Figure 8. Estimation of the amplification constant (A) for representative rods of the three genotypes, from single-photon responses (A) and from families of flash responses (B–D).

A, onset phase of single-photon response, a_var(t), for a representative rod from WT (blue), GRK1-tg (red), and GRK7-tg (green) zebrafish. The parabolas (broken lines) plot the fitted predictions of eqn (6), giving A = 1.5, 0.8, and 1.2 s⁻² respectively. The effective delay time was constrained to t_eff= 48 ms for each cell, according to the fitting for bright flash responses. For the flash intensities used, these single-photon responses yielded ‘functional’ effective collecting areas of A_c= 1.99, 1.8 and 0.28 μm² for the three cells. B–D, families of flash responses (black lines) for the three rods in A, fitted with the predictions of the Lamb & Pugh model including capacitive time constant (coloured lines; using eqn (5) of Smith & Lamb (1997); the end time for the calculation of fit was varied from 130 ms for bright-flash responses to 250 ms for dim-flash responses. In order to provide an appropriate fit to the responses to brighter flashes, the capacitive time constant was set to 5 ms, and the residual delay of the remaining stages was set to 43 ms, giving a total effective delay time of t_eff= 48 ms (used above in A). The numbers of isomerizations were determined by extrapolation from the intensities that elicited a single photoisomerization according to the a_var analysis (which we used above to extract the functional effective collecting area). B, WT rod. Flashes delivered from 1.8–900 photoisomerizations for the collecting area of 1.99 μm²; fitted amplification constant A = 1.8 s⁻². C, GRK1-tg rod. Flashes delivered from 1.7–442 photoisomerizations for the collecting area of 1.8 μm²; fitted amplification constant A = 1.0 s⁻². D, GRK7-tg rod. Flashes delivered from 1.2–252 photoisomerizations for the collecting area of 0.28 μm²; fitted amplification constant A = 1.4 s⁻².