HDC and Arrhythmia: On the other hand, addition of isoprenaline (a β-adrenoceptor agonist) or histamine to the isolated mouse hearts led in nearly all experiments to cardiac arrhythmias (He et al. 2012): When these authors used isolated perfused hearts from HDC-KO animals (from mice with global histidine decarboxylase knockout, where no histamine could be formed because HDC is the pace-making enzyme for histamine formation in vivo), they detected far less, if any, arrhythmias in these isolated perfused mouse hearts after reperfusion (He et al. 2012).