The repolarization phase of cardiac action potential is prone to aberrant excitation that is common in cardiac patients. Here, we demonstrate that this phase is markedly sensitive to Ca 2؉ because of the surprising existence of a Ca 2؉ -activated K ؉ currents in cardiac cells. The current was revealed using recording conditions that preserved endogenous Ca 2؉ buffers. The Ca 2؉ -activated K ؉ current is expressed differentially in atria compared with ventricles. Because of the significant contribution of the current toward membrane repolarization in cardiac myocytes, alterations of the current magnitude precipitate abnormal action potential profiles. We confirmed the presence of a small conductance Ca 2؉ -activated K ؉ channel subtype (SK2) in human and mouse cardiac myocytes using Western blot analysis and reverse transcription-polymerase chain reaction and have cloned SK2 channels from human atria, mouse atria, and ventricles. Because of the marked differential expression of SK2 channels in the heart, specific ligands for Ca 2؉ -activated K ؉ currents may offer a unique therapeutic opportunity to modify atrial cells without interfering with ventricular myocytes.Cardiac action potentials (APs) 1 are shaped predominantly by the interplay between transient inward Na ϩ , Ca 2ϩ , and outward K ϩ currents (1). While the repolarization phase of the AP can be wrought by the kinetics of the principal currents, small and sustained outward currents also define this phase, rendering this region prone to irregular membrane excitation.In humans, delineation of the outward currents that confer the late repolarization phase of the cardiac AP is crucial for our understanding of the etiology of arrhythmias. We provide a novel report that demonstrates that the repolarization phase of cardiac AP shows marked sensitivity toward apamin, an exclusive ligand for a small conductance Ca 2ϩ -activated K ϩ channel (2).Ca 2ϩ -activated K ϩ channels (K Ca ) are present in most neurons and mediate the afterhyperpolarizations following AP (3, 4). However, functional significance of K Ca in the heart has not previously been documented. K Ca channels can be divided into three main subfamilies (3, 5-7). These include the large-conductance Ca 2ϩ -and voltage-activated K ϩ channels (BK), the intermediate-conductance K Ca channels (IK), and the smallconductance K Ca channels (SK), which are sensitive to apamin and scyllatoxin. Among the SK channels, they are encoded by at least three genes, SK1, SK2, SK3 (4, 6), with differential sensitivity toward apamin. SK2 is highly sensitive to apamin, with a half-blocking concentration (IC 50 ) of 60 pmol/liter, whereas SK1 channels are not affected by 100 nmol/liter apamin (2). SK3 channels are intermediate.Here, we report for the first time, the presence of I K,Ca (Ca 2ϩ -activated K ϩ current) in cardiac myocytes that plays a crucial role in cardiac AP profile. Using a combination of electrophysiological recordings and biochemical and molecular biological techniques, we have identified the presence of SK2...

Membrane current abnormalities have been described in human heart failure. To determine whether similar current changes are observed in a large animal model of heart failure, we studied dogs with pacing-induced cardiomyopathy. Myocytes isolated from the midmyocardium of 13 dogs with heart failure induced by 3 to 4 weeks of rapid ventricular pacing and from 16 nonpaced control dogs did not differ in cell surface area or resting membrane potential. Nevertheless, action potential duration (APD) was significantly prolonged in myocytes isolated from failing ventricles (APD at 90% repolarization, 1097 +/- 73 milliseconds [failing hearts, n = 30] versus 842 +/- 56 milliseconds [control hearts, n = 25]; P < .05), and the prominent repolarizing notch in phase 1 was dramatically attenuated. Basal L-type Ca2+ current and whole-cell Na+ current did not differ in cells from failing and from control hearts, but significant differences in K+ currents were observed. The density of the inward rectifier K+ current (IKl) was reduced in cells from failing hearts at test potentials below -90 mV (at -150 mV, -19.1 +/- 2.2 pA/pF [failing hearts, n = 18] versus -32.2 +/- 5.1 pA/pF [control hearts, n = 15]; P < .05). The small outward current component of IKl was also reduced in cells from failing hearts (at -60 mV, 1.7 +/- 0.2 pA/pF [failing hearts] versus 2.5 +/- 0.2 pA/pF [control hearts]; P < .05). The peak of the Ca(2+)-independent transient outward current (Ito) was dramatically reduced in myocytes isolated from failing hearts compared with nonfailing control hearts (at +80 mV, 7.0 +/- 0.9 pA/pF [failing hearts, n = 20] versus 20.4 +/- 3.2 pA/pF [control hearts, n = 15]; P < .001), while the steady state component was unchanged. There were no significant differences in Ito kinetics or single-channel conductance. A reduction in the number of functional Ito channels was demonstrated by nonstationary fluctuation analysis (0.4 +/- 0.03 channels per square micrometer [failing hearts, n = 5] versus 1.2 +/- 0.1 channels per square micrometer [control hearts, n = 3]; P < .001). Pharmacological reduction of Ito by 4-aminopyridine in control myocytes decreased the notch amplitude and prolonged the APD. Current clamp-release experiments in which current was injected for 8 milliseconds to reproduce the notch sufficed to shorten the APD significantly in cells from failing hearts. These data support the hypothesis that downregulation of Ito in pacing-induced heart failure is at least partially responsible for the action potential prolongation. Because the repolarization abnormalities mimic those in cells isolated from failing human ventricular myocardium, canine pacing-induced cardiomyopathy may provide insights into the development of repolarization abnormalities and the mechanisms of sudden death in patients with heart failure.

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