To investigate the basis of interactions between nerves and mast cells, we tested the actions of the neuropeptide substance P (SP) on whole cell current characteristics of RBL-2H3 cells (homologous to mucosal mast cells). Control RBL cells showed a K(+)-dependent inwardly rectified current. SP (10(-6) M) caused transient, frequently repetitive increases in current amplitude, which at a membrane potential (Vm) of -80 mV rose by -1,020.0 +/- 223.4 pA after SP application compared with -6.8 +/- 1.7 pA for control. This response was characterized by a lag phase of 102 +/- 16 s. Seventeen percent of cells showed spontaneous transients in the current amplitude from the beginning of the recording. After SP administration, the amplitude of these transients increased by 6.3 +/- 2.0-fold. Responses to SP were mimicked by the application of ionomycin. For both SP and ionomycin, there was a dose dependency of the lag phase. Removal of extracellular calcium abolished the response for 10(-6) M SP but not for 6.6 x 10(-6) M ionomycin. During current transients, the whole cell current had both inward and outward rectified components with the zero current Vm shifted from -87.3 +/- 3.2 mV at control to -10.8 +/- 1.7 mV. We compare the SP-evoked current responses in mucosal-type mast cells with those described in connective tissue type.