kn-93 and Death--Sudden--Cardiac

Diastolic spontaneous Ca(2+) waves (DCWs) are recognized as important contributors to triggered arrhythmias. DCWs are thought to arise when [Ca(2+)] in sarcoplasmic reticulum ([Ca(2+)](SR)) reaches a certain threshold level, which might be reduced in cardiac disease as a consequence of sensitization of ryanodine receptors (RyR2s) to luminal Ca(2+).. We investigated the mechanisms of DCW generation in myocytes from normal and diseased hearts, using a canine model of post-myocardial infarction ventricular fibrillation (VF).. The frequency of DCWs, recorded during periodic pacing in the presence of a β-adrenergic receptor agonist isoproterenol, was significantly higher in VF myocytes than in normal controls. Rather than occurring immediately on reaching a final [Ca(2+)](SR), DCWs arose with a distinct time delay after attaining steady [Ca(2+)](SR) in both experimental groups. Although the rate of [Ca(2+)](SR) recovery after the SR Ca(2+) release was similar between the groups, in VF myocytes the latency to DCWs was shorter, and the [Ca(2+)](SR) at DCW initiation was lower. The restitution of depolarization-induced Ca(2+) transients, assessed by a 2-pulse protocol, was significantly faster in VF myocytes than in controls. The VF-related alterations in myocyte Ca(2+) cycling were mimicked by the RyR2 agonist, caffeine. The reducing agent, mercaptopropionylglycine, or the CaMKII inhibitor, KN93, decreased DCW frequency and normalized restitution of Ca(2+) release in VF myocytes.. The attainment of a certain threshold [Ca(2+)](SR) is not sufficient for the generation of DCWs. Postrelease Ca(2+) signaling refractoriness critically influences the occurrence of DCWs. Shortened Ca(2+) signaling refractoriness due to RyR2 phosphorylation and oxidation is responsible for the increased rate of DCWs observed in VF myocytes and could provide a substrate for synchronization of arrhythmogenic events at the tissue level in hearts prone to VF.

Topics: Adrenergic beta-Agonists; Animals; Benzylamines; Caffeine; Calcium Channel Agonists; Calcium Signaling; Calcium-Calmodulin-Dependent Protein Kinase Type 2; Cardiac Pacing, Artificial; Death, Sudden, Cardiac; Disease Models, Animal; Dogs; Excitation Contraction Coupling; Female; Isoproterenol; Male; Myocardial Infarction; Myocytes, Cardiac; Oxidation-Reduction; Phosphorylation; Protein Kinase Inhibitors; Reaction Time; Reducing Agents; Ryanodine Receptor Calcium Release Channel; Sarcoplasmic Reticulum; Sulfonamides; Time Factors; Tiopronin; Ventricular Fibrillation

Activation of cellular Ca2+ signaling molecules appears to be a fundamental step in the progression of cardiomyopathy and arrhythmias. Myocardial overexpression of the constitutively active Ca2+-dependent phosphatase calcineurin (CAN) causes severe cardiomyopathy marked by left ventricular (LV) dysfunction, arrhythmias, and increased mortality rate, but CAN antagonist drugs primarily reduce hypertrophy without improving LV function or risk of death.. We found that activity and expression of a second Ca2+-activated signaling molecule, calmodulin kinase II (CaMKII), were increased in hearts from CAN transgenic mice and that CaMKII-inhibitory drugs improved LV function and suppressed arrhythmias. We devised a genetic approach to "clamp" CaMKII activity in CAN mice to control levels by interbreeding CAN transgenic mice with mice expressing a specific CaMKII inhibitor in cardiomyocytes. We developed transgenic control mice by interbreeding CAN transgenic mice with mice expressing an inactive version of the CaMKII-inhibitory peptide. CAN mice with CaMKII inhibition had reduced risk of death and increased LV and ventricular myocyte function and were less susceptible to arrhythmias. CaMKII inhibition did not reduce transgenic overexpression of CAN or expression of endogenous CaMKII protein or significantly reduce most measures of cardiac hypertrophy.. CaMKII is a downstream signal in CAN cardiomyopathy, and increased CaMKII activity contributes to cardiac dysfunction, arrhythmia susceptibility, and longevity during CAN overexpression.

Topics: Amino Acid Sequence; Animals; Animals, Newborn; Apoptosis; Arrhythmias, Cardiac; Benzylamines; Calcineurin; Calcium Signaling; Calcium-Calmodulin-Dependent Protein Kinase Type 2; Calcium-Calmodulin-Dependent Protein Kinases; Cells, Cultured; Death, Sudden, Cardiac; Disease Models, Animal; Enzyme Induction; Hypertrophy, Left Ventricular; Isoproterenol; Mice; Mice, Transgenic; Molecular Sequence Data; Myocardial Contraction; Myocytes, Cardiac; Peptide Fragments; Rats; RNA, Messenger; Sulfonamides; Ventricular Dysfunction, Left