Figure 4. Effect of Cs⁺ on the photovoltage following substantial bleaching of visual pigment.

A, photovoltage was measured from the inner segments of rods in retinal slices of Gnat2^−/− mice in darkness and 1 h after 70% bleaching of the visual pigment (n = 6). Inner segment K⁺ conductances were blocked by substituting Cs⁺ for K⁺ in the pipette solution. Flashes at the time indicated by the downward arrow delivered 2.0–510 photons μm⁻² in darkness, 8.0–1000 photons μm⁻² after the 30% bleach, and 85–11 000 photons μm⁻² after the 70% bleach. B, the sensitivity of rod photoresponses was determined from the flash strength that yielded a half‐maximal response. Sensitivity following pigment bleaching (S _F), relative to the sensitivity in darkness (S _F ^D), is plotted versus the fraction of the visual pigment bleached. The best fit Weber–Fechner relationship for photovoltage responses with Cs⁺ internal solution (dashed line) is with k = 20. The Weber–Fechner relationship for photocurrent (dotted line, k = 24) and photovoltage (solid line, k = 8) are shown for comparison. C, the reduction in the maximal amplitude of the response (R _max), normalized to the maximum amplitude in the dark‐adapted state (R _max ^D), is plotted against the fraction bleached. Data were fit with a line constrained to the value of R _max ^D at 0% bleach. Fits indicate that R _max declines less steeply as a function of fraction bleach for the photovoltage compared to the photocurrent, or to the photovoltage measured with a Cs⁺ internal solution.