Kinetic parameters of X-rhod-1. (A) Baseline-normalized and line-averaged fluorescence F/F₀ of Rhod-2 and X-rhod-1 in a WT muscle cell subjected to a voltage clamp pulse of 50 ms to 30 mV. Fluorescence of Rhod-2 was excited at 514 nm and collected between 540 and 570 nm. The corresponding wavelengths for X-rhod-1 were 594, 610, and 700 nm. The excitation lights were interspersed line by line. (B, solid trace) [Ca²⁺]_c calculated from the fluorescence trace of Rhod-2 in A using Eq. 2, with F_max/F_min determined in our laboratory and kinetic parameters provided by Escobar et al. (1997), including K_d = 1.58 µM and k_OFF = 130 s⁻¹. (dashed trace) [Ca²⁺]_c calculated from the fluorescence trace of X-rhod-1 using Eqs. 2 and 3, with F_max/F_min determined in our laboratory and kinetic parameters adjusted for best fit to the function derived from Rhod-2. Best fit values were k_ON = 2.6 10⁷ M⁻¹s⁻¹ and k_OFF = 100 s⁻¹. ID: 031611b series 35.