Data on the time-course of the light-activation of the cyclic GMP phosphodiesterase and of the GTPase, and results on the influence of cyclic GMP on the disc membrane permeability are presented. On the basis of the kinetic data, it is not possible to separate the light-activation of these two enzymes from the early steps of photoreceptor transduction. In addition, the cyclic GMP increases the permeability of the disc membranes, indicating that a decrease of the endogenous cyclic GMP concentration, consequent to the light-activation of the phosphodiesterase, can decrease the membrane permeability shortly after illumination. © 1980.
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