library_construction_protocol: Cardiomyocytes were isolated using Langendorff perfusion from the left ventricle as previously described. Enzymatic dissociation using Langendorff perfusion was performed with 35 mL enzyme solution (type 2 collagenase 1 mg/mL [Worthington], protease type XIV 0.05 mg/mL [Sigma-Aldrich], NaCl 130 mM, KCl 5.4 mM, MgCl2 0.5 mM, NaH2PO4 0.33 mM, D-glucose 22 mM, HEPES 25 mM; pH 7.4) pre-heated to 37 °C, at a rate of 3 mL/min. To neutralize enzymatic activity, fetal bovine serum (FBS) was added to the solution to a final concentration of 0.2% (v/v). After perfusion of digestion buffer, left ventricle was cut according to each sectional region (IZ+BZ vs RZ) and cardiomyocytes were subsequently isolated. Cell suspensions were filtered through a 100-μm nylon mesh cell strainer and centrifuged at 100 g for 2 min, after which the supernatant was discarded. To prevent hypercontraction, the cardiomyocytes were resuspended in medium (NaCl 130 mM, KCl 5.4 mM, MgCl2 0.5 mM, NaH2PO4 0.33 mM, D-glucose 22 mM, HEPES 25 mM, FBS 0.2%; pH 7.4) containing a low concentration of calcium (0.1 mM). Nuclei were isolated immediately after the collection of cardiomyocytes using Minute™ Detergent-Free Single Nuclei Isolation Kit (NI-024, Invent Biotechnologies, USA) according to the manufacturer's instruction. Single-nucleus cDNA libraries were generated using 5,000 isolated nuclei by the Chromium 3′ v3 chemistry kit (10x Genomics) according to the manufacturer's instruction.