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G. L. Fain, M. L. Woodruff, S. H. Tsang, Sr.; Decrease in Sensitivity Without Acceleration of Response Decay in PDEgamma T35A Mouse Rods Exposed to Background Illumination. Invest. Ophthalmol. Vis. Sci. 2007;48(13):2845.
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© ARVO (1962-2015); The Authors (2016-present)
Recent experiments suggest that in mouse (unlike salamander), acceleration of response decay in backgrounds is caused by modulation of PDE turn-off rate and is unrelated to the change in sensitivity. We have examined this possibility in PDEgamma T35A and T22A/T35A mutant rods, which lack one or both of the two sites of PDEgamma phosphorylation.
Suction-electrode recordings were made from WT, T35A, and T35A/T22A rods with standard techniques.
In WT rods, backgrounds produce an acceleration of time course of decay (tau_REC) and Pepperberg constant (tau_D); in T35A rods both tau_REC and tau_D are slower in darkness than WT but are unaffected by backgrounds, even though T35A rod sensitivity declines in backgrounds nearly as in WT. This has the effect that responses to equal intensity flashes are smaller in backgrounds than in darkness but show little if any change in response waveform. When however the background is turned off, sensitivity increases but response decay suddenly accelerates and only slowly recovers to the dark-adapted value of tau_REC, with a time course much as in WT rods (Krispel et al, JGP 122:702-712, 2003). These effects do not seem to be due to removal of the T35A site per se, since in T35A/T22A mutant rods which lack both gamma phosphorylation sites, the response waveform changes in backgrounds like WT.
Our data together with experiments of the Burns laboratory on WT rods indicate that acceleration of response decay in mouse rods in background light is produced by modulation of the rate-limiting step (TalphaGTP hydrolysis) and is not intimately linked to the change in sensitivity. The rate of cGMP synthesis and Ca exchange may be fast enough in a mouse rod even in darkness so that further acceleration of the cyclase feedback loop and increase in feedback gain with increasing background light has little effect on response waveform. Our results suggest that there may be two mechanisms of waveform modulation, one produced in the presence of the background (and mostly blocked in T35A) and the other becoming apparent after the background has turned off. Experiments which might explain why substitution at the T35 site interrupts the first of these mechanisms but not the second are in progress.
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