Effects of hydroquinone-type and phenolic antioxidants on calcium signals and degranulation of RBL-2H3 cells

We previously reported that a hydroquinone-type antioxidant, 2,5-di(tert-butyl)-1,4-hydroquinone (DTBHQ), increases intracellular free Ca2+ concentration ([Ca2+](i)), causes degranulation together with a protein kinase C activator, phorbol 12-myristate 13-acetate (TPA), and increases antigen-induced degranulation in rat basophilic leukemia (RBL 2H3) cells. In this study, the effects of five hydroquinone-type and phenolic antioxidants (2,5-di(tert-amyl)-1,4-hydroquinone [DTAHQ], 2-tert-butyl-1,4-hydroquinone [MTBHQ], 3,5-di(tert-butyl)-4-hydroxytoluene [BHT], 3,5-di(tert-butyl)-4-hydroxyanisole [DTBHA], and 3-tert-butyl-4-hydroxyanisole [MTBHA]) on [Ca2+](i) and degranulation (beta-hexosaminidase release) were examined and compared with that of DTBHQ. DTAHQ (greater than or equal to 3 mu M) showed effects similar to those of DTBHQ (10 mu M) on [Ca2+](i) elevation, induction of degranulation with TPA, and increase of antigen-induced degranulation. BHT (50 mu M) and DTBHA (50 mu M) caused [Ca2+](i) elevation and increased degranulation in the presence of TPA or antigen, but their effects were less than those of DTBHQ and DTAHQ. MTBHQ and MTBHA had no effect on [Ca2+](i) and degranulation, even at 50 mu M. The degree of Ca2+ response caused by the compounds correlated well with the increase in degranulation, hut not with their antioxidant activity estimated with the first oxidation potential. From these results, it is suggested that the increasing effects of six antioxidants on degranulation in the presence of TPA or antigen were dependent on [Ca2+](i) increase caused by the compounds, probably through their ability to inhibit endoplasmic reticulum Ca2+-ATPase.