Membrane currents activated by bradykinin (500 nM) and by extracellular ATP (50 microM) were studied in voltage-clamped, NGF-treated rat pheochromocytoma (PC12) cells. Under quasiphysiological ionic conditions, both substances caused an outward current due to opening of Ca(2+)-activated K+ channels. Bradykinin caused an additional inward current that could be studied after blockade by internal Cs+ of the initial transient outward current. The inward current became larger when the extracellular Ca2+ concentration was increased. Neither inositol-1,4,5-trisphosphate, dioctanoylglycerol, phorbol 12-myristat 13-acetate, forskolin, GTP, GTP-gamma-S, or pretreatment with pertussis toxin affected this current component. Increasing the internal Ca buffer concentration [EGTA or bis-(o-aminophenoxy)-ethane-N,N,N′,N′- tetra-acetic acid] from 1 to 10 mM had no effect on the inward current as long as the free [Ca2+]i was kept constant. However, it was modulated by the resting free [Ca2+]i. Elevation of [Ca2+]i from nominally 0 to 60 or to 180 nM increased the bradykinin-induced average peak current density from 0.14 to 1.04 or to 2.29 pA/pF, respectively. This regulation may depend on a calmodulin-dependent pathway, since CGS 9343B, a calmodulin inhibitor, blocked the effect of elevated [Ca2+]i. With ATP as an agonist, outward current was preceded by a large inward current that was partially blocked by extracellular Ca2+ in the millimolar range. Extracellular Ca2+ was also found to reduce the single-channel conductance estimated from outside-out patches treated with ATP.