Figure S2.

Decomposition of the amplitude and rate-saturated photocurrents of K_v2.1^−/− rods into three membrane current components: CNG, NCKX, and capacitive. Related to Fig. 7. The traces (heavy black line) are averages of several responses to flashes of at least three intense flash strengths up to 20,000 photons μm⁻². The responses to different flash strengths were interpolated on a 100-µs grid and shifted for maximal congruence with the early rising phase (CNG channel closure) of the most intense flash. The running error (gray shaded region) is ±1 SD. A–H provide three numbers characterizing the theory traces: the saturated photocurrent (PC) amplitude; f_NCKX, the fraction of the photocurrent ascribed to NCKX current; and Ca_rest, the estimated Ca²⁺_i in the dark-adapted rod immediately before the flash. The panels are arranged in order of increasing Ca_D, which ranges from approximately two times normal (A, 560 nM) to >4 µM (G); in the latter case, Ca_rest greatly exceeds the internal binding constant K_ex (1.1 µM) of the NCKX, which serves as a gauge (Fig. 7). The “average” value of Ca_rest is 10-fold higher than the normal WT level. The values of f_NCKX colored red exceed the naive theoretical limit of 0.33, which neglects the effect of hyperpolarization on the NCKX (the value of the half-saturation constant K_ex of the NCKX current limits the reliability of estimating Ca_rest, as it is highly nonlinearly dependent on f_NCKX as the latter approaches its limit; thus, Ca_rest estimates >4 µM were not differentiated). The preprocessing of the photocurrent data of each of the rods was performed with the procedure illustrated in Fig. S1.