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Department of Pharmacology, Graduate School of Medical Sciences, Kyushu University, Fukuoka 812-8582, Japan
We have investigated the effects of hypotonic stress on intracellular calcium concentration ([Ca2+]i) in bovine aortic endothelial cells. Reducing extracellular osmolarity by 5% to 40% elicited a steep Ca2+ transient both in normal Krebs and Ca2+-free solutions. The hypotonic stress-induced Ca2+ transient was inhibited by phospholipase C inhibitors (neomycin and U-73122), a P2-receptor antagonist (suramin), and an ATP-hydrolyzing enzyme (apyrase), suggesting that the hypotonic stress-induced Ca2+ transient is mediated by ATP. A luciferin-luciferase assay confirmed that 40% hypotonic stress released 91.0 amol/cell of ATP in 10 min. When the hypotonic stress-induced fast Ca2+ transient was inhibited by neomycin, suramin, or apyrase, a gradual [Ca2+]i increase was observed instead. This hypotonic stress-induced gradual [Ca2+]i increase was inhibited by a phospholipase A2 inhibitor, 4-bromophenacyl bromide. Furthermore, exogenously applied arachidonic acid induced a gradual [Ca2+]i increase with an ED50 of 13.3 µM. These observations indicate that hypotonic stress induces a dual Ca2+ response in bovine aortic endothelial cells, i.e., an ATP-mediated fast Ca2+ transient and an arachidonic acid-mediated gradual Ca2+ increase, the former being the predominant response in normal conditions.
mechanical stress; adenosine 5'-triphosphate; arachidonic acid.
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