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At least 109 records · Page 6Linked to original sources
PubMed1956

Increased photoreversal of ultraviolet injury by flashing light.

1. Photoreversal of ultraviolet (UV) injury was studied in the ciliate protozoan Tetrahymena pyriformis (geleii) strain W, cultured in the absence of other living organisms. The division pattern of progeny of single animals was followed in hanging drop preparations. 2. A sublethal dose of 450 ergs/mm.(2) of monochromatic UV of wave length 2654 A produces a lag before the first division followed by a period of cessation of fission after the second division. This cessation sometimes lasts as long as 6 weeks, during which time the animals become smaller and rounder and more opaque. Organisms about to resume division increase in size and transparency; after a few divisions the animals regain their normal division rate. 3. The effect of UV ranging in intensity from 5 to 15 ergs/mm.(2)/sec. was found to obey the reciprocity law quite well for the UV effect on the division pattern of T. pyriformis. However, the same dose at lower and at higher intensities was less effective. 4. The effect of a dose of UV delivered at high intensity (19 ergs/mm.(2)/sec.) could be increased by flashing the light, indicating that the system became saturated in the continuous light. 5. A photoreversing dose of monochromatic blue light of wave length 4350 A was found to be more effective when delivered as continuous light at a low intensity, or as intermittent light at a high intensity, rather than as continuous light at the high intensity-indicating that a dark mechanism participates in photoreversal. 6. The time for the dark reaction was determined to be of the order of a few hundredths of a second in experiments in which different lengths of dark period were used while maintaining a constant light period of 0.0025 second. 7. For Colpidium colpoda the efficiency of a given dose of photoreversing light was increased by flashing the light. 8. The present experiments are interpreted in terms of data available in the literature.

Animals↗
PubMed1956

Photoreversal of nuclear and cytoplasmic effects of short ultraviolet radiation on Paramecium caudatum.

1. Irradiation with three short ultraviolet (UV) wave lengths, 226, 233, and 239 mmicro rapidly immobilizes Paramecium caudatum, the dosage required being smaller the shorter the wave length. 85 per cent of paramecia immobilized with wave length 226 mmicro recover completely. Recovery from immobilizing doses is less the longer the wave length. 2. Irradiation continued after immobilization kills the paramecia in a manner which is markedly different for very short (226, 233, and 239 mmicro) and longer (267 mmicro) wave lengths. 3. An action spectrum for immobilization in P. caudatum was determined for the wave lengths 226, 233, 239, 248, and 267 mmicro, and found to resemble the absorption of protein and lipide in the wave length region below 248 mmicro. Addition of these data to those of Giese (1945 b) gives an action spectrum resembling the absorption by albumin-like protein. 4. Division of P. caudatum is delayed by doses of wave lengths 226, 233, and 239 mmicro which cause immobilization, the longest wave length being most effective. 5. Immobilization at any of the wave lengths tested (226, 233, 239, 248, 267 mmicro) is not photoreversible when UV-treated paramecia are concurrently illuminated. 6. Division delay resulting from immobilizing doses of 226, 233, and 239 mmicro is photoreversible by exposure to visible light concurrently with the UV. 7. Division delay induced by exposure to wave length 267 mmicro is reduced by exposure to visible light applied concurrently with UV or immediately afterwards. 8. The data suggest that the shortest UV wave length tested (226 mmicro) affects the cytoplasm selectively, because it is absorbed superficially as indicated by unilateral fluorescence in UV. Consequently it immobilizes paramecia rapidly but has little effect on the division rate because little radiation reaches the nucleus. 9. The data support the view that nuclear effects of UV are readily photoreversed but cytoplasmic effects are not.

Ciliophora↗
PubMed1956

The responses of the pupil of Gekko gekko to external light stimulus.

1. The responses of the pupil of a nocturnal gecko (Gekko gekko) to external light stimulus were studied. 2. The responses of the pupil are determined by light entering the pupil and not by light acting directly on the iris. 3. The responses of the pupil are very uniform in sensitivity including spectral sensitivity for light coming in different directions to the eye. 4. The possible change in area of the pupil is more than 300-fold and probably represents an effort to shield the pure rod retina from saturating light intensities. 5. The pupil continues to contract sharply for changes in external light intensity which give retinal illuminations corresponding to 10(6) quanta/sec. striking a retinal rod. 6. There is a large degree of spatial summation of the response; circular external light fields subtending 5 and 140 degrees giving the same illumination at the pupil give approximately the same pupil response. 7. The spectral sensitivity curve agrees with the absorption curve of an extracted pigment from a closely related gecko described by Crescitelli in the followig paper. It is similar to the human scotopic curve but its maximum is displaced about 20 to 30 mmicro towards the red end of the spectrum. The fall in sensitivity towards the red end of the spectrum is described by the equation See PDF for Equation

Humans↗
PubMed1957

The effect of ultraviolet and white light on growth rate, lysis, and phage production of Bacillus megatherium.

Cultures of megatherium 899a, growing under different conditions, were exposed to ultraviolet or white light. 1. Cultures exposed to ultraviolet light and then to white light continue to grow at the normal rate. Cultures exposed to ultraviolet light and then placed in the dark grow at the normal rate for varying lengths of time, depending on conditions, and then lyse with the liberation of from 5 to 1000 phage particles per cell, depending on the culture medium. 2. Increasing the time of exposure to ultraviolet light results in an increase in the fraction of cells which lyse in the dark. The lysis time decreases at first, remains constant over a wide range of exposure, and then increases. The lysis can be prevented by visible light after short exposure, but not after long exposures. 3. The time required for lysis is independent of the cell concentration. 4. Effect of temperature. After exposure to ultraviolet the cell concentration increases about 4 times at 20 degrees , 30 degrees , or 35 degrees C., but only 1.5 to 2.0 times at 40-45 degrees . This is due to the fact that the growth rate of the culture reaches a maximum at 38 degrees while the lysis rate increases steadily up to 45 degrees . 5. Terramycin decreases the growth rate and lysis rate in proportion. 6. At pH 5.1, the cultures continue to grow slowly in the dark after exposure to ultraviolet light. 7. Megatherium sensitive cells infected with T phage lyse more rapidly than ultraviolet-treated 899a, and visible light does not affect the lysis time. The results agree with the assumption that exposure to ultraviolet results in the production of a toxic (mutagenic) substance inside the bacterial cell. This substance is inactivated by white light.

Bacillus megaterium↗
PubMed1957

Sensitivity to light in the sea anemone Metridium senile (L). II. Studies of reaction time variability and the effects of changes in light intensity and temperature.

1. The reaction time of the photosensitive response of Metridium was found to be composed almost entirely of a sensitization period. If a latent period exists, it is too short to be detected by the methods used and is probably less than a second in duration. The length of the reaction time, therefore, was used as a measure of the radiant energy necessary at any intensity to elicit a response. 2. The length of the reaction time was found to vary randomly by a factor of seven under constant environmental and stimulating conditions. Determination of large numbers of reaction times on several anemones, grouped according to length, gave closely similar distributions resembling Poisson distributions. It was suggested that the variability may be caused by the same factor or mechanism in each individual. An experimental scheme was presented for determining the expected error and variation in statistical quantities when groups of ten observations are used. 3. The means of groups of reaction times determined at different intensities formed a hyperbolic relationship when plotted against intensity, suggesting that the animal obeys the Bunsen-Roscoe law of reciprocity. No marked changes were noted of per cent deviation from the mean or of randomness at different intensities. Stimulation of Metridium requires roughly 5 x 10(9) incident quanta/cm.(2) of blue-green light. 4. No temperature effect could be found on either the means, the per cent deviation from the mean, or the degree of randomness of series of about ten observations, studied over a 17 degrees C. range. It was concluded that the variability in reaction time was not due to changes in any complex physiological state such as muscular tone. 5. A possible use of the photosensitive response is suggested and the potentialities of "integrative" photosensitivity are discussed. 6. Possible mechanisms to explain the variability in reaction time are discussed. In the light of evidence presented, the most likely hypothesis appears to be the uncertainty of quantum capture caused by low concentrations of photosensitive pigment. Assuming the validity of this hypothesis, evidence suggests that the anemones responded to less than 10 quanta of absorbed light. Caution, however, is recommended in accepting the hypothesis because of the indirect nature of the supporting evidence.

Animals↗
PubMed1957

Blue light and the regeneration of human rhodopsin in situ.

Hubbard has found that the photoisomerization of retinene was important for the regeneration of rhodopsin in vitro, and the object of the present investigation was to find whether this was also true for regeneration in the living human eye. In the Appendix is described a device which permits the rhodopsin density to be measured by analysing the light reflected from the fundus oculi in an ophthalmoscopic arrangement, the measurement taking about 5 seconds. Now if a blue and a yellow light viewed scotopically are adjusted in intensity so as to appear identical, they must bleach rhodopsin equally, but the blue will be more than 10 times as effective in isomerizing retinene. Therefore if retinene isomerization is important for rhodopsin regeneration, blue light should cause a more rapid regeneration after bleaching, and during bleaching the equilibrium level attained should be less profound. But, as the figures show, the course of bleaching and regeneration is identical for the matched yellow or blue bleaching lights, therefore isomerization of retinene is not important for rhodopsin regeneration in the living human eye.

Humans↗