The control of mediator release from RBL-2H3 cells: roles for Ca2+, Na+, and protein kinase C1

Antigen-stimulated rat basophilic leukemia (RBL-2H3) cells release serotonin and other inflammatory mediators by a process that requires Ca2+ influx and increased cytoplasmic Ca2+ levels, and is mimicked by Ca2+ ionophores. We report here that the Ca2+ response to antigen and to ionomycin has two components, a Ca2+ spike and a Ca2+ plateau. In nominally Ca2+-free medium, both components of the Ca2+ response are inhibited and secretion does not occur. In Na+-free medium, the initial Ca2+ spike induced by antigen or ionomycin occurs, but the plateau is again absent and secretion is inhibited by 30 to 50%. Secretion is also reduced by 10(-4) M amiloride, an inhibitor of Na+ transport pathways, and by 10(-5) M concentrations of two amiloride analogs with greater activity than amiloride, respectively, against Na+ channels and Na+/Ca2+ exchange. Phorbol esters, which stimulate protein kinase C, enhance the Ca2+ plateau and secretion caused by suboptimal amounts of both antigen and ionomycin; this enhancement depends on extracellular Na+. The Na+ ionophore, monensin, mimics the Ca2+ plateau. From these data, we infer that the Ca2+ spike and plateau reflect separate responses of RBL-2H3 cells to antigen or ionomycin. We propose that the Ca2+ plateau results at least in part from the activation of a Na+-dependent Ca2+ influx pathway. One possible mechanism is that antigen binding stimulates a protein kinase C-regulated Na+ transport system. The resulting influx of Na+ may activate a Na+/Ca2+ antiporter that supports the Ca2+ plateau and mediator release.