Membrane current responses of NG108‐15 mouse neuroblastoma x rat glioma hybrid cells to bradykinin.

1. Membrane current responses to focal application of bradykinin (BK) were recorded in voltage‐clamped NG108‐15 neuroblastoma x glioma hybrid cells. 2. BK produced sequential outward and inward currents at clamp potentials between ‐60 and ‐30 mV, designated IBK(out) and IBK(in), respectively. 3. The outward current IBK(out) was accompanied by an increased membrane conductance. Ramp current‐voltage (I‐V) curves yielded a reversal potential (VBK) of ‐80 +/‐ 5.6 mV (mean +/‐ S.D., n = 9) in 5.4 mM [K+]o. VBK showed a positive shift on raising [K+]o, compatible with a primary increase in K+ conductance. Subtracted I‐V curves indicated that the underlying conductance was not strongly voltage dependent between ‐120 and ‐40 mV. 4. IBK(out) was inhibited by d‐tubocurarine (dTC, 0.1‐0.5 mM) but was insensitive to tetraethylammonium (TEA) below 5 mM. 5. The inward current IBK(in) was accompanied by a fall in membrane conductance. This was associated with the inhibition of a time‐ and voltage‐dependent K+ current, IM. In consequence, IBK(in) was strongly voltage dependent and dissipated, usually without reversal, on hyperpolarizing the cell beyond ‐70 mV in 5.4 mM [K+]o. Reversal to an outward current negative to ‐40 mV could be obtained on raising [K+]o to 54 mM. 5. Both IBK(in) and IBK(out) persisted when ICa was blocked with Co2+ or Cd2+. IBK(out) slowly diminished in Ca2+‐free, Mg2+‐substituted solution. 6. The Ca2+ spike current ICa and the Ca2+‐activated K+ current IAHP were inhibited during IBK(out) or after Ca2+ injections. BK did not affect the voltage‐activated K+ current IK(V) recorded in Co2+ solution. 7. It is concluded that the dual response to BK results from opposing effects on two different species of K+ current. IBK(out) results from activation of a Ca2+‐dependent, voltage‐insensitive K+ conductance, probably mediated by a transient rise in intracellular Ca2+. It is suggested that the Ca2+ is released from an intracellular store. IBK(in) results primarily from inhibition of the Ca2+‐independent, ...