21:T54f,Histamine is a widely distributed biogenic amine involved in the regulation of an array of biological processes. Serum histamine level is markedly elevated in the early stages of acute myocardial infarction, whereas the role it plays remains unclear. Histidine decarboxylase (HDC) is the unique enzyme responsible for histamine production, and cardiac injury is significantly aggravated in HDC knockout mice (HDC^-/-), in which histamine is deficient. We also observed that autophagy was highly activated in cardiomyocytes of HDC^-/- mice post acute myocardial infarction (AMI), which was abolished by compensation of exogenous histamine. The in vivo and in vitro results showed that acting through histamine 1 receptor, histamine increased miR-206 and miR-216b, which worked in concert to target to Atg13, resulting in the reduction of autophagy activation under hypoxia and AMI condition. Further study revealed that Atg13 interacted with FADD to promote the activation of caspase-8 and cell apoptosis. Taken together, these data unveil a novel intracellular signaling pathway involved in histamine regulating myocardial autophagy and apoptosis under hypoxia and AMI condition, which might help to more comprehensively evaluate the usage of histamine receptor antagonists and to develop new therapeutic targets for myocardial infarction.22:T4b1,Histidine decarboxylase (HDC) catalyses the formation of histamine from L-histidine. Histamine is a biogenic amine involved in many physiological and pathological processes, but its role in the regeneration of skeletal muscles has not been thoroughly clarified. Here, using a murine model of hindlimb ischaemia, we show that histamine deficiency in Hdc knockout (Hdc^-/- ) mice significantly reduces blood perfusion and impairs muscle regeneration. Using Hdc-EGFP transgenic mice, we demonstrate that HDC is expressed predominately in CD11b⁺ Gr-1⁺ myeloid cells but not in skeletal muscles and endothelial cells. Large amounts of HDC-expressing CD11b⁺ myeloid cells are rapidly recruited to injured and inflamed muscles. Hdc^-/- enhances inflammatory responses and inhibits macrophage differentiation. Mechanically, we demonstrate that histamine deficiency decreases IGF-1 (insulin-like growth factor 1) levels and diminishes myoblast proliferation via H3R/PI3K/AKT-dependent signalling. These results indicate a novel role for HDC-expressing CD11b⁺ myeloid cells and histamine in myoblast proliferation and skeletal muscle regeneration.

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