BRADYKININ STIMULATES CA2+ RELEASE FROM INOSITOL 1,4,5-TRISPHOSPHATE-SENSITIVE POOL, WHICH IS INSUFFICIENT FOR PROSTAGLANDIN E(2) RELEASE IN HUMAN GINGIVAL FIBROBLASTS

In the absence of external Ca2+, bradykinin evoked a transient increase in intracellular Ca2+([Ca2+](i)) in a dose-dependent manner in fura-2-loaded human gingival fibroblasts, indicating the Ca2+ release from intracellular Ca2+ pool. The concentration of bradykinin that stimulated the increase in [Ca2+](i) induced an increase of inositol 1,4,5-trisphosphate [Ins(1,4,5)P-3]. Thapsigargin, an inhibitor of Ca2+-ATPase in Ins(1,4,5)P-3-sensitive Ca2+ pool, induced an increase in [Ca2+](i). In the cells pretreated with thapsigargin, the effect of bradykinin on [Ca2+](i) was obviously decreased. These results suggest that bradykinin stimulates Ca2+ release from the Ins( 1,4,5)P-3-sensitive Ca2+ pool. Bradykinin has been reported to stimulate the release of prostaglandin E(2) (PGE(2)) from human gingival fibroblasts (12). However, bradykinin or thapsigargin stimulated PGE(2) release to a lesser extent in the absence of external Ca2+, demonstrating that the Ca2+ release from Ins(1,4,5)P-3-sensitive Ca2+ pool is insufficient for the release of PGE(2). In the bradykinin-stimulated cells, Ca2+ entry from the external site was activated. In the presence of external Ca2+, Ca2+ ionophore A23187 evoked the release of PGE(2) despite the absence of bradykinin. These results suggest that bradykinin-mediated PGE(2) release is due to the entry of external Ca2+ rather than Ca2+ release from the intracellular pool.