Roles of fibroblast Na^+/Ca^<2+> exchangers in fibrosis

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We studies the effect of hypoxia on the expression levels of Na^+/Ca^<2+> exchangers and transient receptor potential melastatin(TRPM2) channels in cardiac fibroblasts obtained primarily from rat hearts. Cardiac fibroblasts were cultured under hypoxic condition for 24 hours using Aneropac. When the hypoxia-exposed cells were whole-cell clamped, significant increase in membrane current was observed. The current was a non-selective cation current and was inhibited by clotrimazole, indicating that curren is likely TRPM2 current. mRNA analysis indicated increase in TRPM2 expression in hypoxia exposed fibroblasts. When siRNA was introduced in the cells, hypoxia did not increase the nonselective cation current nor TRPM2 mRNA. H_2O_2 is an activator of TRPM2. When hypoxia-exposed cells were fura-2 loaded, H_2O_2(300μM) increased intracellular Ca^<2+> concentration significantly in hypoxia-exposed fibroblasts but not in normoxia-exposed control cells. Na^+/Ca^<2+> exchanger expression was decreased by hypoxia exposure. We concluded that hypoxia increase TRPM2 expression but decrease Na^+/Ca^<2+> exchanger expression in cardiac fibroblasts. These phenomena may contribute to activation of fibroblasts and acceleration of fibrosis after hypoxia in the heart.