Light-induced APs in Xenopus oocytes coexpressing β1-ChR2 and K_v2.1 with either Na_v1.2 or Na_v1.5. (A and C) Current-clamp recordings. For detailed statistics on the effect of K_v2.1 on Na_v1.5-mediated APs, see Fig. S2 and Table S2. (B and D) Corresponding Na⁺ currents in the absence and presence of TTX. Similarly to Na_v1.4, neuronal Na_v1.2 channels belong to the TTX-sensitive Na⁺ channels; they can be blocked at nanomolar TTX concentrations. Cardiac Na_v1.5 channels are TTX resistant, with IC₅₀ values >1 µM. The peak and plateau currents through β1-ChR2 were I_peak = 768 nA, I_plateau = 193 nA (Na_v1.2-expressing cell in A and B), and I_peak = 957 nA, I_plateau = 193 nA (Na_v1.5-expressing cell in C and D). The K⁺ currents through K_v2.1 were I_{+70 mV} = 14.3 µA (Na_v1.2-expressing cell, A and B), and I_{+70 mV} = 30.9 µA (Na_v1.5-expressing cell, C and D). Duration of the light pulse was 25 ms.