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). This evidence concerns the gene HDC and chronic obstructive pulmonary disease.