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Laboratory of Cell Biology, National Institute of Mental Health, Bethesda, MD 20892.
1. The role of inositol 1,4,5-trisphosphate (InsP3) and diacylglycerol (DAG) as possible mediators of the membrane current responses of NG108-15 neuroblastoma x glioma hybrid cells to bradykinin (BK, Brown & Higashida, 1988b) has been tested using intracellular ionophoresis of InsP3 and external application of phorbol dibutyrate (PDBu) and 1-oleoyl-2-acetylglycerol (OAG). 2. Intracellular ionophoresis of InsP3 into cells clamped at -30 to -50 mV produced (i) a transient outward current, (ii) a transient outward current followed by an inward current, or (iii) an inward current. All currents were accompanied by an increased input conductance. 3. The transient outward current reversed at between -80 and -90 mV. The reversal potential was shifted to more positive potentials on raising extracellular [K+], suggesting that it resulted from an increased K+ conductance. 4. The outward current was inhibited by apamin (0.4 microM) or d-tubocurarine (0.2-0.5 mM); these drugs also inhibit the outward current produced by BK or by intracellular Ca2+ injections (Brown & Higashida, 1988 a, b). The outward current was also slowly reduced in 0 mM [Ca2+] or 0.5 mM [Cd2+] plus 2 mM [Co2+] solution. 5. Ionophoretic injection of inositol 1,3,4-trisphosphate and inositol 1,3,4,5-tetrakisphosphate, guanosine trisphosphate or inorganic phosphate did not evoke an outward current but produced only an inward current with an increased conductance, reversing at between -10 and -20 mV. 6. Bath application of PDBu (10 nM-1 microM) or OAG (1-10 microM) produced an inward current with a fall in input conductance. The inward current was voltage dependent and was accompanied by an inhibition of the time-dependent current relaxations associated with activation or deactivation of the voltage-dependent K+ current, IM. 7. PDBu did not clearly reduce the Ca2+ current or the Ca2+-dependent K+ current recorded in these cells. During superfusion with PDBu, the outward current produced by intracellular ionophoresis of InsP3 was greatly enhanced. 8. The results support the view that the two membrane current responses to BK might both result from accelerated membrane phosphatidylinositide hydrolysis. One product, InsP3, releases Ca2+ and activates an apamin-curare-sensitive outward K+ current; this effect is imitated by intracellular InsP3 ionophoresis. The second product, DAG; activates protein kinase C to inhibit the voltage-dependent K+ current IM and generate an inward current; this effect is imitated by external application of PDBu or OAG.
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