The role of external and internal free Ca2+ concentration on ionomycin induced leukotriene C4 formation in rat basophilic leukemia cells.

Rat basophilic leukemia (RBL-2H3) cells serve as a model to examine the role of external and internal free Ca2+ concentration [Ca2+]i, following ionomycin induced stimulation of leukotriene C4 (LTC4) formation. Brief exposure of RBL cells to Ca(2+)-free medium abolished the effect of ionomycin on elevation of [Ca2+]i (monitored by Quin-2/AM) and on stimulation of LTC4 production. In Ca(2+)-rich medium (1.8 mM) however there was a large increase in both parameters. We showed recently (Her et al., 1990) that hydrocortisone (HC) and dexamethasone markedly suppressed the elevated [Ca2+]i induced by antigen. Following HC pretreatment, there was a modest (35%) suppression of [Ca2+]i elevation induced by submaximal (0.1 microM) as well as maximal (1 microM) doses of ionomycin (nevertheless, 8 fold increase above basal level was still observed), LTC4 formation, however, was only inhibited (47%) by HC when induced by submaximal dose of ionomycin, but not that induced by higher doses of ionomycin. Phorbol ester (TPA) abolished elevation of [Ca2+]i induced by antigen. Short treatment with TPA had a modest inhibitory (28%) effect on elevation of [Ca2+]i and on LTC4 formation (23%) induced by ionomycin. It is proposed that high [Ca2+]i, possibly originated mainly from extracellular source, is essential for induction of LTC4 formation.