Li L, Niederer SA, Idigo W, Zhang YH, Swietach P, Casadei B, Smith NP. A mathematical model of the murine ventricular myocyte: a data-driven biophysically based approach applied to mice overexpressing the canine NCX isoform. Am J Physiol Heart Circ Physiol 299: H1045-H1063, 2010. First published July 23, 2010; doi:10.1152/ajpheart.00219.2010.-Mathematical modeling of Ca 2ϩ dynamics in the heart has the potential to provide an integrated understanding of Ca 2ϩ -handling mechanisms. However, many previous published models used heterogeneous experimental data sources from a variety of animals and temperatures to characterize model parameters and motivate model equations. This methodology limits the direct comparison of these models with any particular experimental data set. To directly address this issue, in this study, we present a biophysically based model of Ca 2ϩ dynamics directly fitted to experimental data collected in left ventricular myocytes isolated from the C57BL/6 mouse, the most commonly used genetic background for genetically modified mice in studies of heart diseases. This Ca 2ϩ dynamics model was then integrated into an existing mouse cardiac electrophysiology model, which was reparameterized using experimental data recorded at consistent and physiological temperatures. The model was validated against the experimentally observed frequency response of Ca 2ϩ dynamics, action potential shape, dependence of action potential duration on cycle length, and electrical restitution. Using this framework, the implications of cardiac Na ϩ / Ca 2ϩ exchanger (NCX) overexpression in transgenic mice were investigated. These simulations showed that heterozygous overexpression of the canine cardiac NCX increases intracellular Ca 2ϩ concentration transient magnitude and sarcoplasmic reticulum Ca 2ϩ loading, in agreement with experimental observations, whereas acute overexpression of the murine cardiac NCX results in a significant loss of Ca 2ϩ from the cell and, hence, depressed sarcoplasmic reticulum Ca 2ϩ load and intracellular Ca 2ϩ concentration transient magnitude. From this analysis, we conclude that these differences are primarily due to the presence of allosteric regulation in the canine cardiac NCX, which has not been observed experimentally in the wild-type mouse heart. calcium transient; C57BL/6 mouse; Na ϩ /Ca 2ϩ exchanger THE RHYTHMIC CONTRACTION of the heart is dependent on tightly regulated intracellular events triggered in a concerted fashion by electrical stimulation. Among a number of regulatory steps, Ca 2ϩ plays, arguably, the most fundamental role in orchestrating the excitation-contraction coupling process. (34, 59). With the large amount of experimental data in the mouse now becoming available, mathematical modeling has the potential power to rationalize the data and provide an integrated understanding of the mouse heart. Previously, Bondarenko and colleagues (6) developed a biophysically based electrophysiology model of murine left ventricular (LV) myocytes. However, several limitations with t...

Background: Whether neuronal nitric-oxide synthase (nNOS) plays a role in the endothelial NOS (eNOS)-dependent negative inotropic effect of ␤ 3 -adrenergic stimulation remains to be established. Results: nNOS knock-out or inhibition leads to increased superoxide production, eNOS uncoupling, and abrogation of ␤ 3 -adrenergic responses. Conclusion: Disabling nNOS disrupts eNOS function and downstream signaling.Significance: nNOS plays a crucial role in preserving myocardial nitroso-redox balance and coupled eNOS activity.

It is believed, but not without dispute, that activation of PKB is essential to obtain cardioprotection by ischemic preconditioning (IP). Here we have investigated the role of PKB activity in ischemic myocardial injury and IP using novel specific PKB inhibitors, examined whether any effect is species-dependent and determined its location in the transduction pathway. The specific PKB inhibitors VIII (0.05, 0.5 and 5mM) and XI (0.1, 1 and 10mM) were co-incubated with rat ventricular myocardium for 20min prior to 90min ischemia/120min reoxygenation at 37 C (n¼6/group). CK release and cell necrosis and apoptosis (% of nuclei) were significantly decreased by more than 60% at all concentrations of both inhibitors. Similar protection was obtained with IP, results that were unaffected by PKB inhibitors. The PI-3K inhibitors LY294002 (10mM) and wortmanin (0.1mM) administered for 20min prior to ischemia induced identical results to those seen with PKB inhibitors. The protection afforded by PKB inhibitor XI was unaffected by the presumed mitoK ATP channel blocker 5-HD (10mM) but was abrogated by the p38MAPK inhibitor SB203580 (10mM). Western Blot and Proteome Profiler studies confirmed a decrease in PKB phosphorylation in myocardium exposed to IP, wortmanin and PKB inhibitor XI. Studies using human myocardium also showed that both PKB inhibitor XI (1mM) and PI-3K inhibitor wortmanin (0.1mM) equally reduced CK release and cell necrosis and apoptosis. The diabetic myocardium, that could not be protected by IP or diazoxide (100mM), was however protected by PKB inhibitor XI and wortmanin, further suggesting that PKB is located beyond the mitochondria. In conclusion, inhibition of PKB activity is protective against ischemic injury of the rat and human myocardium and is as potent as IP. Importantly, PKB is downstream of the 5-HD target but upstream of p38MAPK.