Representative Na_v1.4 current traces at a test potential of −10 mV in Xenopus oocytes. Coexpression of β1 resulted in a faster decay of macroscopic Na⁺ currents through Na_v1.4 channels. To better illustrate the effect on inactivation, we selected whole-cell currents with similar amplitude. Current traces were fitted (red) using a biexponential function: I = A_f × exp(−t/τ_f) + A_s × exp(−t/τ_s), where τ_f and τ_s are the fast and slow time constants, and A_f and A_s are the corresponding amplitudes. Both fast and slow time constants, τ_f and τ_s, were significantly shorter. This well-known β1 property remained preserved in β1-ChR2 and β1-ChR2-ChR2 and was not obtained when coexpressing ChR2 (top). Values for τ_f, A_f, τ_s, and A_s are from at least six different measurements (*, significantly different versus Na_v1.4, P < 0.05). This typical acceleration of inactivation was also obtained at all other test pulses.