THE CGMP-PHOSPHODIESTERASE AND ITS CONTRIBUTION TO SENSITIVITY REGULATION IN RETINAL RODS

We have used the truncated outer segment preparation to measure rod cG MP-phosphodiesterase activity, as well as its modulation by Ca2+, in d arkness and in light. The basal enzyme activity in darkness was simila r to 0-3 s(-1), and was largely independent of Ca2+ concentration from 10 nM to 10 mu M The steady state activity elicited by a step of ligh t (h = 520 nm) was strongly enhanced by Ca2+, increasing from similar to 0.005 s(-1)(h nu mu m(-2) s(-1)) at 10 nM Ca2+ to similar to 0.16 s (-1)/h nu mu m(-2) s(-1)) at 10 mu M Ca2+. Based on these measurements , as well as previous measurements on the effects of Ca2+ on rod guany late cyclase and the cGMP-gated channel, we have calculated the step r esponse-intensity relation for the rod cell in steady state. This rela tion agrees reasonably well with the relation directly measured from i ntact rods. We have also evaluated the relative contributions from the three Ca2+ effects to rod sensitivity. At low background light intens ities, the Ca2+ modulation of the guanylate cyclase appears to be the most important for sensitivity regulation. At higher light intensities , especially above half-saturation of the response, the Ca2+ modulatio n of the light-stimulated phosphodiesterase shows a progressively sive ly important influence on the light response; it also extends the Webe r-Fechner behavior of the cell to higher intensities. The contribution of the Ca2+ modulation of the cGMP-gated channel is slight throughout .