Bradykinin regulates the expression of claudin‐5 in brain microvascular endothelial cells via calcium‐induced calcium release

Abstract

To investigate the mechanism underlying the regulation of claudin-5, a tight junction protein that participates primarily in the constitution of the blood-brain barrier by bradykinin (BK), we established a primary culture of rat brain microvascular endothelial cells (BMECs). BMECs were treated with 10(-5) M BK, and changes in the intracellular Ca(2+) levels were measured by using the sensitive fluorescent dye fluo-3; the expression and distribution of claudin-5 were investigated by immunocytochemistry and Western blot analyses. We did not detect any expression of bradykinin B2 receptors in the BMECs or freshly isolated rat brain microvessels. We found that 10(-5) M BK triggered Ca(2+) transients in BMECs, and further investigations revealed that inositol 1,4,5-trisphosphate receptors (IP3 Rs) and ryanodine receptors (RyRs) on the endoplasmic reticulum (ER) were responsible for the Ca(2+) fluctuation. Consequently, these intracellular Ca(2+) changes that occur in response to BK application were identified as Ca(2+) -induced Ca(2+) release (CICR). Immunocytochemistry and Western blot results demonstrated that 10(-5) M BK could cause the internalization and a decrease in the expression of claudin-5; agonists of IP3 Rs and RyRs, such as IP3 and caffeine, enhanced the BK-induced downregulation of claudin-5, whereas antagonists of IP3 Rs and RyRs, such as 2-APB and ryanodine, abrogated BK's effect on claudin-5. In conclusion, the BK-induced CICR in primary culture BMECs might be the mechanism by which BK modulates claudin-5.