In a previous report, we described the ability of two secretogogues, histamine and nicotine, to stimulate additive effects on catecholamine (CA) release and synapsin I1 phosphorylation in bovine adrenal chromaffin cells (BACC) [Firestone and Browning (1992), J. Neurochem., 58:441447]. We hypothesized that these results were due to the combined effects on cytosolic Ca+ + of the two distinct signalling pathways. We therefore examined the intracellular Ca+ + signals stimulated by histamine and nicotine, alone and together. In Ca++-deficient medium, nicotine-stimulated signals were abolished, whereas histamine-stimulated signals were maintained, demonstrating that nicotine depended entirely on Ca+ ' influx for its effects. Indeed, the nicotine-stimulated signal could also be prevented using a Ca++ channel blocker, nicardipine. Further, the observation that exposure of BACC to thapsigargin reduced histamine-stimulated Ca+ + signals verified that histamine mobilizes Ca++ from intracellular stores. Thus, the two secretogogues mobilize Ca++ from distinct pools. When BACC were stimulated with the two secretogogues together, the resulting Ca++ signal was greater than that from either alone. These data are consistent with a model in which two distinct sources of Ca++ can summate within the cell, producing a greater Ca++ signal and, hence, a greater effect on neurotransmitter release. o 1994 Wiley-Liss, Inc.