Figure 4. . Activation and desensitization of ASIC whole-cell currents by external protons. Currents from oocytes injected with ASIC1α, ASIC2, or ASIC3 were examined with the TEVC in the presence of 150 mM Na+, 1 mM Ca2+ at −60 mV of membrane potential. In the upper trace from A an oocyte expressing ASIC1α was activated by sequential exposure to solutions of pHo 6.0, 5.0, and 4.0. Between each test solution, the bath was returned to 7.4 for 25 s. Peak currents were normalized to the value obtained at pHo 4.0 and the data were fitted with the equation I = Imax/1 + pH50/[pHo], and apparent pH50 of 5.85 ± 0.25. The lower trace shows activation of currents with pHo 6.0 but no effect if the pHo is changed directly to pHo 5.0. (B) Activation of ASIC2a channels by sequential exposure to progressively lower pHos without returning to pHo 7.4. The desensitization rate is pHo dependent being faster at pHo 4.0 than 5.0. The dotted line indicates desensitization at pHo 5. Peak currents obtained at each pHo were used to calculate the apparent pH50, 5.0 ± 0.12. (C) Oocyte expressing ASIC3 was exposed to a protocol similar to the one in A. Indicated in the second part of the experiment, after the vertical dotted line, changes in pHo from 6.0 to 5.0 failed to activate channels. The value of the apparent pH50 was 5.4 ± 0.2. Scales of time and current amplitude are indicated by the bars in each panel. The pH50 ± SD for each type of channel was calculated from five to eight oocytes.
Image published in: Zhang P and Canessa CM (2002)
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