Role of endoplasmic reticulum, an intracellular Ca2+ store, in histamine release from rat peritoneal mast cell

By means of the Ca-antimonate precipitation technique, it was revealed that Ca2+ accumulates in the endoplasmic reticulum (ER) of rat peritoneal mast cells. The ER of rat mast cells was isolated using Percoll density gradient centrifugation, and its characteristics were studied. Although the uptake of 45Ca into the ER was enhanced by adenosine 5'-triphosphate (ATP) at concentrations lower than 2 mM, at higher concentrations ATP induced 45Ca release from the ER, suggesting that bidirectional translocation of Ca2+ takes place in the ER membrane. When apyrase was added to the reaction mixture to decompose all ATP molecules, the amount of 45Ca in the ER was decreased, indicating that ATP is necessary to retain Ca2+ in the ER. Not only inositol 1,4,5-trisphosphate (IP3) but also guanosine 5'-triphosphate (GTP) was effective in releasing Ca2+ from the ER at concentrations higher than 2 microM, while guanosine 5'-[gamma-thio]-triphosphate (GTP-gamma S), a non-hydrolysable analogue of GTP, was not effective. This may indicate that a hydrolysis of GTP is necessary for Ca2+ release from the ER. Intracellular Ca2+ blockers such as 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) and dantrolene sodium were effective in inhibiting Ca2+ release from ER induced by IP3. The Ca2+ release due to IP3 was also inhibited by flunarizine, a Ca2+ channel blocker, and by oxatomide, an antiallergic drug. Since these two compounds are diphenylpiperazine derivatives, it is suggested that a diphenylpiperazine moiety may be effective in inhibiting Ca2+ release from the ER.