## 2 Materials and Methods
### Cell Culture and Growth Curve Determination  
HL-1 atrial myocytes (Research Resource Identifier (RRID): CVCL_0303; Sigma-Aldrich Catalogue No: SCC065) were acquired from Sigma-Aldrich and were cultured as previously described [21]. To determine the growth curves of untreated control (i.e., treated with vehicle; Con) and 7 KCh-treated cells, \(5\times 10^{4}\) cells were seeded in a 12-well culture plate and incubated with vehicle (diluted DMSO) or with different concentrations of 7 KCh for the indicated periods. The 7 KCh concentrations used in the present study corresponded to those encountered under pathophysiological conditions. It is estimated that the blood 7 KCh levels of healthy volunteers range from 1 to 2 uM and are elevated 10-20 fold in heart failure (HF) patients [9]. Cardiac cells were fixed in 3.7% formaldehyde and stained with 5 ug/mL Hoechst 33342. The cell number was determined using IN Cell Analyzer 1000 (GE Healthcare Life Sciences, Chicago, IL, USA) [22]. Global Metabolite Analysis by Ultrahigh-Performance Liquid Chromatography Time-of-Flight Mass Spectrometry (UPLC-TOF-MS)  
Extraction was carried out as previously described [23; 24]. In brief, the medium was removed from the culture plates, and 80% methanol (prechilled at \(-80\)\({}^{\circ}\)C) was immediately added. Cells were scraped from the culture dish. The resulting cell suspension was vortexed and centrifuged at 14,000\(\times\)\(g\) for 15 min. It was re-extracted once more with 80% methanol at \(-\)80 \({}^{\circ}\)C. The samples were pooled and dried under nitrogen gas. They were then dissolved in 200 \(\upmu\)L 50% acetonitrile, and the supernatant was analyzed using UPLC-TOF-MS.  
An ACQUITY BEH Amide (2.1 mm \(\times\) 150 mm, particle size: 1.7 \(\upmu\)m) (Waters Corp., Milford, MA, USA) column was used for liquid chromatographic separation. Separation was maintained at a flow rate of 400 \(\upmu\)L/min and a temperature of 45 \({}^{\circ}\)C. A linear gradient of solvents was used: 0-0.1 min, 1% B; 0.1-7.0 min, 1-70% B; 7.0-7.2 min, 1% B, and 7.2-10.0 min, 1% B for re-equilibration. Solvent A was acetonitrile, solvent B was water, and both solvents contained 0.1% formic acid. Each sample was analyzed in triplicate. Mass spectrometric analysis was performed using a Waters SYNAPT G2-S HDMS TOF-MS (Waters Corp., Milford, MA, USA) operated in the positive or negative ion mode. The desolvation gas flow was set at 800 L/h at a temperature of 500 \({}^{\circ}\)C, and the source temperature was 120 \({}^{\circ}\)C. The capillary voltage and cone voltage were adjusted to 2 kV and 25 V, respectively. All data were analyzed using Progenesis QI software (Nonlinear Dynamics, Newcastle, UK). The identities of metabolites were revealed by searching METLIN [25] and Human Metabolome databases [26] or by spectral comparison with standard compounds. Quantification of Coenzymes and Sterols by Liquid Chromatography Coupled with Tandem Mass Spectrometry (LC-MS/MS)  
Intracellular reduced and oxidized coenzymes and sterols were extracted using a modified method [27]. Briefly stated, the culture medium was removed, and the cells were extracted with 1-propanol containing internal standards (Chol-de, 7 KCh-d7, coenzyme Q10-d9 (CoO10-d9)) for analysis of Chol, 7 KCh and coenzyme Qs (CoQs). For CoQ determination, samples were analyzed using UPLC coupled with Waters Xevo TQ-S MS (Waters Corp.) as previously described with slight modifications [27]. For Chol and 7 KCh determination, samples were analyzed using UPLC coupled with Waters Xevo TQ-S MS (Waters Corp., Milford, MA, USA) according to a modified method [28]. MS was performed in ESI-positive ion multiple reaction monitoring (MRM) mode.