Figure 3

Barium slows the rate of MTSEA modification at M³⁷C. Example of experimental paradigm used to assess Ba²⁺ effects on modification rates. (A) Successive ACh-evoked current traces recorded before and after repeated exposures to MTSEA (5 μM, 15 seconds), showing a decrement in responses to 30 μM ACh. Endpoints of MTSEA modification are determined by prolonged application of 500 μM MTSEA (right). (B) The same protocol, including Ba²⁺ pretreatment and co-application with MTSEA. Current traces are truncated in both (A) and (B) between consecutive MTSEA applications; in all cases the currents were allowed to return to baseline prior to the next application of MTSEA ± ACh. (C) Peak current amplitudes from (A) and (B) are normalized and plotted versus total MTSEA exposure time. Data from this single experiment (no error bars) are fitted to a single-exponential decay (solid line) to extract an apparent pseudo first-order rate constant. The pseudo first-order rate constants calculated in this experiment were 0.011 s^-1 and 0.0019 s^-1 for control (A) and +Ba²⁺ (B) measurements, respectively. Second-order rate constants are calculated from these values (Figures 4–6; Table 2).