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Frans Vinberg, Hanna Heikkinen, Ari Koskelainen; Extensive Modulation of R* but Not PDE* Lifetime by Light in Mouse Rods when Registered in the Intact Retina. Invest. Ophthalmol. Vis. Sci. 2011;52(14):6575.
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The average lifetime of PDE* (τE) in mouse rods is believed to be ~0.2 s and the R* lifetime (τR) is thought to be very short (< 50 ms). Light-mediated decline in [Ca2+]i shortens τR through recoverin, but Ca2+ dependent mechanisms of τE modulation have not been identified. Recent data (Woodruff et al. (2008), J. Neurosci. 28:2064-74), however, suggests that also τE is shortened by background light. We studied the modulation of τR and τE (assuming τD = τE) by light in the intact mouse retina with ex vivo ERG.
Transretinal ERG was recorded from isolated mouse retinas at 37oC. The retinas were perfused at the photoreceptor side with bicarbonate buffered Ames medium or modified Ringer’s supplemented with L-15. The photoreceptor component was isolated with 20 -50 µM DL-AP4 and 50 - 100 µM BaCl2. The dominant time constant (τD) was determined from saturated rod flash photoresponses (100 - 3000 R*) in darkness and during steps of light (400 R*s-1). Modulation of τR by light was investigated with the "step/flash" protocol (Fain et al. (1989), J. Physiol. 416:215-43).
τD was 180 ± 6 ms (n = 15) in darkness and 173 ± 12 ms (n = 7) in light-adapted rods. In the seven experiments where τD was determined both in darkness and during steps of light no significant change of τD was observed during light-adaptation. In the step/flash paradigm (4 retinas) the flash responses showed always a clear shortening of saturation time with increasing step intensity Istep. The saturation time of the rod responses to a constant intensity flash decreased as a linear function of Ln(Istep) within the intensity window 80-600 R*s-1 with a slope kΔT = 59 ± 7 ms.
The τD of mouse rods can be robustly determined with ex vivo ERG from the intact retina, and it is similar to the value determined from single dark-adapted rods by suction electrode recordings. τD was not shortened by backgrounds suppressing up to 70% of the rod saturated response amplitude, indicating that the rate-limiting step of saturated rod photoresponse recovery is not modulated by light. Assuming that τD = τE, the slope kΔT obtained in the step-flash -experiments would correspond to ~30% shortening of τR per e-fold increase in step intensity (Eq. (10) in Nikonov et al. (2000), J. Gen. Physiol. 116:795-824).
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