We studied fluxes of Rb+ ions, using its 86Rb isotope as a radioactive tracer in living rat mucosal mast cell cultures (RBL-2H3 line) grown to high density on beads. Continuously perfused samples of these cells could be immunologically stimulated by antigen clustering of IgE bound to the cells type I Fc epsilon receptors (Fc epsilon RI) and both the cellular response, as measured by the secreted mediators, as well as the uptake of 86Rb+ of the perfused sample could be monitored. The following results were obtained. (i) In resting cells, 86Rb+ influx is observed upon exposure to extracellular 86Rb+. It proceeds with a monoexponential time course (tau = 30.6 +/- 8 min) reaching a steady-state distribution of [86Rb+]int/[86Rb+]ext = 31.6 +/- 6.4 and can be inhibited by ouabain. (ii) Fc epsilon RI clustering-mediated stimulation of these cells causes an immediate and marked increase in both amplitude and rate of 86Rb+ uptake, which also fits a monoexponential function (tau = 26.8 +/- 8.6 min). (iii) This stimulated 86Rb+ uptake can also be inhibited by ouabain. It is not caused by Ca2+ influx or by the exocytotic process as evidenced by the fact that it is also observed in buffer to which no Ca2+ ions were added. Analysis of these results by a simple model taking into account unidirectional 86Rb+ influx by the Na+/K(+)-dependent ATPase and its efflux by K+ channels yields a resting cells unidirectional K+ uptake of 3.0 +/- 1.1 10(7) ions/cell/s, which is increased by ca. 10% upon clustering of the Fc epsilon RI by IgE and antigen. The stimulated influx is suggested to be due to enhanced activity of the Na+/K(+)-dependent ATPase, reflecting increased permeability for Na+ ions.