library_construction_protocol: Explanted rat hearts were arrested with the cardioplegic solution. Tissues (~5mg) were collected from inferior and superior sinoatrial nodefrom the left and right atria,  and left ventricles. The tissue was stored in RNAlater (Invitrogen) overnight at +4 °C and  stored at -80 °C. Total mRNA was extracted with RNeasy Micro Kit (Qiagen). RNA purity and yield were quality checked with Agilent Bioanalyzer to satisfy RIN >7 and total RNA amount 3-5 μg. TruSeq RNA Library Prep Kit (Illumina)