It has been previously suggested that the overexpression of GRK1 in mouse rods does not affect the kinetics of their light responses.
9 However, examination of the transretinal (
Fig. 2C) and single-photon (
Fig. 3D) responses of our Grk1+ rods revealed a surprising acceleration of response termination compared with WT rods. The effect of rhodopsin kinase expression level on the kinetics of response termination of mouse rods could be better appreciated from their normalized dim flash responses (
Fig. 4A). The threefold reduction in rhodopsin kinase on the deletion of one copy of
Grk1 slowed down the response shutoff, as previously shown.
11 On the other hand, contrary to the report by Krispel et al.,
9 the threefold overexpression of rhodopsin kinase in the Grk1+ mice resulted in notable acceleration of the response shut off. Accordingly, the integration time was also significantly higher in Grk1+/− rods (
P < 0.005) and lower in Grk1+ rods (
P < 0.0001) compared with WT controls (
Table 2). The recovery time constant (τ
rec), believed to reflect the rate-limiting step of the rod phototransduction inactivation, was also affected by the expression level of rhodopsin kinase. It was significantly increased in Grk1+/− rods (
P < 0.0005) and decreased in Grk1+ rods (
P < 0.05) (
Table 2, and
Fig. 4B). To ensure that our measurements reflect the recovery time constant and are not affected by reactions early on in the response shut off, we used only the second half of the response shut off for the single exponential fit determining τ
rec (
Fig 4A, inset). Together, these results demonstrate that, unexpectedly, the dim flash response shut off accelerates with increasing expression of GRK1 leading to a reduction in the recovery time constant. Grk1+b rods expressing rhodopsin kinase at twice the WT level also showed a trend of accelerated response shut off, though the difference with WT rods did not reach statistical significance (
Table 1, and
Fig. 4B). Thus, the effect of GRK1 overexpression appears to be dose-dependent so that a clear physiological phenotype becomes detectable only when GRK levels become sufficiently higher than in WT rods.