Figure 7.

The involvement of extracellular La³⁺ in the enhancing effect of TTX on isolated TTXR currents. (a) The graph shows relative amplitudes of isolated TTXR current in modified extracellular solutions, with the bar showing the application of 0.5 μM TTX. TTXR currents were evoked as described in Figure 2. Reducing extracellular Ca²⁺ did not alter the enhancing effect of TTX on TTXR current. Reduction of Ca²⁺ plus removal of La³⁺ substantially reduced any current increase by TTX, suggesting that the effect is dependent on the presence of extracellular La³⁺. In control experiments, recorded in standard extracellular solution, application of TTX-free extracellular solution to the cells had no effect on TTXR current amplitude. (b) The graph shows relative amplitudes of isolated TTXR current recorded in a low extracellular Ca²⁺ solution (50 μM) in the absence of extracellular La³⁺ and TTX. TTXR currents were evoked as described in Figure 2. The external application of 10 μM La³⁺ caused a sustained reduction in TTXR current amplitude of 60–70%. Subsequent addition of 0.5 μM TTX produced a partial reversal of this inhibition. The numbered data points on the graph correspond to the time points of the example traces shown in panel (c). (c) Example traces from data shown in panel b demonstrating the inhibition of TTXR current amplitude by 10μM La³⁺, and the ability of 0.5 μM TTX to reverse this inhibition by La³⁺.