Isolation of primary guinea pig ventricular (GPV) cardiomyocytes

GPV myocytes were isolated as described previously (Pelzmann et al., 2003). Briefly, guinea pigs were killed, hearts were quickly removed, mounted on a Langendorff apparatus and the coronary system was perfused with buffer (composition in mM: NaCl 126, KCl 4.7, KH₂PO₄ 1.2, MgSO₄ 2.5, NaHCO₃ 2.49, HEPES/Na⁺ 0.5 and D(+)‐glucose 5.45, pH 7.4 adjusted with NaOH) containing 2 mM pyruvic acid, 1 mg·mL⁻¹ bovine serum albumin and 100 IU·mL⁻¹ collagenase (Worthington CLS 2) at 37°C (Scheruebel et al., 2014). After enzymatic digestion, cardiomyocytes were isolated from ventricles and Ca²⁺ concentration was raised stepwise to 1.8 mM. Cells were transferred to cell culture medium (M199) containing 50 IU·mL⁻¹ penicillin and 50 μg·mL⁻¹ streptomycin and maintained at 37°C under 5% CO₂. All experiments were performed on the day after isolation.