We have used the truncated outer segment preparation to measure rod cGMP-phosphodiesterase activity, as well as its modulation by Ca2+ in darkness and in light. The basal enzyme activity in darkness was ~0-3 s-1, and was largely independent of Ca2+ concentration from 10 nM to 10 µM. The steady state activity elicited by a step of light (λ = 590 nm) was strongly enhanced by Ca2+, increasing from ~0.005 s-1/(hv µm-2 s-1) at 10 nM Ca2+ to ~0.16 s-1/hv µm-2 s-1) at 10 µM Ca2+. Based on these measurements, as well as previous measurements on the effects of Ca2+ on rod guanylate cyclase and the cGMP-gated channel, we have calculated the step response-intensity relation for the rod cell in steady state. This relation agrees reasonably well with the relation directly measured from intact rods. We have also evaluated the relative contributions from the three Ca2+ effects to rod sensitivity. At low background light intensities, 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+ modulation of the light-stimulated phosphodiesterase shows a progressively important influence on the light response; it also extends the Weber-Fechner behavior of the cell to higher intensities. The contribution of the Ca2+ modulation of the cGMP-gated channel is slight throughout.
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