Xestospongin C, a novel blocker of IP3 receptor, attenuates the increase in cytosolic calcium level and degranulation that is induced by antigen in RBL‐2H3 mast cells

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Abstract
We evaluated the role of the cross‐linking of FcεRI‐mediated inositol 1,4,5‐triphosphate (IP3) in the increase in cytosolic Ca2+ level ([Ca2+]i) using xestospongin C, a selective membrane permeable blocker of IP3 receptor, in RBL‐2H3 mast cells. In the cells sensitized with anti‐dinitrophenol (DNP) IgE, DNP‐human serum albumin (DNP‐HSA) and thapsigargin induced degranulation of β‐hexosaminidase and a sustained increase in [Ca2+]i. Xestospongin C (3 – 10 μM) inhibited both of these changes that were induced by DNP‐HSA without changing those induced by thapsigargin. In the absence of external Ca2+, DNP‐HSA induced a transient increase in [Ca2+]i. Xestospongin C (3 – 10 μM) inhibited this increase in [Ca2+]i. In the cells permeabilized with β‐escin, the application of IP3 decreased Ca2+ in the endoplasmic reticulum (ER) as evaluated by mag‐fura‐2. Xestospongin C (3 – 10 μM) inhibited the effect of IP3. After the depletion of Ca2+ stores due to stimulation with DNP‐HSA or thapsigargin, the addition of Ca2+ induced capacitative calcium entry (CCE). Xestospongin C (3 – 10 μM) inhibited the DNP‐HSA‐induced CCE, whereas it did not affect the thapsigargin‐induced CCE. These results suggest that FcεRI‐mediated generation of IP3 contributes to Ca2+ release not only in the initial phase but also in the sustained phase of the increase in [Ca2+]i, resulting in prolonged Ca2+ depletion in the ER. The ER Ca2+ depletion may subsequently activate CCE to achieve a continuous [Ca2+]i increase, which is necessary for degranulation in the RBL‐2H3 mast cells. Xestospongin C may inhibit Ca2+ release and consequently may attenuate degranulation. British Journal of Pharmacology (2002) 135, 1959–1966; doi:10.1038/sj.bjp.0704662