Intracellular injection of cyclic-GMP increases sodium conductance in gecko photoreceptors.

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Abstract
In order to examine the possibility that cGMP is an internal transmitter in the process of phototransduction, cGMP was iontophoretically injected into a gecko photoreceptor from 1 barrel of a double-barrelled microelectrode with the other barrel monitoring membrane potential and conductance changes. cGMP induced a prolonged depolarization accompanied by a marked increase in membrane conductance. In the Na-free bathing solution (substituted by choline), injection of cGMP did not cause any depolarization. The cells, which showed the cGMP-induced depolarization, produced no significant potential changes with injection of 5''-GMP or cAMP, indicating that the depolarization was cGMP-specific. The results suggest that cGMP is involved in the phototransduction process in the gecko photoreceptors. Uncertainty still remained as to whether the cGMP level directly correlates with the photoreceptor membrane potential.
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