Fig. 5. (A ) Currents evoked by brief voltage steps to 0 mV from the indicated holding potentials (Vh) were differentially sensitive to the blocking action of 10 μm cOA (upper trace on superimposed sweep) under equilibrium conditions. Each trace is the average of five sweeps applied at 0.5 Hz. (B ) The onset of action and reversal of the cOA block exhibited slow kinetics. cOA application is indicated by the horizontal bar. Data depict the mean responses (± SEM) derived from three replicated experiments. (C ) The effects of cOA were concentration-dependent and appeared to approach saturation at 64 μm. Tonic block was clearly influenced by holding potential: the substantial shift in EC²⁰for the two curves and the difference in maximal fractional block for the two holding potentials were both highly significant, as indicated in text. Figures in brackets indicate number of cells examined.