Recovery phase of the murine rod photoresponse reconstructed from electroretinographic recordings

The activation and recovery phases of the murine rod photoresponse were determined from corneal electroretinograms (ERGs) obtained in response to pairs of full-field flashes producing 50-10⁵ photoisomerized rhodopsins (R*) per rod. The a-wave component of the ERG in response to the initial flash provided a well established measure of the activation phase of the red response. The amplitude of the a-wave response to an intense second flash (45,000 R*) delivered 0.2-5 seconds (s) after the first flash was used to reconstruct the recovery phase of the response. For 160-3000 R* rod^-1, recovery curves were isomorphic, translating on the time axis such that each e-fold increase in R* produced an incremental recovery delay of τ(c) = 210 ± 50 ms (mean ± SD). For initial flashes producing >3000 R*, recovery curves lost their initial isomorphism and half-times had intensity dependence exceeding 1 s per e-fold increase in R*. We conclude that for flashes producing <3000 R*, the effective lifetime of these R* is not >210 ms. Two extant and non-mutually exclusive hypotheses are discussed that can account for the sharp increase in recovery times from flashes producing >3000 R*. They are as follows: (1) ~0.03% of R* have a lifetime exceeding 1 s; and (2) the γ subunit of phosphodiesterase (PDE(γ)) serves as a GTPase- activating factor, and 3000 R* produce sufficient activated G-protein (G*) to exceed the total quantity of PDE(γ) subunits such that excess G* must wait for unoccupied PDE(γ) to inactivate via GTP hydrolysis.