Na-Dependent Ca Transport Modulates the Secretory Response to the Fce Receptor Stimulus of Mast Cells

Immunological stimulation of rat mucosal-type mast cells (RBL-2H3 line) by clustering of their Fce receptors (FceRI) causes a rapid and transient increase in free cytoplasmic Ca ion concentration ([Ca]i) because of its release from intracellular stores. This is followed by a sustained elevated [Ca]i, which is attained by Ca 21 influx. Because an FceRIinduced increase in the membrane permeability for Na ions has also been observed, and secretion is at least partially inhibited by lowering of extracellular sodium ion concentrations ([Na]o), the operation of a Na /Ca exchanger has been considered. We found significant coupling between the Ca and Na ion gradients across plasma membranes of RBL-2H3 cells, which we investigated employing Na-NMR, Ca, Sr, and the Ca-sensitive fluorescent probe indo-1. The reduction in extracellular Ca concentrations ([Ca]o) provoked a [Na ]i increase, and a decrease in [Na ]o results in a Ca 21 influx as well as an increase in [Ca]i. Mediator secretion assays, monitoring the released b-hexosaminidase activity, showed in the presence of extracellular sodium a sigmoidal dependence on [Ca]o. However, the secretion was not affected by varying [Ca]o as [Na ]o was lowered to 0.4 mM, while it was almost completely inhibited at [Na ]o 5 136 mM and [Ca ]o , 0.05 mM. Increasing [Na]o caused the secretion to reach a minimum at [Na ]o 5 20 mM, followed by a steady increase to its maximum value at 136 mM. A parallel [Na]o dependence of the Ca 21 fluxes was observed: Antigen stimulation at [Na]o 5 136 mM caused a pronounced Ca influx. At [Na]o 5 17 mM only a slight Ca 21 efflux was detected, whereas at [Na]o 5 0.4 mM no Ca transport across the cell membrane could be observed. Our results clearly indicate that the [Na]o dependence of the secretory response to FceRI stimulation is due to its influence on the [Ca]i, which is mediated by a Na-dependent Ca transport.