Na+-dependent Ca2+ transport modulates the secretory response to the Fc epsilon receptor stimulus of mast cells

Immunological stimulation of rat mucosal-type mast cells (RBL-2H3 line) by clustering of their Fee receptors (Fc epsilon RI) causes a rapid and transi ent increase in free cytoplasmic Ca2+ ion concentration ([Ca2+](i)) because of its release from intracellular stores. This is followed by a sustained elevated [Ca2+](i), which is attained by Ca2+ influx. Because an Fc epsilon RI-induced increase in the membrane permeability for Na+ ions has also bee n observed, and secretion is at least partially inhibited by lowering of ex tracellular sodium ion concentrations ([Na+](o)), the operation of a Na+/Ca 2+ exchanger has been considered. We found significant coupling between the Ca2+ and Na+ ion gradients across plasma membranes of RBL-2H3 cells, which we investigated employing Na-23-NMR, Ca-45(2+), Sr-85(2+), and the Ca2+-se nsitive fluorescent probe indo-1. The reduction in extracellular Ca2+ conce ntrations ([Ca2+](o)) provoked a [Na+](i) increase, and a decrease in [Na+] (o) results in a Ca2+ influx as well as an increase in [Ca2+](i). Mediator secretion assays, monitoring the released beta -hexosaminidase activity, sh owed in the presence of extracellular sodium a sigmoidal dependence on [Ca2 +](o). However, the secretion was not affected by varying [Ca2+](o) as [Na](o), was lowered to 0.4 mM, while it was almost completely inhibited at [N a+](o) = 136 mM and [Ca2+](o) < 0.05 mM. Increasing [Na+](o) caused the sec retion to reach a minimum at [Na+](o) = 20 mM, followed by a steady increas e to its maximum value at 136 mM. A parallel [Na+](o) dependence of the Ca2 + fluxes was observed: Antigen stimulation at [Na+](o) = 136 mM caused a pr onounced Ca2+ influx. At [Na+](o) = 17 mM only a slight Ca2+ efflux was det ected, whereas at [Na+](o) = 0.4 mM no Ca2+ transport across the cell membr ane could be observed. Our results clearly indicate that the [Na+](o) depen dence of the secretory response to Fc<epsilon>RI stimulation is due to its influence on the [Ca2+](i), which is mediated by a Na+-dependent Ca2+ trans port.