Atrial fibrillation (AF), the most common human cardiac arrhythmia, is associated with abnormal intracellular Ca 2+ handling. Diastolic Ca 2+ release from the sarcoplasmic reticulum via "leaky" ryanodine receptors (RyR2s) is hypothesized to contribute to arrhythmogenesis in AF, but the molecular mechanisms are incompletely understood. Here, we have shown that mice with a genetic gain-of-function defect in Ryr2 (which we termed Ryr2 R176Q/+ mice) did not exhibit spontaneous AF but that rapid atrial pacing unmasked an increased vulnerability to AF in these mice compared with wild-type mice.

BACKGROUND-Although defective Ca 2+ homeostasis may contribute to arrhythmogenesis in atrial fibrillation (AF), the underlying molecular mechanisms remain poorly understood. Studies in patients with AF revealed that impaired diastolic closure of SR Ca 2+ -release channels (ryanodine receptors, RyR2) is associated with reduced levels of the RyR2-inhibitory subunit FKBP12.6.

Cardiomyocytes adapt to physical stress by increasing their size while maintaining cell function. The serine/threonine kinase Akt plays a critical role in this process of adaptation. We previously reported that transgenic overexpression of an active form of Akt (Akt-E40K) in mice results in increased cardiac contractility and cell size, as well as improved sarcoplasmic reticulum (SR) Ca 2؉ handling. Because it is not fully elucidated, we decided to study the molecular mechanism by which Akt-E40K overexpression improves SR Ca 2؉ handling. To this end, SR Ca 2؉ uptake and the phosphorylation status of phospholamban (PLN) were evaluated in heart extracts from wild-type and Akt-E40K mice and mice harboring inducible and cardiac specific knock-out of phosphatidylinositol-dependent kinase-1, the upstream activator of Akt. Moreover, the effect of Akt was assessed in vitro by overexpressing a mutant Akt targeted preferentially to the SR, and by biochemical assays to evaluate potential interaction with PLN. We found that when activated, Akt interacts with and phosphorylates PLN at Thr Furthermore, overexpression of SR-targeted Akt in cardiomyocytes improved Ca2؉ handling without affecting cell size. Thus, we describe here a new mechanism whereby the preferential translocation of Akt to the SR is responsible for enhancement of contractility without stimulation of hypertrophy.Akt, also referred to as protein kinase B, is a serine/threonine kinase found as part of the insulin, insulin-like growth factor-1 (IGF-1) 4 /phosphatidylinositol 3-kinase (PI3K)/phosphatidylinositol-dependent kinase-1 (PDK1) pathway (1). Upon activation, Akt phosphorylates a broad range of substrates involved in metabolism, transcription, translation, cell growth, differentiation, proliferation, and survival (2, 3). In the heart the IGF-1/ Akt axis is implicated in the control of physiological cardiac hypertrophy, contractile function, and Ca 2ϩ handling (4 -11). The cardiac effects of Akt have been described in many transgenic (Tg) mouse models, and cardiac specific overexpression of different mutated forms of Akt results in varying phenotypes (5,(12)(13)(14)(15)(16). A common effect observed in most of these Tg mouse lines is increased cardiomyocyte (CM) size and either maintained or improved cardiac function (5,(12)(13)(14), with modest or no activation of either mitogen-activated protein kinases (MAPKs) or fetal genes, two hallmarks of maladaptation to stress (5, 12, 13). Our group has previously described a Tg mouse model (Akt-E40K Tg) expressing an Akt with a mutation (E40K) located in its pleckstrin homology domain. This mutation renders Akt constitutively active in a manner similar to stimulation with growth factor (5). The occurrence of increased CM size with preservation of cardiac contractility constitutes an intriguing aspect of this mouse model. Single cell studies revealed not only increased inotropism but also enhanced lusitropism and an increase in systolic calcium (Ca 2ϩ ) transients in adult CMs isolated from this Tg mouse (5, 15)....

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Calcium release from intracellular stores plays an important role in the regulation of muscle contraction and electrical signals that determine the heart rhythm. The ryanodine receptor (RyR) is the major calcium (Ca 2+ ) release channel required for excitation-contraction coupling in the heart. Recent studies have demonstrated that RyR are macromolecular complexes comprising of 4 pore-forming channel subunits, each of which is associated with regulatory subunits. Clinical and experimental studies over the past 5 years have provided compelling evidence that intracellular Ca 2+ release channels play a pivotal role in the development of cardiac arrhythmias and heart failure. Changes in the channel regulation and subunit composition are believed to cause diastolic calcium leakage from the sarcoplasmic reticulum, which could trigger arrhythmias and weaken cardiac contractility. Therefore, cardiac RyR have emerged as potential therapeutic targets for the treatment of heart disease. Consequently, there is a strong desire to identify and/or develop novel pharmacological agents that may target these Ca 2+ signaling pathways. Pharmacological agents known to modulate RyR in the heart, and their potential application towards the treatment of heart disease are discussed in this review.