Aminoglycosides are toxic to sensory hair cells (HCs). Macroautophagy/autophagy is an essential and highly conserved self-digestion pathway that plays important roles in the maintenance of cellular function and viability under stress. However, the role of autophagy in aminoglycoside-induced HC injury is unknown. Here, we first found that autophagy activity was significantly increased, including enhanced autophagosome-lysosome fusion, in both cochlear HCs and HEI-OC-1 cells after neomycin or gentamicin injury, suggesting that autophagy might be correlated with aminoglycoside-induced cell death. We then used rapamycin, an autophagy activator, to increase the autophagy activity and found that the ROS levels, apoptosis, and cell death were significantly decreased after neomycin or gentamicin injury. In contrast, treatment with the autophagy inhibitor 3-methyladenine (3-MA) or knockdown of autophagy-related (ATG) proteins resulted in reduced autophagy activity and significantly increased ROS levels, apoptosis, and cell death after neomycin or gentamicin injury. Finally, after neomycin injury, the antioxidant N-acetylcysteine could successfully prevent the increased apoptosis and HC loss induced by 3-MA treatment or ATG knockdown, suggesting that autophagy protects against neomycin-induced HC damage by inhibiting oxidative stress. We also found that the dysfunctional mitochondria were not eliminated by selective autophagy (mitophagy) in HEI-OC-1 cells after neomycin treatment, suggesting that autophagy might not directly target the damaged mitochondria for degradation. This study demonstrates that moderate ROS levels can promote autophagy to recycle damaged cellular constituents and maintain cellular homeostasis, while the induction of autophagy can inhibit apoptosis and protect the HCs by suppressing ROS accumulation after aminoglycoside injury.

Sustained cardiac hypertrophy represents one of the most common causes leading to cardiac failure. There is emerging evidence to implicate the involvement of NF-κB 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-κB-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 pressure-induced 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-κB 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-κB activation.

Small-conductance Ca²⁺-activated K⁺channels (SK channels, K_Cachannels) 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 (K_Ca2.2) channels in human and mouse cardiac myocytes. Here, we report cloning of SK1 (K_Ca2.1) and SK3 (K_Ca2.3) 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 NH₂terminus 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.

Small conductance Ca²⁺‐activated K⁺ channels (SK channels) have been reported in excitable cells, where they aid in integrating changes in intracellular Ca²⁺(Ca²⁺_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.