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Biomedical subjects

D G Birch

Publications and source records attributed to D G Birch.

At least 91 records · Page 5Linked to original sources
PubMed1988

Ammonium Assimilation Requires Mitochondrial Respiration in the Light : A Study with the Green Alga Selenastrum minutum.

Mass spectrometric analysis of O(2) and CO(2) exchange in the green alga Selenastrum minutum (Naeg. Collins) provides evidence for the occurrence of mitochondrial respiration in light. Stimulation of amino acid synthesis by the addition of NH(4)Cl resulted in nearly a 250% increase in the rate of TCA cycle CO(2) efflux in both light and dark. Ammonium addition caused a similar increase in cyanide sensitive O(2) consumption in both light and dark. Anaerobiosis inhibited the CO(2) release caused by NH(4)Cl. These results indicated that the cytochrome pathway of the mitochondrial electron transport chain was operative and responsible for the oxidation of a large portion of the NADH generated during the ammonium induced increase in TCA cycle activity. In the presence of DCMU, ammonium addition also stimulated net O(2) consumption in the light. This implied that the Mehler reaction did not play a significant role in O(2) consumption under our conditions. These results show that both the TCA cycle and the mitochondrial electron transport chain are capable of operation in the light and that an important role of mitochondrial respiration in photosynthesizing cells is the provision of carbon skeletons for biosynthetic reactions.

Journal Article↗
PubMed1988

Simultaneous Transport of CO(2) and HCO(3) by the Cyanobacterium Synechococcus UTEX 625.

A mass spectrometer was used to simultaneously follow the time course of photosynthetic O(2) evolution and CO(2) depletion of the medium by cells of the cyanobacterium Synechococcus leopoliensis UTEX 625. Analysis of the data indicated that both CO(2) and HCO(3) (-) were simultaneously and continuously transported by the cells as a source of substrate for photosynthesis. Initiation of HCO(3) (-) transport by Na(+) addition had no effect on ongoing CO(2) transport. This result is interpreted to indicate that the CO(2) and HCO(3) (-) transport systems are separate and distinctly different transport systems. Measurement of CO(2)-dependent photosynthesis indicated that CO(2) uptake involved active transport and that diffusion played only a minor role in CO(2) acquisition in cyanobacteria.

Journal Article↗
PubMed1987

Dependence of cone b-wave implicit time on rod amplitude in retinitis pigmentosa.

Computer-averaged full-field rod and cone electroretinograms (ERGs) were evaluated in 63 patients with retinitis pigmentosa and 116 normal subjects. Cone b-wave implicit times to 30 Hz white flicker and to 1.0 Hz white flashes on a white background were both inversely proportional to the log amplitude of the dark-adapted rod b-wave, even when controlling for the log amplitude of the dark-adapted cone b-wave. Cone ERGs were also obtained from a subset of 12 patients with retinitis pigmentosa in the presence of photopically-matched red and blue backgrounds that allowed direct assessment of the effect of rod function on cone b-wave implicit time in the patients. The effectiveness of the blue background in reducing cone b-wave implicit time relative to that obtained with the red background varied with the amplitude of the rod ERG. The results of both experiments suggest that delays in cone b-wave implicit time obtained with 30 Hz white flicker or with background adaptation in patients with retinitis pigmentosa are due, at least in part, to rod disease.

Adolescent↗
PubMed1987

Pupillometric measures of retinal sensitivity in infants and adults with retinitis pigmentosa.

Steady-state pupil diameter was measured in darkness and over a 9 log unit range of Ganzfeld illumination in 20 normal adults, 28 normal infants, 59 adults with retinitis pigmentosa, and 30 children at risk for retinitis pigmentosa. Pupil threshold was defined as the retinal illuminance necessary to decrease pupil diameter by 1.0 mm relative to the diameter in darkness. Spectral measurements in normal adults confirmed that pupil threshold was determined primarily by rods with this procedure. Pupil thresholds of newborns and infants did not differ significantly from those of normal adults. In patients with retinitis pigmentosa, elevations in pupil threshold were related to rod loss as indexed by reductions in rod ERG amplitude. Electroretinographic follow-up of 13 of the children at risk for retinitis pigmentosa showed normal rod ERG function in 5 children who had pupil thresholds within the normal range and significant rod ERG loss in 8 children who had elevated pupil thresholds.

Adolescent↗
PubMed1987

Effect of monocular light exclusion on the Stiles-Crawford function.

Contrary to previously published studies, monocular light exclusion did not alter the shape of the Stiles-Crawford function in a substantial manner in four subjects who were patched in this laboratory for one full week. In order to rule out methodological differences, an experimenter from each of the two sets of experiments (current and prior) patched one eye for 1 week, then tested each other. For one subject, there was little effect of patching. For the other, patching had a measurable effect, but it was different from that originally reported. However, the much larger, symmetrical broadening of the Stiles-Crawford function reported in earlier studies was not replicated. A testable working hypothesis to explain the earlier results is discussed.

Female↗
PubMed1987

The relationship between rod perimetric thresholds and full-field rod ERGs in retinitis pigmentosa.

Dark-adapted static perimetric fields and rod full-field electroretinograms (ERGs) were measured in normal subjects and 15 patients with retinitis pigmentosa. Psychophysical thresholds for short-wavelength stimuli were obtained from 76 locations across the retina on a modified Octopus 201 perimeter; threshold differences to short- and long-wavelength stimuli were used to determine whether rods were mediating threshold at each retinal location. Full-field rod ERG peak-to-peak amplitude as a function of retinal illuminance was analyzed by determining the parameters of the best-fit Naka-Rushton equation. Rod ERG thresholds and mean rod perimetric thresholds were significantly correlated with a slope of approximately 1.0. Log k was positively correlated with threshold elevation in the region of peak field sensitivity and log Vmax was positively correlated with log rod visual field area. These results demonstrate correspondence between parameters derived from rod visual fields and parameters derived from the Naka-Rushton analysis of the full-field rod ERG.

Adult↗
PubMed1987

Diurnal rhythm in the human rod ERG: retinitis pigmentosa.

Rod ERGs were measured at three times of day over an extensive range of retinal illuminances in six light-entrained patients with autosomal recessive or isolate forms of retinitis pigmentosa and at five times of day in six light-entrained normal volunteers. B-wave amplitude versus retinal illuminance functions from each time of day were described by determining the parameters of the best-fit Naka-Rushton function. Results from normal subjects showed that rod ERG threshold (defined as the log retinal illuminance necessary to elicit a 2.0 microV response) was elevated 1 1/2 hr after daily light-onset due to both an increase in log k (semi-saturation constant) and a decrease in log Vmax (maximum rod amplitude). The magnitude of the threshold elevation 1 1/2 hr after light-onset was comparable in patients with retinitis pigmentosa and normal subjects. Whereas thresholds returned to pre-light exposure levels rapidly during the light-phase of the daily cycle in normal subjects, thresholds continued to rise in patients with retinitis pigmentosa due primarily to a further increase in log k. These findings are consistent with abnormal rod photoreceptor disc renewal mechanisms in retinitis pigmentosa.

Adolescent↗
PubMed1987

Rod ERGs in retinitis pigmentosa and cone-rod degeneration.

Rod amplitudes as a function of retinal illuminance were analyzed by determining the parameters of the best fit Naka-Rushton function for 15 normal subjects, 25 patients with rod-cone degeneration, and 10 patients with cone-rod degeneration. Rod b-wave implicit times were analyzed by determining the best linear fit to log retinal illuminance. Retinal illuminances necessary for a criterion b-wave implicit time and for a half-maximum amplitude response (log k) were highly correlated indices of rod sensitivity. Both measures were normal or near normal in patients of all ages with cone-rod degeneration, but significantly elevated in patients with rod-cone degeneration. These results suggest that the underlying mechanisms of rod degeneration are fundamentally different in these two forms of hereditary retinal degeneration.

Adolescent↗
PubMed1986

A comparison of visual function tests in eyes with maculopathy.

Several recently developed tests of visual function, including the Potential Acuity Meter (PAM), laser interferometer (LI), white-light interferometer (WLI), blue field entoptic phenomenon, and focal electroretinogram (ERG) were compared in 81 eyes with clear media and known macular disease. The results indicate that the PAM, the LI, and the WLI overread relative to Snellen acuity. Laser interferometric acuity values differed from Snellen acuity by at least 1.5 octaves in approximately 40% of all eyes, regardless of stimulus size (2, 5, or 8 degrees). Similar results were obtained with the WLI. Agreement with Snellen acuity was better for the PAM, with 91% of eyes falling within 1.5 octaves of Snellen acuity. Blue field and focal ERG results were categorized as normal or abnormal. While not producing Snellen equivalents, abnormal results from the blue field and focal ERG corresponded with poor Snellen acuity (less than 20/40) in 65% and 91% of eyes, respectively. Assuming that media opacities do not prevent adequate retinal stimulation, the present results suggest that the PAM and focal ERG are the most reliable for evaluating macular function when maculopathy is present.

Electroretinography↗
PubMed1986

Photosynthetic Adaptation by Synechococcus leopoliensis in Response to Exogenous Dissolved Inorganic Carbon.

Synechococcus leopoliensis was grown over a wide range of dissolved inorganic carbon (DIC) concentrations (4-25,000 micromolar) which were obtained by varying culture pH (6.2-9.6) and the CO(2) concentration of the gas stream (36-50,000 microliters per liter). The [DIC] required to half-saturate photosynthesis (K((1/2)) (DIC)) was found to vary depending upon the ambient DIC concentration at which the cells were grown. Low [DIC] grown cells exhibited low values of K((1/2)) (DIC) (4.7 micromolar) whereas cells grown at high [DIC] exhibited high values of K((1/2)) (DIC) (1-2.5 millimolar). Intermediate concentrations of DIC produced intermediate values. Changes in K((1/2)) (DIC) appeared to be solely a function of [DIC] and were independent of both culture pH and CO(2) concentration. As changes in K((1/2)) (DIC) occur in response to DIC concentrations commonly found in natural systems we suggest this adaptation may be of ecological significance.

Journal Article↗
PubMed1986

Nitrate and Ammonium Induced Photosynthetic Suppression in N-Limited Selenastrum minutum: II. Effects of NO(3) and NH(4) Addition to CO(2) Efflux in the Light.

The effects of nitrate and ammonium addition on net and gross photosynthesis, CO(2) efflux and the dissolved inorganic carbon compensation point of nitrogen-limited Selenastrum minutum Naeg. Collins (Chlorophyta) were studied. Cultures pulsed with nitrate or ammonium exhibited a marked decrease in both net and gross photosynthetic carbon fixation. During this period of suppression the specific activity of exogenous dissolved inorganic carbon decreased rapidly in comparison to control cells indicating an increase in the rate of CO(2) efflux in the light. The nitrate and ammmonium induced rates of CO(2) efflux were 31.0 and 33.8 micromoles CO(2) per milligram chlorophyll per hour, respectively, and represented 49 and 48% of the rate of gross photosynthesis. Nitrate addition to cells at dissolved inorganic carbon compensation point caused an increase in compensation point while ammonium had no effect. In the presence of the tricarboxylic acid cycle inhibitor fluoroacetate, the nitrate-induced change in compensation point was greatly reduced suggesting the source of this CO(2) was the tricarboxylic acid cycle. These results are consistent with the mechanism of N-induced photosynthetic suppression outlined by Elrifi and Turpin (1986 Plant Physiol 81: 273-279).

Journal Article↗
PubMed1986

Diurnal rhythm in the human rod ERG. Relationship to cyclic lighting.

Five normal subjects were monitored with periodic full-field ERG testing during entrainment to a 14L:10D lighting cycle. On the second day of entrainment, mean ERG threshold was significantly higher 1.5 hr after light onset than at other times of day and was comparable to a previously reported threshold elevation following 7 days of entrainment. This diurnal rhythm was not detectable in eyes entrained for 3 days that were then maintained in darkness for 1 day. The findings support the idea that 2 days of entrainment are required to maximize the diurnal rhythm in the human rod ERG and that an adapting light may be required to trigger this rhythm.

Adaptation, Ocular↗
PubMed1985

Natural course of retinitis pigmentosa over a three-year interval.

Ninety-four patients, 6 to 49 years old, with progressive forms of retinitis pigmentosa were examined at baseline and annually for three consecutive years with respect to visual acuity, kinetic visual fields, dark-adaptation thresholds, computer-averaged electroretinograms, and fundus photographs. A subset was recalled within two months of a given visit to measure intervisit variability and to develop criteria for what constitutes significant (P less than .01) functional change. Over a three-year interval full-field electroretinograms declined significantly in 66 of 86 patients (77%) with detectable responses at baseline. Patients lost an average of 16% to 18.5% of remaining electroretinographic amplitude per year and 4.6% of remaining visual field area per year. Bone spicule pigmentation increased in 41 of 76 patients for whom we could make comparisons over a three-year interval (54%). Visual acuity and dark-adaptation thresholds remained relatively stable.

Adolescent↗
PubMed1984

Diurnal rhythm in the human rod ERG.

Six normal subjects, monocularly entrained to a 14 hr light/10 hr dark cycle, showed a diurnal rhythm in the rod ERG of the entrained eye but not the unentrained eye. Thresholds in the entrained eye were significantly higher 1 1/2 hrs after light onset than at other times. This threshold increase occurred at a time when rod disc shedding has been observed histologically to be maximal in light-entrained animals.

Adaptation, Ocular↗
PubMed1982

Psychophysical studies of cone optical bandwidth in patients with retinitis pigmentosa.

Estimates of variation in the alignment of cones with the pupil were obtained from the fovea and parafovea in normal subjects and in patients with retinitis pigmentosa known to have reduced directional sensitivity. The degree of cone disarray was inferred from differences in sensitivity between Stiles-Crawford functions obtained with a stimulus alone and with the stimulus superimposed on a background entering the pupil at a point corresponding to the peak of the Stiles-Crawford function. The magnitude of the selective effect of the background in normals was consistent with previous estimates of variations in photoreceptor alignment in the macula. In contrast, the selective adaptation effect was not present in patients with retinitis pigmentosa. These results exclude increased disarray among cones with normal optical bandwidths, but rather, suggest that morphological abnormalities increasing the optical bandwidths of individual cones underlie reduced directional sensitivity in these patients.

Adaptation, Ocular↗
PubMed1982

Adaptation to polarized light in humans.

Recent investigators reported large adaptational effects in human vision dependent on the plane of linearly polarized light. In the present study, such adaptational effects were observed to be small or insignificant. Our observers adapted to intense homogenous, foveal fields of linearly polarized, orange-red light produced by a beam that entered the side of the pupil. Periodically the plane of polarization of the beam suddenly shifted 90 degrees without there being any change in the spectral distribution of the light or position of the beam in the pupil plane. The change of radiance of the beam could be adjusted to be as low as approx. 0.5%. Signal detection experiments revealed that observers typically could not detect a change of polarization of the field. Results obtained with the method of adjustment showed that the threshold of an increment flash presented on the intense field was not affected by changing the polarization of the field. Physiological implications of these results are discussed.

Adaptation, Ocular↗
PubMed1982

The Stiles-Crawford effect in retinitis pigmentosa.

Stiles-Crawford functions were obtained from the maculas of 22 patients with different genetic types of retinitis pigmentosa and visual acuity of 20/40 or better. Reduced cone directional sensitivity was seen in the fovea with both focal cone electroretinographic testing and psychophysical testing. Functions from the parafovea determined with psychophysical testing showed either significant flattening or displaced peak locations. Results are discussed in terms of factors know to affect photoreceptor orientation and histologic findings in retinitis pigmentosa.

Adult↗