Background: Claudin-5, claudin-9, and claudin-11 are expressed in endothelial cells to constitute tight junctions, and their deficiency may lead to hyperpermeability, which is the initiating process and pathological basis of cardiovascular disease.Although tongxinluo (TXL) has satisfactory antianginal effects, whether and how it modulates claudin-5, claudin-9, and claudin-1 1 in hypoxia-stimulated human cardiac microvascular endothelial cells (HCMECs) have not been reported.Methods: In this study, HCMECs were stimulated with CoCl2 to mimic hypoxia and treated with TXL.First, the messenger RNA (mRNA) expression of claudin-5, claudin-9, and claudin-l 1 was confirmed.Then, the protein content and distribution of claudin-9, as well as cell morphological changes were evaluated after TXL treatment.Furthermore, the distribution and content histone H3K9 acetylation (H3K9ac) in the claudin-9 gene promoter, which guarantees transcriptional activation, were examined to explore the underlying mechanism, by which TXL up-regulates claudin-9 in hypoxia-stimulated HCMECs.Results: We found that hypoxia-suppressed claudin-9 gene expression in HCMECs (F=7.244;P =0.011) and the hypoxia-suppressed claudin-9 could be reversed by TXL (F=61.911;P =0.000), which was verified by its protein content changes (F=29.142;P =0.000).Moreover, high-dose TXL promoted the cytomembrane localization of claudin-9 in hypoxia-stimulated HCMECs, with attenuation of cell injury.Furthermore, high-dose TXL elevated the hypoxia-inhibited H3K9ac in the claudin-9 gene promoter (F=37.766;P =0.000), activating claudin-9 transcription.Conclusions: The results manifested that TXL reversed the hypoxia-suppressed claudin-9 by elevating H3K9ac in its gene promoter, playing protective roles in HCMECs.