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PubMed1979

The quantum yield of flash-induced proton release by bacteriorhodopsin-containing membrane fragments.

The quantum yield of proton release by bacteriorhodopsin was measured from volume changes after excitation of purple membrane fragments by short flashes. At low ionic strengths, about 0.25 mol of protons is released per einstein absorbed. This agrees well with quantum yields reported recently for the conversion of bacteriorhodopsin into a metastable state (M) that absorbs near 412 nm. However, the quantum yield of proton release increases gradually with increasing ionic strength; it plateaus with a value of 0.43 +/- 0.03 at ionic strengths above 200 mM. Changing the ionic strength has no detectable effect on the quantum yield of formation of the M spectral state. It thus appears that as many as two protons can be released and rebound in each photochemical cycle at high ionic strengths. The quantum yield of proton release is essentially independent of pH over the range 6.0-8.75. The quantum yield decreases with increasing flash strength, apparently due to photoreversal of the initial photochemical reaction.

Bacteriorhodopsins↗

PubMed1992

Quaternary structure and geminate recombination in hemoglobin: flow-flash studies on alpha 2CO beta 2 and alpha 2 beta 2CO.

The kinetics of geminate recombination for the diliganded species alpha 2CO beta 2 and alpha 2 beta 2CO of human hemoglobin were studied using flash photolysis. The unstable diliganded species were generated just before photolysis by chemical reduction in a continuous flow reactor from the more stable valency hybrids alpha 2CO beta 2+ and alpha 2+ beta 2CO, which could be prepared by high pressure liquid chromatography. Before the flash photolysis studies, the hybrids had been characterized by double-mixing stopped-flow kinetics experiments. At pH 6.0 in the presence of inositol hexaphosphate (IHP) both of the diliganded species show second order kinetics for overall addition of a third CO that is clearly characteristic of the T state (l' = 1-2 x 10(5) M-1 s-1), whereas at higher pH and in the absence of IHP they show combination rates characteristic of an R state. The kinetics of geminate recombination following photolysis of a bound CO, however, showed little dependence on pH and IHP concentration. This surprising observation is explained on the basis that the kinetics of geminate recombination of CO primarily depends on the tertiary structure of the ligand binding site, which apparently does not differ much between the R state and the liganded T state formed on adding IHP in this system. Since this explanation requires distinguishing different tertiary structures within a particular quaternary structure, it amounts to a contradiction to the two-state allosteric model.

Chromatography, High Pressure Liquid↗

PubMed1994

Effect of photoregeneration on the calculation of the amount of rhodopsin bleached by small flashes.

A sample of rhodopsin that is exposed to a series of small light flashes of equal intensity is expected to bleach in successively smaller decrements in proportion to the remaining unbleached rhodopsin. The exponential depletion law describing this effect has been used as a rapid, convenient, and intuitive method for determining the fraction of rhodopsin bleached per flash. This method is commonly assumed to be free of error provided the amount bleached is small, so that there is no significant photoregeneration. We show here, however, that if there is any photoregeneration, the bleach fraction calculated in this manner can be in error by a factor of two or more, no matter how little rhodopsin is bleached. This flaw occurs insidiously, without perturbing the expected exponentiality of the bleaching decrements, thereby escaping ready notice. The erroneous bleach values readily propagate as underestimates of metarhodopsin and accompanying G-protein equilibrium and kinetic constants. We derive equations for correcting such errors and illustrate how empirical constants can be obtained from experiments that permit the true fraction bleached to be determined.

Animals↗

PubMedDate not supplied

Flash freezing of erythrocyte suspensions.

Freezing whole blood in bulk usually results in severe cellular destruction through the action of ice crystals and osmotic effects in the freezing liquid. The potential of flash freezing blood aerosols onto a liquid nitrogen surface as a means of inhibiting cellular damage was studied in this work. Three commercial spraying devices were employed to spray-freeze either whole blood or concentrated erythrocyte suspensions, using hydroxyethyl starch (HES) as a cryoprotectant. The integrity and viability of the processed cells were assessed by measuring gross rheological properties and the extent of hemolysis. Cells were found to be susceptible to the very high shear stresses imposed by some of the spraying devices. Bulk freezing of blood, even in the presence of cryoprotectant, resulted in complete cellular destruction. Whereas flash freezing was capable of substantially reducing the level of hemolysis to 12.6% and preserving the cellular deformability.

Aerosols↗

PubMed1992

Flash and pattern reversal visual evoked responses in normal and demented elderly.

P1 and P2 components of visual evoked responses (VERs) to flash stimuli were compared between patients diagnosed as suffering from Senile Dementia of the Alzheimer Type (AD) and elderly controls. Additionally the P100 component of the VER to pattern reversal stimuli at high and low contrast and using large and small check patterns was compared. Significant differences between moderate AD patients and the normal elderly were found for the P2 component of the flash stimulus and for the P100 component of the high spatial frequency pattern reversal stimuli at low contrast. These findings suggest a deficit in primary visual processing and a selective deficit in secondary visual processing in moderate cases of dementia. This indicates that visual processing capacities of AD patients decline as the disease progresses in line with the continual degeneration of neuronal populations involved with vision.

Aged↗

PubMed1998

Reactivity of beta-carotene towards peroxyl radicals studied by laser flash and steady-state photolysis.

Peroxyl radicals, as model for peroxyl radicals formed during autoxidation of lipids, have been generated in three solvent systems (cyclohexane, tetrahydrofuran and tert-butanol/water) by steady-state and laser flash photolysis, and their reaction with beta-carotene studied. Steady-state photolysis experiments showed that alkyl, alkoxyl and alkylperoxyl radicals all react with beta-carotene. However, laser flash photolysis experiments indicated that the reaction with peroxyl radicals (second-order rate constant estimated to be less than 10(6) M(-1) s(-1)) is slower than with alkyl and alkoxyl radicals, and that beta-carotene is hence a poor direct scavenger of peroxyl radicals. Scavenging of peroxyl radicals by beta-carotene is suggested not to proceed via electron transfer but rather by adduct formation and/or hydrogen abstraction. For different phenoxyl radicals, differences in reactivity towards beta-carotene seem to be correlated with standard reduction potential.

Cyclohexanes↗

PubMed1998

Exposure of Synechocystis 6803 cells to series of single turnover flashes increases the psbA transcript level by activating transcription and down-regulating psbA mRNA degradation.

Exposure of Synechocystis sp. PCC 6803 cells to series of single turnover flashes increases specifically the level of psbA and psbD2 messages, encoding the D1 and D2 proteins of photosystem II, as compared to light exposed cells. This increase is due to maintenance the transcription rate as high as in growth light and to the down-regulation of transcript degradation as in darkness. Inhibition of the plastoquinone pool reduction by DCMU or its oxidation by DBMIB does not diminish the transcription of the psbA gene under growth conditions. However, the degradation rate of psbA transcript, as well as of other transcripts encoding proteins of thylakoid complexes, is down-regulated in all conditions leading to the oxidation of the plastoquinone pool. We conclude that single turnover flashes are sensed as 'light' by transcription machinery of the cells irrespective of the plastoquinone pool reduction state and as 'dark' by the transcript degradation system.

Cyanobacteria↗

PubMed2000

On-column refractive index detector for flash chromatography.

A universal detector for on-column analysis in flash chromatography is reported. The detection scheme takes advantage of refractive index changes as analytes move through an illuminated region of the column. The column packing material is a diffuse scattering medium when the refractive index of the solvent is significantly different than that of the packing material. The magnitude of the 'signal' depends on the degree to which the refractive index mismatch is changed. An empirical model that qualitatively accounts for the observed trends is presented. This detection scheme provides a simple, inexpensive means to monitor the end of a flash chromatography column in order to determine the exit time of the species of interest, thus greatly reducing the post-column analysis time. Additionally, the detector is movable along the length of the column, offering the potential to monitor separations as they occur.

Calibration↗

PubMed2000

Crossed-uncrossed difference in simple reaction times to lateralized flashes: between- and within-subjects variability.

In unimanual reaction times (RT) to lateralized flashes, contralateral responses tend to be slower than ipsilateral responses. This has been called Crossed-Uncrossed Difference (CUD). The CUD tends to show variability across subjects and across studies, but until now the stability of the CUD in an individual subject has not been investigated. To address the role of inter- and intra-subject variability in the CUD, three normal right handers were tested over 50 experimental sessions of 800 trials each, for a total of 40,000 trials of simple reaction times to lateralized flashes. In each subject, CUDs were computed for each session, over two, three, or more sessions, and over the entire dataset. These CUDs were then compared to the CUDs obtained in a group of 15 normal right handers, each tested once in a single session. Results show that: (i) CUD variability across several sessions in a single subject mimics the variability observed in a sample of subjects tested in a single session; (ii) this variability is considerably reduced when the CUD is computed over at least 2400 trials per subject; (iii) CUDs computed over 2400 and up to 12,000 of trials tend to be extremely similar ( approximately 2 ms) across the three subjects tested here; (iv) when reaction times are ordered from the fastest to the slowest and divided into bins, the CUD is remarkably stable over the entire reaction time distribution; and (v) in contrast to the variability of the CUD, the variability for crossed and uncrossed responses across several sessions in a single subject is small and does not mimic the variability observed in a sample of subjects tested in a single session. Taken together, these data suggest that the intersubject variability in the CUD observed in single experimental sessions does not represent a reliable intersubject difference and that the CUD computed over thousands of trials reflects hard-wired mechanisms of callosal transmission.

Adult↗

PubMed1997

A quantitative estimate of flash-induced Ca(2+)- and Na(+)-influx and Na+/Ca(2+)-exchange in blowfly Calliphora photoreceptors.

The flash-induced Ca(2+)- and Na(+)-influx and Na+/Ca(2+)-exchange activity in blowfly Calliphora photoreceptors were investigated. The change in membrane potential, induced by a bright flash, was intracellularly measured in vivo. Based on a biophysical photoreceptor model, the Na(+)- and Ca(2+)-currents and concentration changes were determined from the first transient depolarization phase of the photoreceptor response. The activity of Na+/Ca(2+)-exchange was determined from the after depolarization phase. It appeared that the Na(+)-influx by Na+/Ca(2+)-exchange is about twice that through light-activated channels, suggesting a substantial contribution of Na+/Ca(2+)-exchange to Na(+)-regulation.

Animals↗

PubMed1998

Motion detection on flashed, stationary pedestal gratings: evidence for an opponent-motion mechanism.

Contrast thresholds were measured for discriminating left vs right motion of a vertical, 1 c/deg luminance grating lasting for one cycle of motion. This test was presented on a 1 c/deg stationary grating (pedestal) of twice-threshold, flashed for the duration of the test motion. Lu and Sperling [(1995). Vision Research, 35, 2697-2722] argue that the visual system detects the underlying, first-order motion of the test and is immune to the presence of the stationary pedestal (and the 'feature wobble' which it induces). On the contrary, we observe that the stationary pedestal has large effects on motion detection at 7 and 15 Hz, and smaller effects at 0.9-3.7 Hz, evidenced by a spatial phase dependency between the stationary pedestal and moving test. At 15 Hz the motion threshold drops as much as five-fold, with the stationary pedestal in the optimal spatial phase (i.e., pedestal and test spatially in phase at middle of motion), and the perceived direction of the test motion reverses with the pedestal in the opposite phase. Phase dependency was also explored using a very brief (approximately 1 msec) static pedestal presented with the moving test. The pedestal of Lu and Sperling (flashed for the duration of the test) has a broad spectrum of left and right moving components which interact with the moving test. The pedestal effects can be explained by the visual system's much higher sensitivity to the difference of the contrast of right vs left moving components than to either component alone.

Contrast Sensitivity↗

PubMed1999

Properties of the flash visual evoked potential recorded in the cat primary visual cortex.

The flash visual evoked potential (F-VEP), elicited by a 100 ms diffuse light flash presented at 2 Hz, was examined in the cat primary visual cortex (Area 17). Intracortical F-VEP depth profiles were recorded to characterize waveform changes with electrode depth. A positive surface component, with a latency of 200 ms, was the dominant waveform feature within the cortex, reversing in polarity and increasing in magnitude as the cortex was penetrated. Other prominent components with latencies of 30, 50, 100, and 125 ms were also observed. Changes in the waveform with stimulus duration and illumination were examined and revealed the sensitivity of prominent components to stimulus parameters.

Animals↗

PubMed1999

Effects of dizocilpine (MK-801) on flash-evoked potentials, body temperature, and locomotor activity of hooded rats.

The present study examined the effects of Dizocilpine (MK-801; a noncompetitive N-methyl-D-aspartate receptor antagonist) on flash-evoked potentials recorded from both the visual cortex (VC) and superior colliculus (SC) of chronically implanted hooded rats. The potentials were recorded at 5, 20, and 35 min following i.p. injections of saline, and of 0.1, 0.3, and 1.0 mg/kg MK-801 on separate days. The amplitude of VC component P1 was unaltered following drug treatment. N1 was increased in amplitude by the 0.1-, 0.3-, and 1.0-mg/kg doses, while two other negative peaks in the VC emerged, beginning with the 0.1-mg/kg dose, to complicate the waveform. One negative peak developed between N1 and P2, while the other effectively split peak P2 (forming P2A and P2B). P2A was depressed at all doses, while P2B was depressed at 0.1 mg/kg but augmented at the 1.0-mg/kg dose. N2 was elevated by the 0.3- and 1.0-mg/kg doses, while P3 was increased in amplitude by all doses. N3 was transiently enhanced by the 0.3-mg/kg dose. SC amplitudes were less affected, with P3 and N4 reduced in amplitude by the 0.3- and 1.0-mg/ kg doses. The latencies of most components in both structures were decreased, often with all doses, but generally at the later recording times. A second experiment demonstrated significant MK-801-induced hyperthermia at all of the above doses, although a higher dose of 3.0 mg/kg MK-801 caused hypothermia. The reduction in component latencies may, therefore, result at least in part from a drug-induced hyperthermia. A third experiment demonstrated MK-801-induced changes in locomotor activity in rats in an open field. The effects were both dose and time dependent. The 0.3-mg/kg dose of MK-801 produced significant increases in the number of line crossings from 20-60 min in comparison to the saline condition. Increases in the number of line crossings with the 1.0-mg/kg dose peaked at 15 min, and then gradually declined. It is unlikely, however, that these changes in movement can account for the effects of MK-801 on evoked potentials. In conclusion, the results show that blockade of the ion channel associated with the NMDA receptor produces profound changes in the activity of the neural pathways that are reflected in the middle components of the flash-evoked potential recorded from the VC.

Animals↗

PubMed1999

Visual evoked potentials in school children: a comparative study of transient and steady-state methods with pattern reversal and flash stimulation.

OBJECTIVE: Flash visual evoked potentials (VEPs) are commonly used in pediatrics, because children are sometimes uncooperative. We performed a comparative study of transient and steady-state VEPs with pattern reversal (PR) and flash (light-emitting diode, LED) stimulation. METHODS: We recorded VEPs in 15 boys and 17 girls (aged 6-12 years) using 4 different stimulus conditions. The latency and amplitude of transient VEPs (T-VEPs) were measured. Steady-state VEPs (S-VEPs) were Fourier analyzed, and both the phase and amplitude of the major components were obtained. RESULTS: The mean P100 latency of LED T-VEPs was longer and had a greater variability than that of PR T-VEPs. The LED T-VEPs had an amplitude of about double that of PR T-VEPs. The first harmonic response in the LED and second harmonic in PR were the major components of S-VEPs. The phases of PR and LED S-VEPs had narrow angular dispersions and amplitudes showed marked intersubject variability. Sex and age had no significant effect on both T-VEPs and S-VEPs. CONCLUSIONS: Reproducible VEPs with 4 stimulus conditions can be obtained in school children. T-VEPs and S-VEPs are clinically useful because these methods provide complementary information.

Brain↗

PubMed2002

Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers: a laser flash photolysis study.

The riboflavin (RF, Vitamin B2) and flavin adenine dinucleotide (FAD)-sensitized photooxidation of tryptophan (TrpH) and tyrosine (TyrOH) were studied by laser flash photolysis. TrpH and TyrOH quench triplet flavin sensitizers to produce reduced flavin radicals (FlH*) and oxidized radicals of TrpH or TyrOH (Trp* and TyrO*. Although Trp* and TyrO* cannot be observed directly by the laser flash photolysis, N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), as a probe, was added to the system to result in the formation of radical cations of TMPD (TMPD*+) via quenching of Trp* and TyrO*, which provides more definitive proof of electron transfer in the photosensitization process than only direct observation of reduced flavin radicals. Electron transfer from TrpH and TyrOH to oxidized radicals of riboflavin and FAD with similar rate constants to the triplet flavins was observed for the first time, which may be a new way of TrpH and TyrOH damage. These results may shed new light on future application of flavins in photodynamic therapy, and imply that flavins might be applied potentially to photosensitization of oxygen deficiency or under high-intensity pulsed laser irradiation.

Electron Transport↗

PubMed1998

Analysing functional connectivity in brain slices by a combination of infrared video microscopy, flash photolysis of caged compounds and scanning methods.

We evaluate a novel set-up for scanning functional connectivity in brain slices from the somatosensory cortex of the rat. Upright infrared video microscopy for targeted placement of electrodes is combined with rapid photolysis of bath-applied caged neurotransmitter induced by a xenon flash lamp. Flash photolysis of caged glutamate and electrical stimulation produce comparable field potential responses and demonstrate that the viability of the submerged slices exceeds several hours. Glutamate release leads to field potential responses whose two phases are differentially affected by selective blockade of N-methyl-D-aspartate- and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate-type glutamate receptors with DL-2-amino-5-phosphonovaleric acid and 1,2,3,4-tetrahydro-6-nitro-2,3-dioxo-benzo[f]quinoxaline-7-sulphonamide, respectively. Rapid computer-controlled scanning of hundreds of distinct stimulation sites with simultaneous recordings at a fixed reference site allows construction of functional input maps from peak amplitudes and delays to peak of field potential responses. Selective laminar expansion of the functional input maps after bicuculline application demonstrates that the combination of this conveniently assembled set-up with pharmacological and physical manipulations can provide insights into the determinants of functional connectivity in brain slices.

2-Amino-5-phosphonovalerate↗

PubMedDate not supplied

[Temporal variations of excitability in photosensitive papio papio after series of intermittent light flashes (author's transl)].

After several trains of intermittent photic stimulation (IPS) each lasting 10 seconds and sometimes inducing paroxysmal electrical activity, isolated flashes or salves of 2 or 3 light flashes are capable of inducing bifrontal paroxysmal discharges in photosensitive Papio papio, an effect which is augmented by allylglycine or the use of highly photosensitive animals. Use of these salves repeated every second enabled the authors to demonstrate two types of changes in cortical excitability after intermittent photic stimulation: 1. responses which were more frequent and of greater amplitude appearing in the first 3 or 4 seconds after IPS; after paralysis of the animal amplitude and frequency of the responses are augmented. 2. if trains of intermittent photic stimulation are repeated regularly a seizure occurs after 5 to 15 trains. In the paralysed animal, more trains are required although large responses are seen after each train whereas in the non-paralysed animal the seizure occurs rapidly but the responses are on average weaker and less frequent than in 1. The relationship between photosensitive epilepsy and these responses is discussed as well as that of somatic afferents.

Animals↗

PubMed1981

[Visual evoked responses with flash pattern in normal subjects (author's transl)].

Visual evoked responses to flash pattern simulations have been observed in young healthy adults. Stimulations of whole visual fields (37 subjects) and half-fields (11 subjects) have been performed. These responses are reproductible from one subject to another and show very clear waves in the occipital and central regions. On central leads, whatever the stimulation (stimulation of total visual fields or half-fields), the responses are always the same: a negative peak at 70 msec. followed by a positive peak at 90 msec, then a negative peak at 116-120 msec. On occipital leads, stimulation of the whole visual field shows a diphasic response: a positive wave at 100 msec and a negative wave at 140-150 msec. However, half-field stimulation shows different responses on the ipsi- and contralateral hemispheres, with a disappearance of the positive 100 msec wave in the ipsilateral occipital region. Thus, flash pattern stimulations seem to be useful in clinical practice because they evoke different types of responses in occipital and central regions.

Adult↗