Light-induced APs in Xenopus oocytes coexpressing Nav1.4 and β1-ChR2. All recordings shown were performed on a single oocyte. Light pulses are illustrated as blue bars. (A) Current-clamp recording. The membrane potential was set to −80 mV, and the change of membrane voltage was followed on a 25-ms light pulse delivered to the whole oocyte. For AP variability and for detailed statistics, see Fig. S1 and Table S1. (B) Corresponding whole-cell current through β1-ChR2 channels recorded in the voltage-clamp mode. Pulse duration was 1 s. (C) Nav1.4 whole-cell current of the same oocyte recorded in the voltage-clamp mode (holding potential, −120 mV; test pulse, −25 mV). (D) Current-clamp recording in the presence of 400 nM TTX (red). The remaining depolarization was due to ChR2 activity. Washout restored the fast upstroke to nearly +20 mV (black). (E) TTX did not affect β1-ChR2 activity (voltage-clamp recording as in B). (F) Nav1.4 channels were completely blocked in the presence of 400 nM TTX (voltage-clamp mode as in C). Black line shows the Na+ current after washout of TTX.