S O Hemilä | PubMed Health

PubMed1979

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 (DeltaR) 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 I(t)/I(0) = W exp (-t/tau)+P, where W is the extrapolated value for log I(t)/I(0)-P at t = 0 and P is log I(t)/I(0) for t = infinity. When DeltaR increases from 2 to 10% W increases from ca. 2.6 to ca. 5. The time constant tau is about 13 min at 9 degrees C and 7 min at 14 degrees C (DeltaR = 5-10%).3. When the bleaching period is extended, keeping the amount bleached (Ixt) 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 (Q(10) approximately 3).5. The permanent log threshold rise P is approximately proportional to DeltaR after small bleaches; when more than about 10% is bleached the slope of the curve P(DeltaR) decreases. P is considerably larger (about 2.5-fold) for the same fraction bleached in experiments at 14 degrees C as compared to experiments at 9 degrees 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.

Animals↗

PubMed1979

Dark-adaptation in frog rods: changes in the stimulus-response function.

1. Aspartate-isolated photoresponses of the frog's rods to weak and strong flashes have been recorded during dark-adaptation after bleaching a fraction of rhodopsin (generally 4--30%). Stimulus--response functions were measured before the bleach and in the steady state after dark-adaptation. 2. The movements of the operating curve, i.e. the stimulus--response function plotted in a log-log diagram, are interpreted in terms of a model of outer segment adaptation, where the adaptation processes are associated with the transmitter release (Q-adaptation), the number of active sodium channels and leakage channels in the plasma membrane of the outer segment (M-adaptation), and the transmitter background (c1-adaptation). 3. A small bleach in a fully dark-adapted, non-bleached retina brings about a displacement of the operating curve predominantly to the right. The shift back to the left is approximately exponential, typical time constants being 6--12 min. 4. A strong exposure (bleaching 15--30% of rhodopsin) in a previously partially bleached retina brings about a nearly vertical displacement of the operating curve: after the bleach the maximum photoresponse is strongly reduced, and during intermediate adaptation the operating curve returns mainly upwards. 5. Cumulatively increasing permanent displacements of the operating curve are observed in the steady states after successive dark-adaptation transients. The permanent displacements are predominantly to the right and they increase with increasing temperature. 6. The experimental results, as interpreted according to the model, indicate that the Q-adaptation process is dominant in physiological conditions (small or moderate bleaches), whereas the M-adaptation becomes important only after rather large bleaches and especially after several successive bleaches in an isolated retina.

Animals↗