In NH15-CA2 cells head activator (HA) stimulates cell proliferation by acting in the G2/M transition. Cells in mitosis were analyzed by flow cytometry 2-4 h after HA application. HA in a dose-dependent manner stimulated mitosis. Mitosis was prevented by preincubation of cells with pertussis toxin identifying the HA receptor as being Gi-protein coupled. As second effect of HA, an increase in intracellular calcium concentration was observed. This increase in calcium concentration was abolished by inhibiting calcium influx from the extracellular space into NH15-CA2 cells either by chelating extracellular calcium with EGTA or by blocking calcium channels. The increase in intracellular calcium concentration led to an activation of calcium-dependent potassium channels. The higher potassium conductance resulted in hyperpolarization of NH15-CA2 cells. Blocking calcium channels with nickel chloride or potassium channels with tetraethylammonium chloride inhibited the effect of HA on cell proliferation. HA-induced mitosis was inhibited by charybdotoxin and apamin, but not by alpha-dendrotoxin confirming the notion that Ca(2+)-dependent potassium channels are involved in mediating the effect of HA on cell division.