Light-dependent effects of a hydrolysis-resistant analog of GTP on rod photoresponses in the toad retina.

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Abstract
Responses to 100 ms flashes were recorded intracellularly from dark- and light-adapted rod photoreceptors in the isolated retina of the toad, Bufo marinus. Properties of photoresponses were analyzed under each condition of adaptation when retinas were superfused with 1.0 mM guanosine 5''-[.beta.,.gamma.-methylene]triphosphate (p[CH2]ppG), a hydrolysis-resistant analog of GTP. When applied to retinas that previously were subjected to intense light (.simeq. 30% bleach), p[CH2]ppG increased both the amplitude and duration of photoresponses. By contrast, treatment of dark-adapted retinas with p[CH2]ppG did not alter these response parameters. When similarly applied to dark- or light-adapted retinas, GTP had no effect on amplitude or duration of photoresponses. Results are discussed in terms of GTP-dependent mechanisms for rod adaptation.