Abstract: | Antigenic stimulation of rat basophilic leukemia cells (RBL-3H3) elevates intracellular free Ca2+ concentration (Ca2+]i) and induces production of leukotriene C4 (LTC4). This model was used to examine the role of Ca2+ in LTC4 formation, and inhibition by hydrocortisone (HC). HC, at a physiological concentration (2×10−7M), selectively prevented the stimulatory effect of the antigen on LTC4 production whereas the response to calcium inophore (A23187) remained unimpaired. The inhibition by HC was time-dependent: half maximal response was reached at 2 hour and maximal response at 3 hours. Addition of arachidonic acid (3 μg/ml) did not overcome the inhibitory action of HC. An elevated Ca2+]i is known to be essential for the activation ob both 5-lipoxygenase and phospholipase A2. The stimulatory effect of the antigen on LTC4 production was abolished when the cells were incubated in Ca2+-deficient medium. Likewise, calcium ionophore stimulation shows dependence on extracellular Ca2+. Half maximal stimulation by the antigen and calcium ionophore was observed at external Ca2+ concentration of 150 μM and 40 μM respectively. Treatment with HC largely prevented the antigen-induced rise in Ca2+]i, measured by Quin 2. In addition, HC reduced by 70% the accumulation of 45Ca2+ induced by the antigen. Collectively, these results demonstrate for the first time that HC reduces antigen-induced elevation of Ca2+]i, and this may be associated with the inhibitory action of HC on LTC4 formation. This property could be partly responsible for the antiallergic and antiinflammatory activities of HC. |