Transgenic (TG) Ca(2+)/calmodulin-dependent protein kinase II (CaMKII) δ(C) mice develop systolic heart failure (HF). CaMKII regulates intracellular Ca(2+) handling proteins as well as sarcolemmal Na(+) channels. We hypothesized that CaMKII also contributes to diastolic dysfunction and arrhythmias via augmentation of the late Na(+) current (late I(Na)) in early HF (8-week-old TG mice). Echocardiography revealed severe diastolic dysfunction in addition to decreased systolic ejection fraction. Premature arrhythmogenic contractions (PACs) in isolated isometrically twitching papillary muscles only occurred in TG preparations (5 vs. 0, P < 0.05) which could be completely terminated when treated with the late I(Na) inhibitor ranolazine (Ran, 5 μmol/L). Force-frequency relationships revealed significantly reduced twitch force amplitudes in TG papillary muscles. Most importantly, diastolic tension increased with raising frequencies to a greater extent in TG papillary muscles compared to WT specimen (at 10 Hz: 3.7 ± 0.4 vs. 2.5 ± 0.3 mN/mm²; P < 0.05). Addition of Ran improved diastolic dysfunction to 2.1 ± 0.2 mN/mm² (at 10 Hz; P < 0.05) without negative inotropic effects. Mechanistically, the late I(Na) was markedly elevated in myocytes isolated from TG mice and could be completely reversed by Ran. In conclusion, our results show for the first time that TG CaMKIIδ(C) overexpression induces diastolic dysfunction and arrhythmogenic triggers possibly via an enhanced late I(Na). Inhibition of elevated late I(Na) had beneficial effects on arrhythmias as well as diastolic function in papillary muscles from CaMKIIδ(C) TG mice. Thus, late I(Na) inhibition appears to be a promising option for diastolic dysfunction and arrhythmias in HF where CaMKII is found to be increased.

Chronic hemodynamic overload on the heart results in pathological myocardial hypertrophy, eventually followed by heart failure. Phosphatase calcineurin is a crucial mediator of this response. Little is known, however, about the role of calcineurin in response to acute alterations in loading conditions of the heart, where it could be mediating beneficial adaptational processes. We therefore analyzed proteome changes following a short-term increase in preload in rabbit myocardium in the absence or presence of the calcineurin inhibitor cyclosporine A. Rabbit right ventricular isolated papillary muscles were cultivated in a muscle chamber system under physiological conditions and remained either completely unloaded or were stretched to a preload of 3 mN/mm(2), while performing isotonic contractions (zero afterload). After 6 h, proteome changes were detected by two-dimensional gel electrophoresis and ESI-MS/MS. We identified 28 proteins that were upregulated by preload compared to the unloaded group (at least 1.75-fold regulation, all P < 0.05). Specifically, mechanical load upregulated a variety of enzymes involved in energy metabolism (i.e., aconitase, pyruvate kinase, fructose bisphosphate aldolase, ATP synthase alpha chain, acetyl-CoA acetyltransferase, NADH ubiquinone oxidoreductase, ubiquinol cytochrome c reductase, hydroxyacyl-CoA dehydrogenase). Cyclosporine A treatment (1 micromol/l) abolished the preload-induced upregulation of these proteins. We demonstrate for the first time that an acute increase in the myocardial preload causes upregulation of metabolic enzymes, thereby increasing the capacity of the myocardium to generate ATP production. This short-term adaptation to enhanced mechanical load appears to critically depend on calcineurin phosphatase activity.