Figure 4

PDE-mediated cGMP hydrolysis in rods and cones. (A) The time courses for PDE activities (the number of cGMP molecules hydrolyzed per VP*) in the membrane of carp rods and cones in response to light stimulations (a, 0.024% for rods (open circle); b, 0.46% for cones (open circle)) in the presence of ATP (0.25 mM), cGMP (2500 μM) and either GTP (open closed circle) or GTPγS (filled triangle) determined by biochemical experiments^17,27). Corresponding simulation results (solid lines (with GTPγS) and dotted lines (with GTP) in a and b) were superimposed onto the experimental data. The continuous presence of saturating levels of cGMP was assumed for the simulation study, and thus βdark was set to 0. (B) Light-induced peak PDE activity as a function of flash intensity in rods (circle) and cones (triangle) either in the presence of ATP (a, 0.25 mM) or without ATP (b) and either GTP (open symbols) or GTPγS (filled symbols) are normalized to maximum values. Simulated PDE activity of the rods and cones reproduced under the corresponding experimental conditions (with either GTP (dotted lines) or GTPγS (solid lines)) were superimposed onto the experimental results. The value for the rate constant, k_decay, for cGMP hydrolysis was set to 0 for the simulation of the experimental results obtained with GTPγS.