The mast cell plays a pivotal role in initiating allergic inflammation by secreting several cytokines including TNF-alpha, in addition to granule mediators such as histamine. Anti-allergic drugs including azelastine prevent immediate-type hypersensitivity by inhibiting mast cell degranulation, as well as blocking histamine H1 receptors. However, their effects on cytokine release from mast cells remain unknown. In a rat mast RBL-2H3 cell line, azelastine inhibited Ag- and ionomycin-induced TNF-alpha release with IC50 values of 25.7 +/- 3.4 microM and 1.66 +/- 0.45 microM, respectively. These effects were observed at lower concentrations than needed for the inhibition of degranulation. In Ag-stimulated cells, azelastine also inhibited TNF-alpha mRNA expression, TNF-alpha protein synthesis and release, and, possibly related to these effects, Ca2+ influx. In ionomycin-stimulated cells, however, azelastine inhibited TNF-alpha release to a greater extent than mRNA expression/protein synthesis and Ca2+ influx, suggesting that azelastine inhibits the release process more potently than transcription or production of TNF-alpha by interfering with a signal other than Ca2+. Azelastine added 1 h after ionomycin stimulation also immediately blocked subsequent release of TNF-alpha, which had been produced in the cells, without affecting Ca2+ influx. Pretreatment with 1 microM azelastine inhibited ionomycin-induced, but not Ag-induced, protein kinase C translocation to the membranes. These results suggest that the release process of TNF-alpha in mast cells is regulated by a mechanism distinct from that of degranulation, and that in Ca2+-ionophore-stimulated cells, it is also different from that of transcription/production, and possibly involves protein kinase C activation.