DARK-ADAPTATION OF THE ASPARTATE-ISOLATED ROD RECEPTOR POTENTIAL OF THE FROG RETINA - THRESHOLD MEASUREMENTS

1. The dark‐adaptation of the aspartate‐isolated rod receptor potential of the isolated and perfused frog retina has been measured after bleaching about 5‐10% of the rhodopsin. The fraction bleached (ΔR) and the decay of rhodopsin photoproducts were determined using alternating measurements with a photometric technique (Donner & Hemilä, 1975). 2. The dark‐adaptation time course of the log threshold elevation is exponential, log It/I0 = W exp (‐t/τ)+P, where W is the extrapolated value for log It/I0‐P at t = 0 and P is log It/I0 for t = ∞. When ΔR increases from 2 to 10% W increases from ca. 2·6 to ca. 5. The time constant τ is about 13 min at 9 °C and 7 min at 14 °C (ΔR = 5‐10%). 3. When the bleaching period is extended, keeping the amount bleached (I×t) constant, dark‐adaptation is completed earlier. 4. The time course of dark‐adaptation and the decay of the photoproduct ‘retinal’ are similar, as is also their dependence on temperature (Q10 ≈ 3). 5. The permanent log threshold rise P is approximately proportional to ΔR after small bleaches; when more than about 10% is bleached the slope of the curve P(ΔR) decreases. P is considerably larger (about 2·5‐fold) for the same fraction bleached in experiments at 14 °C as compared to experiments at 9 °C. 6. A comparison with previously obtained corresponding values for dark‐adaptation after small bleaches at the ganglion cell level shows a close agreement between the time constants for the dark‐adaptation curve, its range and the dependence of threshold on the fraction of rhodopsin bleached. © 1979 The Physiological Society