86Rb+ ion fluxes in resting and immunologically stimulated mucosal mast cells

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 Fee receptors (FcεRI) 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 (τ = 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) FcεRI 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 (τ = 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 unidirectionel K⁺ uptake of 3.0 ± 1.1 10⁷ ions/cell/s, which is increased by ca. 10% upon clustering of the FcεRI 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.