The effect of the type V/VI-selective phosphodiesterase inhibitor, Zaprinast, (200 μM) on the light-evoked extracellular field potential (EFP) in the isolated rat retina was tested under dark- and light-adapted conditions at two different temperatures. Peak enhancement EFP in dark- (344±70%; mean±SEM) and light-adapted (182±31%) retina at 37°C was reached within 3 min of treatment with Zaprinast (200 μM) followed by a slower decrease to a level of 85±14 and 26±7% in dark- and light-adapted retina, respectively. The effect of Zaprinast (20 μM) on the pharmacologically-isolated photoreceptor component of the EFP was lost with increasing levels of background light. This may suggest that there is a slow time scale (minutes) shift in the steady state level of cGMP during light-adaptation.
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