In the heart, Ca2+ is a key element in excitation-contraction coupling and a pivotal second messangerin trascriptional regulation involved in cardiomyocyte growth . cGMP/PKG signaling is involved in Ca2+ cycling since it stimulates phospholamban and L-type calcium channels (LTCC) phosphorylation .cGMP levels are modulated by members of the phosphodiesterases (PDEs) superfamily . In the heart, the most of cGMP concentrations are hydrolyzed by phosphodiesterase 5A (PDE5A).
In our lab was demonstrated that inhibition of PDE5A with Sildenafil attenuates contraction rate and calcium transients, following cAMP generation induced by ßAR agonists, in neonatal cardiomyocytes.
The involvement of PDE5A in [Ca2+]i regulation in adult heart was not clarified, to this we take advantage of the PDE5A knockout mouse model (Pde5A-/- ) generated by our research group.
The aim of this project was to investigate the involvement of PDE5A in Ca2+ regulation in cardiomyocyte contraction and signaling pathways Ca2+-dependent activation.
In our preliminary results we observed that adult cardiomyocytes isolated from Pde5A-/- mice display reduction of Ca2+ transients amplitude and shortening. The expression of LTCC, sarcoplasmic reticulum Ca2+-ATPase (SERCA2a) pump and Ryanodin Receptors Ser2808 phosphorylation was reduced. Furthemore, Calsequestrin was increased in Pde5A-/- mice hearts, that may explain a lower availability of [Ca2+]ifor an effective contraction. In addition, Pde5A-/- mice exhibit a significant reduction in the cardiomyocytes size compared to cardiomyocytes from Pde5A+/+ mice. The analysis of the main Ca2+-dependent pathways involved in cardiac growth showed a reduction of CaMKII pathway activation in hearts from Pde5A-/- mice.
Taken together these data suggest that PDE5A may play a role in the modulation of [Ca2+]i and an alteration of Ca2+-dependent cardiomyocytes growth pathways could be responsible of the defective growth of cardiomyocytes in Pde5A-/- mice.
In the last years, the cardioprotective effect of PDE5A inhibition was studied both in human and mouse models. PDE5A inhibitors, such as Sildenafil, showed their efficacy to ameliorates cardiac performance during a cardiomyopathy (Kukreja R.C. et al., 2012, J Am Coll Cardiol; Pokreisz P., et al. 2009, Circ.; Nagayama T., et al. 2009, J Mol Cell Cardiol; Giannetta E. et al., 2012, Circ). Until now, several hypothesis was proposed about the cardioprotective molecular mechanism of Sildenafil, but these hypothesis are still under study.
The role of PDE5A has been studied mainly through its inhibition in cardiomyopathies Takimoto E., et el. 2005, Nat Med; Tsai E.J. and Kass D.A. 2009, Pharm Ther),, but its role in physiological conditions is still unclear.
From the data obtained until now, PDE5A would seem involved in the Ca2+ regulation that controls cardiomyocytes contraction and activation of Ca2+-regulated pathways.
Pde5A knockout mouse models is a excellent model for studying the role of PDE5A in non-pathological conditions.
In the present study has been investigated, for the first time, the impact of the absence of PDE5A on Ca2+ regulation in cardiomyocytes.
These results will lead further clarification on the involvement of PDE5A and its target pathway cGMP/PKG in basal conditions.