course, it appears that early intervention is required to prevent pathological cellular and ultrastructural remodeling of the RV – a finding with important translational implications.