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At least 469 records · Page 26Linked to original sources
PubMed2003

Decreased temporal precision of auditory signaling in Kcna1-null mice: an electrophysiological study in vivo.

The voltage-gated potassium (Kv) channel subunit Kv1.1, encoded by the Kcna1 gene, is expressed strongly in the ventral cochlear nucleus (VCN) and the medial nucleus of the trapezoid body (MNTB) of the auditory pathway. To examine the contribution of the Kv1.1 subunit to the processing of auditory information, in vivo single-unit recordings were made from VCN neurons (bushy cells), axonal endings of bushy cells at MNTB cells (calyces of Held), and MNTB neurons of Kcna1-null (-/-) mice and littermate control (+/+) mice. Thresholds and spontaneous firing rates of VCN and MNTB neurons were not different between genotypes. At higher sound intensities, however, evoked firing rates of VCN and MNTB neurons were significantly lower in -/- mice than +/+ mice. The SD of the first-spike latency (jitter) was increased in VCN neurons, calyces, and MNTB neurons of -/- mice compared with +/+ controls. Comparison along the ascending pathway suggests that the increased jitter found in -/- MNTB responses arises mostly in the axons of VCN bushy cells and/or their calyceal terminals rather than in the MNTB neurons themselves. At high rates of sinusoidal amplitude modulations, -/- MNTB neurons maintained high vector strength values but discharged on significantly fewer cycles of the amplitude-modulated stimulus than +/+ MNTB neurons. These results indicate that in Kcna1-null mice the absence of the Kv1.1 subunit results in a loss of temporal fidelity (increased jitter) and the failure to follow high-frequency amplitude-modulated sound stimulation in vivo.

Acoustic Stimulation↗
PubMed1994

Improved flow-cytometric detection of low P-glycoprotein expression in leukaemic blasts by histogram subtraction analysis.

Expression of the drug efflux pump P-glycoprotein (PGP) was determined by flow cytometry in human lung cancer cell lines and in leukaemic blasts derived from 60 patients with acute myeloid leukaemia (AML). Cells from the PGP-negative parent cell line H69/P and the multidrug resistant (MDR)-variant H69/LX4 could be clearly distinguished by immunostaining with the anti-PGP monoclonal antibody MRK16. In leukaemic blasts, the differences in fluorescence intensities between samples incubated with the idiotypic nonspecific (control sample) and specific antibody (test sample) were small, resulting in nondisjunct distributions. Only in a few leukaemia specimens were PGP-expressing cells detectable by simple subtraction of histograms using a threshold. Therefore, an improved histogram subtraction analysis, based on curve fitting and a statistical test, was applied to distinguish antigen-positive from antigen-negative cells. Moreover, a multiparametric staining procedure employing propidium iodide (PI) and Hoechst 33342 was used to reduce staining artefacts. By this approach, leukaemic cells with low expression of PGP were detected in 39 out of 60 cases. Subpopulations with strong PGP expression, resulting in disjunct fluorescence distributions, were not observed. Only in 5 out of 60 specimens were PGP expressing cells detected by a conventional subtraction of histograms using a threshold. Comparison of data obtained with or without the multiparametric gating procedure indicated that the increase in sensitivity was mainly due to the application of the data analysis. However, exclusion of cell debris using PI and Hoechst staining properties reduced the deviation of data from mean values. No relation between PGP expression and cell cycle position was observed in either cell lines or in leukaemic blasts.(ABSTRACT TRUNCATED AT 250 WORDS)

ATP Binding Cassette Transporter, Subfamily B, Mem↗
PubMedDate not supplied

Simultaneous in utero assessment of AV nodal and ventricular electrophysiologic parameters in the fetal sheep heart.

BACKGROUND: Fetal tachyarrhythmias are usually of supraventricular origin. To investigate whether specific electrophysiologic properties of the fetal heart contribute to this preponderance by either favoring supraventricular tachycardias or by rendering ventricular tachycardias unlikely, we measured fetal electrophysiologic parameters in utero using transuterine fetal transesophageal electrocardiograms in fetal sheep. Since overdrive pacing may help to establish the mechanism of an arrhythmia and may be used to treat fetal tachycardias, different modes of transesophageal pacing in utero were also assessed. METHODS AND RESULTS: Decapolar electrophysiology catheters were fetoscopically inserted into the esophagus of 9 fetal sheep (pregnancy duration 94- 105 days, term = 145 days). Electrocardiograms were recorded simultaneously from all adjacent bipoles and from two pacing wires sutured onto the fetal shoulders. Pacing was attempted either via two adjacent electrodes of the intraesophageal catheter or via the most distal and most proximal electrode. Fetal cycle length, PQ, and QT intervals were close to (approx. 75 %), but fetal QRS duration was < 20 % of maternal values, thus shifting the relation between activation and repolarization towards longer excitation wave lengths. Fetal QT dispersion was small (< or = 10 ms). Atrial pacing was achieved in all fetuses using distant electrodes, and with lower thresholds when compared to closely spaced bipolar electrodes (p < 0.05). CONCLUSIONS: (I) An altered relation between ventricular activation and repolarization and a low dispersion of ventricular repolarization may protect the fetal heart against ventricular reentrant tachycardias. (II) Relatively normal fetal AV nodal conduction delay already provides one of the prerequisites for supraventricular reentrant tachycardias involving the AV node at this stage of fetal development. (III) High-rate esophageal pacing of the fetal atria is best achieved using widely spaced bipolar pacing electrodes.

Animals↗
PubMed2002

Lead within ecosystems on metalliferous mine tailings in Wales and Ireland.

A comparative study of the concentrations of lead in ecosystems developed on metalliferous mine tailings was undertaken. Mine soils, vegetation, ground-dwelling invertebrates and Apodemus sylvaticus from nine abandoned mines in Wales and a modern Irish mine site were sampled in order to evaluate and compare exposure risks to wildlife. The mine sites had a wide range of relatively high concentrations of total lead in their tailings (from 1058 to 46,630 mg kg(-1)) but the extractable lead fractions were extremely variable and not clearly related or proportional to the total values. The high soil concentrations were reflected in vegetation collected from most of the sites with the exception of the modern mine, but there was no statistical relationship, on a site basis, between available soil lead and that in plant leaf samples. The highest plant concentrations were found in litter, which in all but one of the Welsh sites exceeded the threshold guideline value of 150 mg kg(-1). Food-chain transfer was shown by high concentrations of lead in invertebrates and A. sylvaticus from the abandoned Welsh mines. A highly significant relationship existed between lead in grass and the grasshopper, Chorthippus brunneus. Adverse effects on soil invertebrates, essential to the decomposition processes and cycling of essential nutrients, were identified as probably the major obstacle to natural ecosystem development on the abandoned Welsh sites. Toxicological risk of lead to the small mammals from the Welsh sites, but not the modern Irish tailings, is indicated given the high lead concentrations in dietary items and the resultant residues in kidney with some evidence of renal oedema in animals from two sites. The absence of a significant relationship between the estimated dietary lead concentration. calculated on a site basis, and the total body concentration in A. sylvaticus, was attributed, in part, to the large size of the home range and the partial feeding of individual animals off the contaminated mine site.

Animals↗
PubMed1987

Ventricular fibrillation thresholds in Langendorff perfused rabbit hearts: all or none effect of low potassium concentration.

The median single pulse ventricular fibrillation threshold of 56 determinations in eight isolated rabbit hearts perfused with a Krebs-Henseleit solution was 36 mA, with a range from 12.5 to above 50 mA. The fibrillation threshold remained in the same range when the potassium concentration in the perfusing fluid was decreased from its normal 5.6 mmol.litre-1 to as low as 1.8 mmol.litre-1. A further decrease in potassium concentration reduced the fibrillation threshold to a median value of 1.6 mA in nine determinations on six hearts. With a 1.6 mmol.litre-1 potassium concentration, thresholds were in either the high or the low range depending on the individual heart. A pronounced shortening of refractory period and an earlier vulnerable period were associated with the sharp change in fibrillation threshold; the diastolic extrasystolic threshold, cycle length, and RT interval duration increased in a more linear manner as the potassium concentration was lowered.

Animals↗
PubMed1999

Level ground and uphill cycling ability in professional road cycling.

PURPOSE: To evaluate the physiological capacities and performance of professional road cyclists in relation to their morphotype-dependent speciality. METHODS: 24 world-class cyclists, classified as flat terrain (FT, N = 5), time trial (TT, N = 4), all terrain (AT, N = 6). and uphill (UH, N = 9) specialists, completed an incremental laboratory cycling test to assess maximal power output (Wmax), maximal oxygen uptake (VO2max), lactate threshold (LT), and onset of blood lactate accumulation (OBLA). RESULTS: UH had a higher frontal area (FA):body mass (BM) ratio (5.23 +/- 0.09 m2 x kg(-1) x 10(-3)) than FT and TT (P < 0.05). FT showed the highest absolute Wmax (481 +/- 18 W), and UH the highest Wmax relative to BM (6.47 +/- 0.33 W x kg(-1)). WLT and W(OBLA) values were significantly higher in FT (356 +/- 41 and 417 +/- 45 W) and TT (357 +/- 41 and 409 +/- 46 W) than in UH (308 +/- 46 and 356 +/- 41). Scaling of these values relative to FA and BM exponents 0.32 and 0.79 minimized group differences, but considerable differences among mean group values remained. FT and TT had the highest Wmax per FA unit (1300 +/- 62 and 1293 +/- 57 W x m2), whereas TT had the highest absolute W x kg(-0.32) and W x kg(-0.79), as well as W x kg(-0.32), W x kg(-0.79), and W x m2 at the LT and OBLA. CONCLUSIONS: i) Scaling of maximal and submaximal physiological values showed a performance advantage of TT over FT, AT, and UH in all cycling terrains and conditions; and ii) mass exponents of 0.32 and 1 were the most appropriate to evaluate level and uphill cycling ability, respectively, whereas absolute Wmax values are recommended for performance-prediction in short events on level terrain, and W(LT) and W(OBLA) in longer time trials and uphill cycling.

Adult↗
PubMed2005

Population variability in biological adaptive responses to DNA damage and the shapes of carcinogen dose-response curves.

Carcinogen dose-response curves for both ionizing radiation and chemicals are typically assumed to be linear at environmentally relevant doses. This assumption is used to ensure protection of the public health in the absence of relevant dose-response data. A theoretical justification for the assumption has been provided by the argument that low dose linearity is expected when an exogenous agent adds to an ongoing endogenous process. Here, we use computational modeling to evaluate (1) how two biological adaptive processes, induction of DNA repair and cell cycle checkpoint control, may affect the shapes of dose-response curves for DNA-damaging carcinogens and (2) how the resulting dose-response behaviors may vary within a population. Each model incorporating an adaptive process was capable of generating not only monotonic dose-responses but also nonmonotonic (J-shaped) and threshold responses. Monte Carlo analysis suggested that all these dose-response behaviors could coexist within a population, as the spectrum of qualitative differences arose from quantitative changes in parameter values. While this analysis is largely theoretical, it suggests that (a) accurate prediction of the qualitative form of the dose-response requires a quantitative understanding of the mechanism, (b) significant uncertainty is associated with human health risk prediction in the absence of such quantitative understanding and (c) a stronger experimental and regulatory focus on biological mechanisms and interindividual variability would allow flexibility in regulatory treatment of environmental carcinogens without compromising human health.

Adaptation, Physiological↗
PubMed2002

Influence of caffeine ingestion on autonomic nervous activity during endurance exercise in humans.

The effects of caffeine ingestion on the activities of the autonomic nervous system (ANS) during endurance exercise at low intensity were investigated using a power spectrum analysis of heart rate variability. Placebo or caffeine (300 mg) capsules were randomly administered to the subjects. Each subject ingested the samples 2 h before cycling on an ergometer for 30 min at an intensity corresponding to 40%-50% of his ventilatory threshold. The electrocardiogram, blood pressure (BP) and gas exchange parameters were monitored during rest and exercise. The results indicated that there were no significant differences in heart rate and systolic blood pressure between the trials. The spectrum integrated values of the low frequency power and total power components in the caffeine trial were significantly greater than in the placebo trial during exercise, which implied that activities of the ANS were augmented by caffeine. Caffeine also induced enhanced lipid oxidation as shown by the significantly lower respiratory gas exchange ratio and increases in diastolic blood pressure during exercise. The results shed some light upon the relationship between the activity of the ANS, energy metabolism and BP. In conclusion, the results suggest that caffeinated beverages have a potential to be useful supplements to the prescription of exercise for individuals who experience a depressed activity of the ANS. The results also suggest that the experiment protocol used in this study is a sensitive and noninvasive method for evaluating the effects of various foods and nutrients on the activity of the ANS.

Adult↗
PubMed1992

Relation between repolarization and refractoriness in the human ventricle: cycle length dependence and effect of procainamide.

The cycle length dependence of the action potential duration and the effective refractory period of the right ventricular endocardium were investigated in 24 patients undergoing electrophysiologic studies for suspected ventricular tachycardia. The action potential duration at 90% repolarization and the effective refractory period at twice diastolic threshold strength were measured at the same catheter site at steady state cycle lengths of 350 to 600 ms. Both measurements decreased linearly with decreasing cycle length, maintaining a parallel relation. When the relation between action potential duration and effective refractory period was expressed as the effective refractory period-action potential duration difference, nearly constant values (range -12 to -15 ms) were obtained at all cycle lengths. To determine whether sodium channel blocking drugs influence the effective refractory period-action potential duration relation in humans, measurements of these two variables were obtained in 15 patients before and during the infusion of procainamide. Procainamide prolonged the action potential duration at each cycle length by a near constant amount over baseline values (p less than 0.001). Procainamide also increased the effective refractory period at each cycle length but with a greater incremental increase at the shorter cycle lengths. The rate-dependent increase in the effective refractory period-action potential duration difference became significant at cycle lengths less than or equal to 400 ms; at these high rates, the effective refractory period-action potential duration difference became positive (1.6 ms, p less than 0.01 compared with baseline). Thus, in the human ventricle, the action potential duration and the effective refractory period have a close relation that remains fixed over a wide range of cycle lengths.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1987

The suppression of nocturnal pineal melatonin in the Syrian hamster: dose-response curves at 500 and 360 nm.

It has recently been shown that wavelengths in the near-UV range (UV-A, 320-400 nm) are capable of influencing pineal melatonin content in the hamster. The purpose of this study was to compare the capacities of monochromatic visible and UV radiation for suppressing nocturnal pineal melatonin. Groups of male Syrian hamsters adapted to a 14-h light, 10-h dark cycle (lights on, 1700-0700 h) were exposed to irradiances of 500 or 360 nm light for 5 min during their dark phase. Both wavelengths suppressed pineal melatonin in a dose-related manner. The resultant fluence-response curves were similar in shape, although their corresponding threshold irradiances were markedly different. The calculated ED50 values for 500 and 360 nm light were 0.022 microW/cm2 (1.66 X 10(13) photons/cm2) and 0.306 microW/cm2 (1.66 X 10(14) photons/cm2), respectively. These data show that the induction of a 50% depression of pineal melatonin requires 10 times the number of 360-nm photons compared to 500-nm photons at the level of the cornea. Despite this difference in sensitivity to wavelength, environmental irradiances of UV-A are well above the threshold for melatonin suppression in the hamster. These results thus demonstrate the importance of considering UV-A, in addition to the visible wavelengths, in the regulation of hamster pineal physiology.

Animals↗
PubMed1990

Ventilatory threshold and maximal oxygen uptake during cycling and running in triathletes.

VO2max and the ventilatory threshold (Tvent) were measured during cycle ergometry (CE) and treadmill running (TR) in a group of 10 highly trained male triathletes. Tvent was indicated as the VO2 at which the ventilatory equivalent for oxygen increased without a marked rise in the ventilatory equivalent for carbon dioxide. Triathletes achieved a significantly higher VO2max for TR (75.4 +/- 7.3 ml.kg-1.min-1) than for CE (70.3 +/- 6.0 ml.kg-1.min-1). Mean CE VO2max was 93.2% of the TR value. Average VO2max values for CE and TR compared favorably with values reported for elite single-sport athletes and were greater than those previously reported for other male triathletes. CE Tvent occurred at 3.37 +/- 0.32 l.min-1 or 66.8 +/- 3.7% of CE VO2max, while TR Tvent was detected at 3.87 +/- 0.33 l.min-1 or 71.9 +/- 6.6% of TR VO2max. The VO2 (l.min-1) at which Tvent occurred for TR was significantly higher than for CE (P less than 0.001). Although the VO2 values at TR Tvent expressed as a percentage of VO2max were consistently higher than for CE, the difference between the means did not reach statistical significance (P greater than 0.05). The average Tvent for CE (as %VO2max) was nearly identical to Tvent values reported in the literature for competitive male cyclists, whereas TR Tvent was lower than recently reported values for elite distance runners and marathoners. We speculate that triathlon training results in general (cross-training) adaptations which enhance maximal oxygen uptake values, whereas anaerobic threshold adaptations occur primarily in the specific muscle groups utilized in training.

Bicycling↗
PubMed1997

The relationship between preferred and optimal positioning during submaximal cycle ergometry.

This study was designed to determine how changes in oxygen uptake (VO2) and heart rate (HR) during submaximal cycle ergometry were determined by changes in cycle geometry and/or lower-limb kinematics. Fourteen trained cyclists [Mean (SD): age, 25.5 (6.4) years; body mass 74.4 (8.8) kg; peak VO2, 4.76 (0.79) 1 x min(-1) peak] were tested at three seat-tube angles (70 degrees, 80 degrees, 90 degrees) at each of three trunk angles (10 degrees, 20 degrees, 30 degrees) using a modified Monark cycle ergometer. All conditions were tested at a power output corresponding to 95% of the VO2 at each subject's ventilatory threshold while pedalling at 90 rpm and using aerodynamic handlebars. Sagittal-view kinematics for the hip, knee, and ankle joints were also recorded for all conditions and for the subjects' preferred positioning on their own bicycles. No combination of seat-tube and trunk angle could be considered optimal since many of the nine conditions elicited statistically similar mean VO2 and HR values. Mean hip angle (HA) was the only kinematic variable that changed consistently across conditions. A regression relationship was not observed between mean VO2 or HR and mean hip angle values (P > 0.45). Significant curvilinear relationships were observed, however, between deltaVO2 (VO2 - minimum VO2) and deltaHA (mean HA - preferred HA) using the data from all subjects (R = 0.45, SEE = 0.13 1 x min(-1)) and using group mean values (R = 0.93, SEE = 0.03 1 x min(-1)). In both cases deltaVO2 minimized at deltaHA = 0, which corresponded to the subjects' preferred HA from their own bicycles. Thus, subjects optimized their VO2 cost at cycle geometries that elicited similar lower-limb kinematics as the preferred geometries from their own bicycles.

Adult↗
PubMed2000

The effect of age on the power/duration relationship and the intensity-domain limits in sedentary men.

The time to fatigue (t) in response to high-intensity constant-load exercise decreases hyperbolically with increasing power (W), at least in active and younger individuals [i.e. (W - thetaF)t = W', where thetaF is the critical power asymptote and W' is the curvature constant]. Little is known, however, about the combined effects of age and sedentarity on these parameters. We therefore evaluated 17 non-trained males (9 aged 60-75 years and 8 aged below 30 years) who underwent ramp-incremental cycle ergometry and, on different days, 4 high-intensity constant-load tests to t. Compared to their younger counterparts, the older subjects presented significantly lower maximum oxygen uptake (i.e. the maximum value of oxygen uptake attained at the end of a progressive exercise with the subject exerting a presumably maximal effort, muVO2), estimated lactate threshold (VO2thetaL), VO2thetaF, and W' (P < 0.05). Interestingly, however, both VO2thetaL and VO2thetaF, when expressed as a percentage of muVO2, were higher in older than in younger men [61.8 (6.2)% versus 45.4 (4.6)% and 87.8 (7.3)% versus 79.0 (8.2)%, P < 0.05, respectively]. Therefore, age was associated with an increase in the relative magnitude of the "moderate", sub-thetaL exercise-intensity domain (+30.4%), mainly at the expense of the "very-heavy", supra-thetaF domain (-56%). Our results demonstrate that age and sedentarity are associated with: (1) marked reductions in both the aerobic (thetaF) and anaerobic (W') determinants of the W/t relationship, and (2) changes in either the absolute or relative magnitudes of the exercise-intensity domains. These findings are consistent with the notion that endurance-related parameters are less diminished with ageing than the maximal capacity, thereby mitigating the deleterious effects of senescence in the functional capacity.

Adult↗
PubMed1985

Effects of ACNU, a water-soluble nitrosourea derivative, on survival and cell kinetics of cultured HeLa S3 cells.

Effects of a water-soluble nitrosourea, 1(4-aminomethylpyrimidine-5yl)methyl-3-(2-chloroethyl)-3-nit rosourea hydrochloride (ACNU) was investigated on cultured HeLa S3 cells with regard to their lethality and cell progression through the cell cycle. The survival curve of exponentially growing cells exposed to increasing concentrations of the drug for 1 hr was characterized by a threshold type of response (Do = 7.0 micrograms/ml X 1 hr, Dq 3.5 micrograms/ml X 1 hr). Throughout the cell cycle, ACNU exerted its main killing effect on cells in the G1 and G2 + M phases, whereas cells in S were resistant to the drug. The change in their age-response was due to the Do value of the dose-survival curve rather than the Dq. Effects of cell progression were also examined at a low concentration of ACNU (5.0 micrograms/ml), which allowed 80% of treated cells to survive. Delayed transit was observed in the S phase, and more markedly in the G2 + M phase. The magnitude of these perturbations depended on the position of the cell cycle at which the drug was administered. Cells treated in the G1 and early S phases showed a much longer duration of S and G2 + M phases than cells treated in mid-S phase. Cells treated in the late S and G2 phases could normally pass through mitosis, but were subsequently blocked in the G2 phase following a prolonged S phase in the next cell cycle. These studies revealed that there seems to be some positive relationship of the effect of ACNU on cell progression and cell killing throughout the cell cycle.

Cell Cycle↗
PubMed2001

Exercise intensity and load during mass-start stage races in professional road cycling.

PURPOSE: To evaluate exercise intensity and load during mass-start stages in professional road cycling, using competition heart rate (HR) recordings. METHODS: Seventeen world-class cyclists performed an incremental laboratory test during which maximal power output (Wmax), maximal HR (HRmax), onset of blood lactate accumulation (OBLA), lactate threshold (LT), and a HR-power output relationship were assessed. An OBLAZONE (HROBLA +/- 3 beats.min-1) and an LTZONE (HRLT +/- 3 beats.min-1) were described. HR was monitored during 125 flat (< 13 km uphill, < 800-m altitude change; FLAT), 99 semi-mountainous (13-35 km uphill, 800- to 2000-m altitude change; SEMO), and 86 high-mountain (> 35 km uphill, > 2000-m altitude change; HIMO) stages. Each cyclist's competition power output was estimated from competition HR and individual HR-power output relationships. Competition training impulse (TRIMP) values and time spent at "easy," "moderate," and "hard" zones were estimated from HR and race duration. RESULTS: Average %HRmax were 61 +/- 5%, 58 +/- 6%, and 51 +/- 7% in HIMO, SEMO, and FLAT stages, respectively, and estimated average power outputs were 246 +/- 44, 234 +/- 43, and 192 +/- 45 W. Competition HR values relative to HROBLA and HRLT were, respectively, 69 +/- 6, 79 +/- 9% in HIMO; 65 +/- 7, 74 +/- 11% in SEMO; and 57 +/- 8, 65 +/- 10% in FLAT stages. The amount of TRIMP in HIMO, SEMO, and FLAT stages were, respectively, 215 +/- 38, 172 +/- 31, and 156 +/- 31. Percentage time spent in the "moderate" and "hard" zones was highest in HIMO (22 +/- 14, 5 +/- 6%) followed by SEMO (15 +/- 13, 5 +/- 5%) and FLAT (9 +/- 7, 2 +/- 2%) stages. CONCLUSIONS: %HRmax, time distribution around HROBLA and HRLT, TRIMP, and load zones reflected the physiological demands of different mass-start cycling stage categories. The knowledge of these demands could be useful for planning precompetition training strategies.

Adult↗
PubMed1975

Quantitative dose-response of growth and development in Arabidopsis thaliana exposed to chronic gamma-radiation.

The response of Arabidopsis thaliana (L.) Heynh. (Cruciferae) to a gradient of chronic gamma-radiation was examined under field conditions. Plants that were initially introduced to the gamma field as dry seeds received exposures of 1-34 to 18 800 R/20 hour day from the time of seed germination. Regression analysis demonstrated a significant, but non-linear, response for three variables, number of seedlings emerging, number of plants flowering, and plant volume; the response of a fourth variable, number of leaves per plant, was not related to daily exposure. LD50 values ranged from 66 R/20 hour day for plant volume to 1231 R/20 hour day for seedling emergence. Flowering and plant volume were the most sensitive indicators of radiation exposure. The demonstration of a variable threshold at low levels of exposure indicates that in nature A. thaliana may be exposed to environmental radiation throughout its life-cycle without significant modification of growth or development.

Dose-Response Relationship, Radiation↗
PubMed1996

The Free Fall Machine--a ground-based facility for microgravity research in life sciences.

A device is described in which a biological specimen is periodically accelerated for a short period. Each event is followed by a variable period of free fall. Assuming that the g-dose (expressed g s) has to surpass a certain minimal value to be perceived by cells, and in addition, there is a minimal time threshold for sensing a change in gravity, it is conceivable that conditions are created in which cells do not detect the periodic acceleration, and only experience the periodic free-fall movement as a long-term weightlessness condition. Using the cell-cycle progression of the unicellular green alga Chlamydomonas as an example, it is shown that with this device effects can be generated which are similar to those observed in satellite flights.

Acceleration↗
PubMed2003

Dynamic responses of O2 uptake at the onset and end of exercise in trained subjects.

Inconsistencies about dynamic asymmetry between the on- and off-transient responses in .VO2 are found in the literature. Therefore the purpose of this study was to examine .VO2on- and off-transients during moderate- and heavy-intensity cycling exercise in trained subjects. Ten men underwent an initial incremental test for the estimation of ventilatory threshold (VT) and, on different days, two bouts of square-wave exercise at moderate ( VT) intensities. .VO2 kinetics in exercise and recovery were better described by a single exponential model ( VT). For moderate exercise, we found a symmetry of .VO2 kinetics between the on- and off-transients (i.e., fundamental component), consistent with a system manifesting linear control dynamics. For heavy exercise, a slow component superimposed on the fundamental phase was expressed in both the exercise and recovery, with similar parameter estimates. But the on-transient values of the time constant were appreciably faster than the associated off-transient, and independent of the work rate imposed ( VT). Our results do not support a dynamically linear system model of .VO2 during cycling exercise in the heavy-intensity domain.

Adult↗