Amino acid exchange is associated with inward charge movement. (A) Mechanism of photolysis of MNI-caged alanine. (B and C) Current in response to liberation of ∼200 µM of free alanine from 3 mM of MNI-caged alanine at t = 0 in the presence of internal K⁺ (B) or Na⁺/alanine (C). The anion was Mes⁻ in all cases (V = 0 mV). (D) Current response to alanine application at t = 0 at membrane potentials of −60 mV (red), 0 mV (black), and +60 mV (blue). (E) Charge movement as a function of the transmembrane potential. The charge was obtained by integrating the transient current response over time. The red line shows a fit to the Boltzmann equation (see legend to Fig. 3; z_Q = 0.58 and V_Q = 41 mV).