Figure 3.

Kinetics of the reaction of MTSEA with the Cys at α244 exemplified with α⁰ ₂β⁰γ⁰δ⁻. MTSEA was applied to oocytes expressing α⁰ ₂β⁰γ⁰δ⁻ in both the closed (A and B) and open (C and D) states at a holding potential of −50 mV. The sequence of applications to the oocytes was: 200 μM ACh for 10 s; bath solution for 3 min; 5 mM MTSEA for 2–16 s (beginning at arrows) for the closed state (A) or 50 μM MTSEA together with 200 μM ACh for 5–20 s (arrows) for the open state (C); bath solution for 3 min. This sequence was repeated several times, resulting in cumulative applications of MTSEA for the times shown under the peaks of the test responses to ACh. There was no decline in current during the 10-s applications of ACh; the return of the current to baseline began with the wash out of ACh. The peak current was normalized and plotted against the cumulative MTSEA exposure time in the absence (B) and presence (D) of ACh. Solid lines indicate least squares single exponential fits to the data, which yield the pseudo first- and second-order rate constants for the reactions (see materials and methods).