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K O Donner

Publications and source records attributed to K O Donner.

15 recordsLinked to original sources
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↗
PubMed1978

Excitation and adaptation in the vertebrate rod photoreceptor.

The excitatory processes and the reactions of the rhodopsin photoproducts in vertebrate rods are summarized, and an adaptation model of the rod is outlined. The transmitter factor Q describes the ability of the discs to release a transmitter carrying the signal from the site of light adaptation to the plasma membrane of the receptor. If Q decreases appropriately when the background light intensity increases, the response range of the rod is preserved and the rod obeys Weber's law. This kind of receptor adaptation is experimentally observed in several species. A photoisomerization in a disc apparently affects the release of a transmitter also in neighbouring discs. The background light also affects the neuronal network of the retina, preserving the response range of the ganglion cell. After an exposure bleaching an appreciable fraction of rhodopsin the sensitivity of the rod returns in two phases: a fast intermediate adaptation and a slow opsin adaptation. After a rather small bleach the photoresponse is fairly well preserved (mainly Q-adaptation), but large bleaches cause a strong reduction in the response range. Different experimental methods utilized when correlating rhodopsin photoproducts and intermediate adaptation are discussed.

Adaptation, Ocular↗
PubMed1977

Dark adaptaion processes in the amphibian rod.

Rod dark adaptation in the amphibian retina appears to be due to three processes: 1. background adaptation, occurring immediately after the extinction of an adapting or bleaching light, 2. intermediate adaptation, that frequently lasts 30 min or more and 3. opsin adaptation, which in the isolated retina where regeneration of rhodopsin is insignificant, is observed a a permanent loss of sensitivity after the completion of intermediate adaptation. Intermediate adaptation is characterized by a linear relation between log threshold and the amount of "retinal" present, a similar relation is obtained between log threshold and the amount of rhodopsin bleached in opsin adaptation. These adaptation processes are discussed in terms of a model of the rod outer segment.

Amphibians↗
PubMed1968

Visual adaptation of the rhodopsin rods in the frogs retina.

1. The threshold of the discharge from single ganglion cells in the excised and opened frog's eye has been measured with on/off stimuli and test parameters that make it possible to activate the rhodopsin rods only. The test stimuli have been restricted to the central part of the receptive field, where no nervous reorganization can be observed with changes in the state of adaptation.2. When such thresholds and the intensities of the background lights are expressed in terms of the number of quanta absorbed per unit time, it is found that three factors can be correlated with the thresholds measured in various states of light- and dark-adaptation: (i) the intensity of a steady background, (ii) the rate of regeneration of rhodopsin, and (iii) the amount of metarhodopsin II present in the rods.3. The threshold is found to be proportional both to the intensity of a background and to the rate of regeneration, whereas there is a linear relationship between the logarithm of the threshold and the amount of metarhodopsin II.4. The presence of metarhodopsin elevates all thresholds, the absolute threshold, increment thresholds and the thresholds elevated by regenerating rhodopsin in the same way.5. The saturation of the rods at high background intensities is found to be correlated with the accumulation of significant amounts of metarhodopsin in the rods, caused by the bleaching effect of the background.6. The effect of metarhodopsin on the threshold is independent of the amount of rhodopsin present in the rods.7. The combined effect of all three factors can be expressed in a general formula, given as eqn. (7) on p. 74.8. A background not only reduces the signals from the rods illuminated, but also those from neighbouring unilluminated rods. This effect is rapidly decreased with increasing distance from rods covered by the background. This kind of lateral spread in the retina probably occurs also when the rate of regeneration affects the threshold. The effect of metarhodopsin, on the other hand, appears restricted to those receptors that contain this substance.

Adaptation, Ocular↗