Spectrofluorimetric characterization of the K⁺ indicator APG-1.

(A) Emission spectra recorded in the presence of different [K+] in intracellular-like solutions following excitation at 515 nm. Emission maximum was ∼540 nm. (B) Fluorescence emission plotted as a function of [K+] showing a monotonic relationship of APG-1 fluorescence with increasing [K+] (circles). The same analysis was performed on APG-2, a related indicator with identical spectral properties (diamonds) but lower Kd for K+. The plots show that APG-2 fluorescence becomes saturated at [K+]>80 mM, which is not the case with APG-1. (C) Na+ dependency of APG-1 fluorescence measured in intracellular-like solution containing 135 mM K+ (see also Fig. S1). (D) pH dependency of APG-1 fluorescence measured in intracellular-like solution containing 135 mM K+. The pH of each solution was adjusted using NMDG. This pH analysis was repeated three times. Data are presented as means ± SEM of triplicate measurements.