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Heart failure is associated with sympatho-βAR (β-adrenoceptor) activation and cardiac fibrosis. Gal-3 (galectin-3) and KCa3.1 channels that are upregulated in diverse cells of diseased heart are implicated in mediating myocardial inflammation and fibrosis. It remains unclear whether Gal-3 interacts with KCa3.1 leading to cardiac fibrosis in the setting of βAR activation. We tested the effect of KCa3.1 blocker TRAM-34 on cardiac fibrosis and inflammation in cardiac-restricted β2-TG (β2AR overexpressed transgenic) mice and determined KCa3.1 expression in β2-TG×Gal-3-/- mouse hearts. Mechanisms of KCa3.1 in mediating Gal-3 induced fibroblast activation were studied ex vivo. Expression of Gal-3 and KCa3.1 was elevated in β2-TG hearts. Gal-3 gene deletion in β2-TG mice decreased KCa3.1 expression in inflammatory cells but not in fibroblasts. Treatment of β2-TG mice with TRAM-34 for 1 or 2 months significantly ameliorated cardiac inflammation and fibrosis and reduced Gal-3 level. In cultured fibroblasts, Gal-3 upregulated KCa3.1 expression and channel currents with enhanced membrane potential and Ca2+ entry through TRPV4 (transient receptor potential V4) and TRPC6 (transient receptor potential C6) channels leading to fibroblast activation. In conclusion, βAR stimulation promotes Gal-3 production that upregulates KCa3.1 channels in noncardiomyocyte cells and activates KCa3.1 channels in fibroblasts leading to hyperpolarization of membrane potential and Ca2+ entry via TRP channels. Gal-3-KCa3.1 signaling mobilizes diverse cells facilitating regional inflammation and fibroblast activation and hence myocardial fibrosis. [ABSTRACT FROM AUTHOR]