GCAPs regulate the dark flash sensitivity of rod photoreceptors. (
A) Population mean single photon responses from wild-type and GCAP-deficient rods. Flashes were delivered at
t = 0. (
B) Dark-adapted flash sensitivity (
S
) and the time constant of the Na
+/Ca
2+, K
+ exchange current in wild-type, GCAPs−/− and GCAPs−/− GCAP2+ rods. The rate of Ca
2+ extrusion in GCAPs−/− GCAP2+ rods was highly variable, suggesting that the expression of GCAP2 varied widely in different cells. (
C) Representative responses of wild-type and GCAP-deficient rods to a bright flash (560 photons/μm
2). Traces are averages of two responses. Dark currents (in pA) were 13.1 (wild type, +/+), 12.4 (GCAPs+/−), 13.3 (GCAPs−/−), and 10.8 (GCAPs−/− GCAP2+). The time constant of Na
+/Ca
2+, K
+ exchange in this GCAPs−/− GCAP2+ rod was slightly longer than that of wild-type rods (100.2 ms; see Fig. 5
C), suggesting there was overexpression of GCAP2 in this rod.