Figure 4.

Low Ca²⁺ exposure eliminates sensitivity regulation and minimizes the operating range of GCAP^−/− and DKO mouse rods. Dim-flash responses (in microvolts; see scale bar on the left) in darkness and during steps of light in GCAP^−/− (A; I_B range: 17–1,600 R* s⁻¹ per rod, indicated on the right side of each response) and DKO (B; I_B range: 17–1,600 R* s⁻¹ per rod, indicated on the right side of each response) mouse rods in low Ca²⁺ solution. Flash strengths ranged from 6 to 190 R* per rod and from 12 to 190 R* per rod in GCAP^−/− and DKO mice, respectively, as indicated by the numbers preceding each response. Timing of the flash, 5 s after the background onset at t = 0 s, is indicated by an arrow. Average (±SEM) sensitivities normalized with s_F,D as a function of background light intensity (I_B) are plotted for four GCAP^−/− (C) mouse retinas in normal (black marks) and low (red marks) Ca²⁺ solution and for three DKO (D) mouse retinas in normal (blue marks) and low (magenta marks) Ca²⁺ solution. Dashed lines plot Eq. 3 under low Ca²⁺ conditions for GCAP^−/− (C) and DKO (D) rods (error bars represent mean ± SEM). Insets in C (GCAP^−/−) and D (DKO) show r_sat/r_sat,dark as a function of I_B in normal (black, GCAP^−/−; blue, DKO) and low Ca²⁺ (red, GCAP^−/−; magenta, DKO). Mean ± SEM (n = 7).