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K Hamdorf

Publications and source records attributed to K Hamdorf.

11 recordsLinked to original sources
PubMed1990

Microvillar components of light adaptation in blowflies.

The process of light adaptation in blowfly photoreceptors was analyzed using intracellular recording techniques and double and triple flash stimuli. Adapting flashes of increasing intensity caused a progressive reduction in the excitability of the photoreceptors, which became temporarily suppressed when 3 x 10(6) quanta were absorbed by the cell. This suppression was confirmed by subsequently applying an intense test flash that photoactivated a considerable fraction of the 10(8) visual pigment molecules in the cell. The period of temporary desensitization is referred to as the refractory period. The stimulus intensity to render the receptor cell refractory was found to be independent of the extracellular calcium concentration over a range of 10(-4) and 10(-2) M. During the refractory period (30-40 ms after the adapting flash) the cell appears to be "protected" against further light adaptation since light absorption during this period did not affect the recovery of the cell's excitability. Calculations showed that the number of quantum absorptions necessary to induce receptor refractoriness is just sufficient to photoactivate every microvillus of the rhabdomere. This coincidence led to the hypothesis that the refractoriness of the receptor cells is due to the refractoriness of the individual microvilli. The sensitivity of the receptor cells after relatively weak adapting flashes was reduced considerably more than could be accounted for by the microvilli becoming refractory. A quantitative analysis of these results suggests that a photoactivated microvillus induces a local adaptation over a relatively small area of the rhabdomere around it, which includes several tens of microvilli. After light adaptation with an intense flash, photoactivation of every microvillus by the absorption of a few quanta produced only a small receptor response whereas photoactivation of every rhodopsin molecule in every microvillus produced the maximum response. The excitatory efficiency of the microvilli therefore increases with the number of quanta that are absorbed simultaneously.

Adaptation, Ocular↗
PubMed1987

Photoreconvertible fluorophore systems in rhabdomeres, Semper cells and corneal lenses in the compound eye of the blowfly.

1. The primary aim of the experiments described in this article was to localize the origin of the complex fluorescence in the compound eye of flies. The eye tissue was dissected and the fluorescence from cells and cell organelles was recorded by microspectrofluorometry. Using this technique, fluorophore systems were detected in the rhabdomeres, Semper cells and corneal lenses. The fluorophore systems are photoreconvertible by UV and blue light. 2. The fluorophore systems in the rhabdomeres and Semper cells are similar. The intensity of the fluorescence from the microvilli is enhanced up to 29 X by adaptation to UV light. The enhancement is inversely related to the rhodopsin content in the microvilli, indicating that the chromophoric group of the fluorophore is not a vitamin A derivative. 3. The enhancement of the fluorescence by UV light strongly depends on pH, suggesting that the photoreconvertible fluorophore systems in the microvilli and Semper cells are photosensitive redox pigments. These redox systems are probably located in the membranes of the microvilli in the photoreceptors, and in the endoplasmic reticulum of the Semper cells, or they are coupled to filaments in the cytoskeleton of both cell types. 4. Preliminary reaction schemes for the photoreactions based on the recorded excitation and emission spectra and photokinetics were developed. A primary pigment in the microvillous structure, AR, or in organelles in the Semper cells, AS, is converted by UV light into an excited state AR* or AS*, which either relaxes to the primary pigment by photon emission, or converts into an intermediate X, which by proton uptake changes into stable products, BR or BS. Blue illumination converts BR and BS into the excited states BR* and BS*, which either relax by photon emission to BR or BS, or convert into an intermediate Y, which after deprotonation reconverts into the primary pigment AR or AS. 5. Estimation of the molecular density showed that the concentration of the fluorophore in the microvilli presumably is almost equal to maximal rhodopsin concentration. The high density suggests that the fluorophores have a specific function in transduction or adaptation of the visual process.

Animals↗
PubMed1983

Alkaline phosphatase. A dominant enzyme of microvillus structure of cephalopod photoreceptors.

The rhabdomeres of cephalopod photoreceptors, which are built up mainly of rhodopsin and phospholipid molecules, show a very high alkaline phosphatase activity. The enzyme has been partially characterized in purified rhodopsin vesicle fractions of the rhabdomeres by the following kinetic data: pH optimum 8.7; activation energy 9100 cal . m-1; Vmax = 2.5 mumol . min-1 . mg-1; Km = 1.5 x 10(-4) M; its activity depends on Mg2+. There is good evidence that the alkaline phosphatase is a membrane-bound enzyme with receptor sites presumably located on the inside of the membrane. This enzyme has not been purified but its high activity compared to that of other known alkaline phosphatases (see Table I) indicates that each mirovillus, the structural unit of the rhabdomere, contains 1-20 enzyme molecules. This finding supports the hypothesis that the alkaline phosphatase is involved in the biochemical amplification process of excitation, or adaptation.

Alkaline Phosphatase↗
PubMed1983

Two late response components in on-off ganglion cells of the frog retina: the delayed response-generated by red rods; the second off-response-generated by green rods.

Frog on-off retinal ganglion cells react to diffuse, large-sized stimuli not only with an "early response" and a "delayed response" but also with a "second off-response". The intensity at which the delayed response becomes visible coincides with the quantum amount necessary for saturating the red rod response. The spectral sensitivity of the delayed response fits the nomogram for the 502 nm pigment. Unlike the delayed response, the second off-response appears earlier with increasing stimulus intensity and follows the 433 nm nomogram. Therefore, it is most probably generated by the green rods.

Animals↗
PubMed1980

Reversible events in the transduction process of photoreceptors.

In photoreceptors, a latency of many milliseconds elapses between the absorption of a light quantum and the occurrence of the late receptor potential, even for strong light stimuli. Surprisingly, this is much longer than the time necessary for conductance changes such as occur in membranes of neurones or muscles, mediated by chemical transmitters. There are several possible explanations for the long photoreceptor latency. (1) It may be due to properties of the visual pigment molecules. For instance, the temporal coincidence of the occurrence of metarhodospin II with the receptor signal indicates that the meta I-meta II transition might be the trigger for the electrical response in vertebrate photoreception. (2) It may be explained by properties of transport processes. Such a time consuming process could be the diffusion of an internal 'transmitter substance', which diffuses to a 'pore' in the receptor membrane. (3) A third possibility is the time needed to produce and accumulate chemical substances. The light-induced change of the visual pigment molecule might trigger a chemical reaction chain, in which the product of an earlier step triggers the next one. The experiments described here show that a considerable part of the long latency in photoreception is due to processes that are localised at the level of the visual pigment molecule.

Animals↗
PubMedDate not supplied

Rhodopsin particles in the photoreceptor membrane of an insect.

Electron-microscopic examination of freeze-fractured fly retinae has revealed the presence of particles, 80 to 100 A in diameter, on the photoreceptor membrane. Flies which were raised on a vitamin-A deficient diet show a substantial reduction in the density of such particles. The reduction in particle density is in agreement with the reduction in visual-pigment concentration as measured spectrophotometrically for these flies. These results suggest that the particles are identical with molecules of the visual pigment, rhodopsin.

Animals↗