Figure 8.

Predicted onset of Ca²⁺-dependent feedback reactions after bright illumination of frog photoreceptors. The time course of Ca²⁺ decline and the predicted increases in guanylate cyclase and rhodopsin kinase activities in rod photoreceptors after exposure to bright illumination is shown. (A) Change in free Ca²⁺ after saturating light. The curve was calculated from the sum of three exponential processes (McCarthy et al., 1996): Ca²⁺(t) = 25e^{−t/ 0.25} + 35e^{−t/ 1.35} + 40e^{−t/ 6.75}. (B) The onset of the cyclase activity. Cyclase activity was calculated by the numerical solution of this equation and Eq. 1 in the text using the computer program Mathcad (version 6.0; MathSoft, Inc., Cambridge, MA). A 0.2-s delay was imposed between the Ca²⁺ decline and the change in the cyclase rate. The upper curve was calculated assuming a dark Ca²⁺ level of 200 nM and the lower curve with a dark Ca²⁺ of 400 nM. (C) Onset of rhodopsin kinase activity. In this case, the delay of 0.11 s found in Fig. 4 was imposed. The Ca²⁺ dependence of the kinase activity was taken from calculations in Klenchin et al. (1995). The solid line assumes a Ca²⁺ K _1/2 for kinase regulation of 270 nM. The dashed lines above and below the solid line assume Ca²⁺ K _1/2 values of 540 and 135 nM, respectively.