We investigated the effects of high concentrations of cytoplasmic cyclic GMP on the photocurrent kinetics and light sensitivity of the tiger salamander rod both in intact cells and in detached outer segments. Photoreceptors were internally perfused with cGMP by applying patch pipettes containing cGMP to the inner or outer segment. Large increases in the concentration of cGMP in the outer segment cytoplasm were achieved only when the patch pipette was applied directly to the outer segment. The dark-current amplitude increased with increasing cGMP concentrations up to approximately 1,400 pA. Internal perfusion with 5.0 mM cGMP introduced a delay of 1-3 s in the photocurrent. The magnitude of the delay was inversely proportional to the light intensity. In addition, the photocurrent time course was slowed down and the light sensitivity, measured 1 s after the flash, was decreased approximately 100-fold when compared with that of the intact cell. The observed effects of cGMP were compared with those predicted by a model that assumes that the initial photocurrent time course is determined by the kinetics of the light-activated phosphodiesterase (PDE) and the cGMP dependence of the light-sensitive channels. At high concentrations of cGMP, the experimental data were similar to those predicted by the model and based on the known biochemical properties of the light-activated PDE and cGMP-activated channels.

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