Sustained cardiac hypertrophy represents one of the most common causes leading to cardiac failure. There is emerging evidence to implicate the involvement of NF-B in the development of cardiac hypertrophy. However, several critical questions remain unanswered. We tested the use of soluble epoxide hydrolase (sEH) inhibitors as a means to enhance the biological activities of epoxyeicosatrienoic acids (EETs) to treat cardiac hypertrophy. sEH catalyzes the conversion of EETs to form the corresponding dihydroxyeicosatrienoic acids. Previous data have suggested that EETs may inhibit the activation of NF-B-mediated gene transcription. We directly demonstrate the beneficial effects of several potent sEH inhibitors (sEHIs) in cardiac hypertrophy. Specifically, we show that sEHIs can prevent the development of cardiac hypertrophy using a murine model of pressureinduced cardiac hypertrophy. In addition, sEHIs reverse the preestablished cardiac hypertrophy caused by chronic pressure overload. We further demonstrate that these compounds potently block the NF-B activation in cardiac myocytes. Moreover, by using in vivo electrophysiologic recordings, our study shows a beneficial effect of the compounds in the prevention of cardiac arrhythmias that occur in association with cardiac hypertrophy. We conclude that the use of sEHIs to increase the level of the endogenous lipid epoxides such as EETs may represent a viable and completely unexplored avenue to reduce cardiac hypertrophy by blocking NF-B activation.epoxyeicosatrienoic acids ͉ NF-B

Small conductance Ca2+ -activated K + channels (SK channels) have been reported in excitable cells, where they aid in integrating changes in intracellular Ca 2+ (Ca 2+ i ) with membrane potential. We have recently reported the functional existence of SK2 channels in human and mouse cardiac myocytes. Moreover, we have found that the channel is predominantly expressed in atria compared to the ventricular myocytes. We hypothesize that knockout of SK2 channels may be sufficient to disrupt the intricate balance of the inward and outward currents during repolarization in atrial myocytes. We further predict that knockout of SK2 channels may predispose the atria to tachy-arrhythmias due to the fact that the late phase of the cardiac action potential is highly susceptible to aberrant excitation. We take advantage of a mouse model with genetic knockout of the SK2 channel gene. In vivo and in vitro electrophysiological studies were performed to probe the functional roles of SK2 channels in the heart. Whole-cell patch-clamp techniques show a significant prolongation of the action potential duration prominently in late cardiac repolarization in atrial myocytes from the heterozygous and homozygous null mutant animals. Morover, in vivo electrophysiological recordings show inducible atrial fibrillation in the null mutant mice but not wild-type animals. No ventricular arrhythmias are detected in the null mutant mice or wild-type animals. In summary, our data support the important functional roles of SK2 channels in cardiac repolarization in atrial myocytes. Genetic knockout of the SK2 channels results in the delay in cardiac repolarization and atrial arrhythmias.

Small-conductance Ca2+-activated K+ channels (SK channels, KCa channels) have been reported in excitable cells, where they aid in integrating changes in intracellular Ca2+ with membrane potential. We recently reported for the first time the functional existence of SK2 (KCa2.2) channels in human and mouse cardiac myocytes. Here, we report cloning of SK1 (KCa2.1) and SK3 (KCa2.3) channels from mouse atria and ventricles using RT-PCR. Full-length transcripts and their variants were detected for both SK1 and SK3 channels. Variants of mouse SK1 channel (mSK1) differ mainly in the COOH-terminal structure, affecting a portion of the sixth transmembrane segment (S6) and the calmodulin binding domain (CaMBD). Mouse SK3 channel (mSK3) differs not only in the number of polyglutamine repeats in the NH2 terminus but also in the intervening sequences between the polyglutamine repeats. Full-length cardiac mSK1 and mSK3 show 99 and 91% nucleotide identity with those of mouse colon SK1 and SK3, respectively. Quantification of SK1, SK2, and SK3 transcripts between atria and ventricles was performed using real-time quantitative RT-PCR from single, isolated cardiomyocytes. SK1 transcript was found to be more abundant in atria compared with ventricles, similar to the previously reported finding for SK2 channel. In contrast, SK3 showed similar levels of expression in atria and ventricles. Together, our data are the first to indicate the presence of the three different isoforms of SK channels in heart and the differential expression of SK1 and SK2 in mouse atria and ventricles. Because of the marked differential expression of SK channel isoforms in heart, specific ligands for Ca2+-activated K+ currents may offer a unique therapeutic opportunity to modify atrial cells without interfering with ventricular myocytes.

Triclosan (TCS), a high-production-volume chemical used as a bactericide in personal care products, is a priority pollutant of growing concern to human and environmental health. TCS is capable of altering the activity of type 1 ryanodine receptor (RyR1), but its potential to influence physiological excitation–contraction coupling (ECC) and muscle function has not been investigated. Here, we report that TCS impairs ECC of both cardiac and skeletal muscle in vitro and in vivo. TCS acutely depresses hemodynamics and grip strength in mice at doses ≥12.5 mg/kg i.p., and a concentration ≥0.52 μM in water compromises swimming performance in larval fathead minnow. In isolated ventricular cardiomyocytes, skeletal myotubes, and adult flexor digitorum brevis fibers TCS depresses electrically evoked ECC within ∼10–20 min. In myotubes, nanomolar to low micromolar TCS initially potentiates electrically evoked Ca 2+ transients followed by complete failure of ECC, independent of Ca 2+ store depletion or block of RyR1 channels. TCS also completely blocks excitation-coupled Ca 2+ entry. Voltage clamp experiments showed that TCS partially inhibits L-type Ca 2+ currents of cardiac and skeletal muscle, and [ 3 H]PN200 binding to skeletal membranes is noncompetitively inhibited by TCS in the same concentration range that enhances [ 3 H]ryanodine binding. TCS potently impairs orthograde and retrograde signaling between L-type Ca 2+ and RyR channels in skeletal muscle, and L-type Ca 2+ entry in cardiac muscle, revealing a mechanism by which TCS weakens cardiac and skeletal muscle contractility in a manner that may negatively impact muscle health, especially in susceptible populations.

Sarcolipin is a novel regulator of cardiac sarcoplasmic reticulum Ca 2؉ ATPase 2a (SERCA2a) and is expressed abundantly in atria. In this study we investigated the physiological significance of sarcolipin in the heart by generating a mouse model deficient for sarcolipin. The sarcolipin-null mice do not show any developmental abnormalities or any cardiac pathology. The absence of sarcolipin does not modify the expression level of other Ca 2؉ handling proteins, in particular phospholamban, and its phosphorylation status. Calcium uptake studies revealed that, in the atria, ablation of sarcolipin resulted in an increase in the affinity of the SERCA pump for Ca 2؉ and the maximum velocity of Ca 2؉ uptake rates. An important finding is that ablation of sarcolipin resulted in an increase in atrial Ca 2؉ transient amplitudes, and this resulted in enhanced atrial contractility. Furthermore, atria from sarcolipinnull mice showed a blunted response to isoproterenol stimulation, implicating sarcolipin as a mediator of ␤-adrenergic responses in atria. Our study documented that sarcolipin is a key regulator of SERCA2a in atria. Importantly, our data demonstrate the existence of distinct modulators for the SERCA pump in the atria and ventricles.atria ͉ calcium uptake ͉ sarcoplasmic reticulum Ca 2ϩ ATPase 2 S arcolipin (SLN), a low-molecular-weight protein (31 aa), is expressed in both cardiac and skeletal muscles (1-5). It colocalizes with sarcoplasmic reticulum (SR) Ca 2ϩ ATPase (SERCA) in the cardiac SR (3) and physically interacts with the SERCA pump (6). Amino acid composition and structural analysis have suggested that SLN and phospholamban (PLB) may belong to the same family of proteins with similar functions (1,(7)(8)(9). Consistent with the notion, in vitro studies have shown that SLN can inhibit the SERCA activity by decreasing the apparent Ca 2ϩ affinity of the pump (7, 10). Protein expression analyses have demonstrated that within the heart there are chamber-specific differences in the expression pattern of SLN and PLB (4). SLN is predominantly expressed in the atrial compartment, whereas PLB is abundant in the ventricles. In addition to atria, SLN is expressed in skeletal muscle tissues (4). SLN expression is regulated during cardiac and skeletal muscle development (2-4). Furthermore, SLN expression levels are altered in atria during cardiac pathology both in animal models (2, 4, 11-13) and in humans (14), suggesting that SLN levels may play an important role in maintaining atrial Ca 2ϩ homeostasis during cardiac pathophysiology.The importance of SLN as a regulator of the cardiac SERCA pump was recently demonstrated by using adenoviral gene transfer into adult rat ventricular myocytes (3) and transgenic overexpression of SLN in the heart (15-17). These studies suggest that overexpression of SLN into ventricular myocytes resulted in decreased rates of SR Ca 2ϩ uptake, Ca 2ϩ transient amplitude, and myocyte contractility. Overexpression of SLN in the PLB-null heart revealed that SLN can inhibit SERCA pump activity in...