The hydrolysis rates in rods of the GCAPs+/+ background level out into plateaus that continue until the SPR peak (∼120 ms; indicated by the transition from solid to dashed colored lines, Figure 5). In contrast, the hydrolysis rates in rods of the GCAPs−/− background begin to decline shortly after reaching their peaks. Analysis of the spatiotemporal

profiles reveals that Y-27632 supplier the decline in hydrolysis rate arises because of substrate depletion: the absence of calcium-activated synthesis causes depletion of cGMP in the regions flanking the disc where R∗ and G∗-E∗s reside, thereby lowering the local hydrolysis rate (βdarkcG) ( Figure S3). Unlike the step-like rates of steady hydrolysis in the

GCAPs+/+ background, the light-driven increases in the cGMP synthesis rates (Figure 5B) rise on delayed ramps whose slopes are in approximately the same ratios (1:2:3) as the hydrolysis plateau magnitudes. To determine the underlying causes of the delayed ramps of synthesis activity, we examined the space-averaged changes in calcium influx and efflux (Figures 5D and 5E). In the dark, calcium influx and efflux Proteases inhibitor are perfectly balanced. During the initial 35 ms of the SPR (pink region), the calcium influx decreases as CNG channels close, but there is little change in the rate of calcium efflux at this early time. From about 35 ms onward, the fall in free calcium causes its efflux to slow (Figure 5E). As a result, the net calcium flux for each genotype is a fairly

symmetric bell-shaped curve (Figure 5F). Given a constant calcium buffer power, the change in free calcium (not shown) is directly proportional to the integral of the bell-shaped curve, giving rise to ramping decreases in calcium. As a consequence, the time course of cGMP synthesis (Figure 5B), which is approximately proportional to the decrease in free calcium (Equation 4), is also ramp-like. To complete the picture detailing the mechanism of GCAPs-mediated feedback contribution to SPR amplitude stability, we now consider the net rate of change of cGMP (i.e., the rate MTMR9 of synthesis minus that of hydrolysis) for each genotype (Figure 5C). The three color-coded rate functions share a common initial trajectory from which they diverge as the ramping synthesis overtakes the step-like hydrolysis time course. Consequently, the predicted times of the SPR peaks (given by the zero-crossings, the times at which cGMP synthesis balances hydrolysis) are nearly identical for the three genotypes, as observed in the experimental SPRs (Table 1). In order to achieve the similarity in time to peak for different R∗ lifetimes, the cGMP synthesis rate must rise in proportion to the steady hydrolysis rates.