Figure 3. Altered RetGC activity in GCAP1^−/− mouse retinas.

Total (A) and normalized (B) cGMP synthetic activity in WT (•, n = 5) and GCAP1^−/− (○, n = 4) retinas as a function of free Ca²⁺ concentration. Notice that sensitivity shifted to lower levels of Ca²⁺ in GCAP1^−/− retinas. In panel B, the activities in each series were normalized by the corresponding maximal RetGC activity measured in each genotype and averaged for each group. The data were fitted by the equation, A = (A_max – A_min)/(1+([Ca]/[Ca]_1/2)^h) + A_min; where A_max and A_min are the maximal and the minimal activity of guanylyl cyclase, respectively, [Ca]_1/2 is the concentration of Ca²⁺ producing 50% inhibition, and h is a Hill coefficient. RetGC activity was assayed as described in Materials and Methods. A_max values for the WT and GCAP1^−/− retinas were 0.6 and 0.8 nmol cGMP min⁻¹ retina⁻¹, [Ca]_1/2 values were 81 and 46 nM, and h values were 1.8 and 1.6, respectively.