Finally our experiments shown that depletion of intracellula

Finally, our experiments shown that depletion of intracellular Ca stores after [cAMP]i elevation is not high and/or fast enough to activate a measurable entry of extracellular Ca through SOCE. In good agreement, Tosun et al. [52] reported that elevated [Ca]c due to store depletion is dissociated from contraction in rat aorta.
Conclusion Our results suggest that a rise of [cAMP]i increases basal [Ca]c in vascular smooth muscle cells by depletion of intracellular thapsigargin-sensitive Ca stores. This effect reduces the amount of Ca ready to be released from intracellular stores via activation of PKA and Epac. Also, Epac and PKA participate in the inhibition of SOCE by cAMP. Both effects may explain, at least in part, the cAMP endothelium-independent vasorelaxant effects.
Disclosures
Acknowledgements This work was supported in part by grants from the Ministerio de Ciencia e Innovación, Spain (SAF2010-22051) and Xunta de Galicia, Spain (INCITE08PXIB203092PR).
Introduction Ca is a key element in cardiac excitation–contraction (EC) coupling. In each heartbeat, membrane depolarization during an Medroxyprogesterone activates L-type Ca channels located in the sarcolemma. Ca entry through these channels activates intracellular Ca release channels, named ryanodine receptors (RyRs), which are located in the membrane of the sarcoplasmic reticulum (SR). RyRs amplify the initial Ca signal via Ca-induced Ca release (CICR), providing enough Ca to activate contractile myofibrils. Relaxation then occurs when intracellular Ca concentration ([Ca]i) returns to diastolic values, due mainly to Ca pumped back into the SR by the Ca-ATPase (SERCA) and extrusion from the cell via Na+/Ca exchange (NCX) [1]. While Ca in EC coupling is physiologically and pathophysiologically relevant, new roles for cardiac myocyte Ca are being elucidated. For instance, prohypertrophic signaling seems to be activated by perinuclear activation of Ca/calmodulin dependent protein kinase II (CaMKII) promoted by local elevation of nuclear [Ca] ([Ca]n) [2]. By analogy to EC coupling, this process has been named excitation–transcription (ET) coupling. However, it is still not fully understood how [Ca]n variations may be dissociated from bulk [Ca]i oscillations during contraction–relaxation cycles. We recently showed that Epac (exchange protein directly activated by cAMP) could activate SR Ca release in ventricular myocytes, via a CaMKII-dependent phosphorylation of the RyR [3], and we hypothesized that Epac-dependent Ca signaling may also be implicated in cardiac hypertrophy [4], [5]. Epac induces cardiomyocyte hypertrophy, both in cultured neonatal ventricular myocytes [4] and in adult myocytes [5]. Epac is a guanylyl exchange protein (GEF) [6], [7] widely distributed in the organism, including the heart, whose functional roles are just beginning to be defined [8], [9]. The hypertrophic effects of Epac are independent of the classical effector of cAMP, protein kinase A (PKA), but rather involve the CaMKII [5]. In addition, Epac expression is increased in experimental animal models of cardiac hypertrophy [10] and contributes to the hypertrophic effect of β-adrenergic receptor [5]. CaMKII is also known to activate nuclear export of class II histone deacetylases (e.g. HDAC4 and 5) [2], [11], an effect which derepresses myocyte enhancer factor 2 (MEF2) driven transcription, and contributes to hypertrophic remodeling.