Diurnal variations in cycling kinematics

Physiological and biomechanical constraints as well as their fluctuations throughout the day must be considered when studying determinant factors in the preferred pedaling rate of elite cyclists. The aim of this study was to monitor the diurnal variation of spontaneous pedaling rate and movement kinematics over the crank cycle. Twelve male competitive cyclists performed a submaximal exercise on a cycle ergometer for 15 min at 50% of their W(max). Two test sessions were performed at 06:00 and 18:00 h on two separate days to assess diurnal variation in the study variables. For each test session, the exercise bout was divided into three equivalent 5-min periods during which subjects were requested to use different pedal rates (spontaneous cadence, 70 and 90 rev min(-1)). Pedal rate and kinematics data (instantaneous pedal velocity and angle of the ankle) were collected. The results show a higher spontaneous pedal rate in the late afternoon than in the early morning (p < 0.001). For a given pedal rate condition, there was a less variation in pedal velocity during a crank cycle in the morning than in the late afternoon. Moreover, diurnal variations were observed in ankle mobility across the crank cycle, the mean plantar flexion observed throughout the crank cycle being greater in the 18:00 h test session (p < 0.001). These results suggest that muscular activation patterns during a cyclical movement could be under the influence of circadian fluctuations.     

Sulfur cycling in Catenococcus thiocyclus

Abstract: Since its isolation from marine volcanic areas, Catenococcus thiocyclus has been known to be able to oxidize thiosulfate to tetrathionate, but the benefits gained from the reaction were unknown. The energy to be gained from such a reaction is so small (1 electron per mol of thiosulfate, compared with 8 electrons if the thiosulfate is oxidized to sulfate) that it seemed unlikely to be a useful metabolic reaction. However, continuous culture experiments have now revealed that C. thiocyclus is able to gain metabolically useful energy from this oxidation (biomass yields increased by approximately 20% after the addition of 7.75 mM thiosulfate to medium containing 20 mM acetate) by combining it with the chemical reduction of the tetrathionate by sulfide. The enzymes for thiosulfate oxidation appear to be constitutive. Moreover, with a suitable primary energy source (e.g. glucose), C. thiocyclus can reduce sulfur (S°) to sulfide and Fe³⁺ to Fe²⁺. A chemical reaction then generates FeS. Such reactions may have important implications for the sulfur cycle at oxic:anoxic interfaces in marine and freshwater systems.     

Dynamic regulation of hematopoietic stem cell cycling

Comment on: Takizawa H, et al. J Exp Med 2011; 208:273-84.     

Estradiol cycling drives female obesogenic adipocyte hyperplasia

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Catalytic cycling of human mitochondrial Lon protease

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Relationship between cycling mechanics and core stability

Core stability has received considerable attention with regards to functional training in sports. Core stability provides the foundation from which power is generated in cycling. No research has described the relationship between core stability and cycling mechanics of the lower extremity. The purpose of this study was to determine the relationship between cycling mechanics and core stability. Hip, knee, and ankle joint kinematic and pedal force data were collected on 15 competitive cyclists while cycling untethered on a high-speed treadmill. The exhaustive cycling protocol consisted of cycling at 25.8 km x h(-1) while the grade was increased 1% every 3 minutes. A core fatigue workout was performed before the second treadmill test. Total frontal plane knee motion (test 1: 15.1 +/- 6.0 degrees ; test 2: 23.3 +/- 12.5 degrees), sagittal plane knee motion (test 1: 69.9 +/- 4.9 degrees ; test 2: 79.3 +/- 10.1 degrees), and sagittal plane ankle motion (test 1: 29.0 +/- 8.5 degrees ; test 2: 43.0 +/- 22.9 degrees) increased after the core fatigue protocol. No significant differences were demonstrated for pedaling forces. Core fatigue resulted in altered cycling mechanics that might increase the risk of injury because the knee joint is potentially exposed to greater stress. Improved core stability and endurance could promote greater alignment of the lower extremity when riding for extended durations as the core is more resistant to fatigue.     

Torque-velocity relationship in isokinetic cycling exercise

Seven healthy female subjects performed brief (less than 10 s) periods of maximal exercise on a constant-velocity cycle ergometer, over the functional range of pedaling velocities, and an isometric contraction with each leg. There was an inverse relationship between peak torque and pedal crank velocity in all subjects; isometric torque was (mean +/- SE) 19.8 +/- 8.3% greater than the torque recorded at the slowest velocity of 11 rpm. The torque-velocity relationship was described best by a single exponential equation: y = 189.6 X e-0.0834x, where y is peak torque in Newton . meters and x is crank velocity in revolutions per minute. Peak power was a parabolic function of crank velocity; the data were fitted suitably by a second-order polynomial equation: y = -0.0589x2 + 14.504x + 47.092, where y is peak power in watts and x is crank velocity in revolutions per minute. Maximal peak power occurred at crank velocities ranging from 120 to 160 rpm, when the torque was 0.36 +/- 0.06 of the maximal isometric tension. These results demonstrate the importance of recording velocity in measurements of dynamic maximal power.     

Meiotic drive and sex chromosome cycling

Sex-linked meiotic drive is found in a broad variety of taxa, including insects, birds, and mammals. In populations of some species, we see four types of sex chromosomes segregating: normal and driving X chromosomes and susceptible and resistant Y chromosomes. A theoretical analysis shows that a stable four-chromosome equilibria is a more common outcome in these systems than previously recognized. Cycling of sex chromosome frequencies and associated changes in the sex ratio are other predicted outcomes. The absence of cycling in nature may be due to migration among populations.     

Age-regulated cycling metabolites are relevant for behavior

Circadian cycles of sleep:wake and gene expression change with age in all organisms examined. Metabolism is also under robust circadian regulation, but little is known about how metabolic cycles change with age and whether these contribute to the regulation of behavioral cycles. To address this gap, we compared cycling of metabolites in young and old Drosophila and found major age-related variations. A significant model separated the young metabolic profiles by circadian timepoint, but could not be defined for the old metabolic profiles due to the greater variation in this dataset. Of the 159 metabolites measured in fly heads, we found 17 that cycle by JTK analysis in young flies and 17 in aged. Only four metabolites overlapped in the two groups, suggesting that cycling metabolites are distinct in young and old animals. Among our top cyclers exclusive to young flies were components of the pentose phosphate pathway (PPP). As the PPP is important for buffering reactive oxygen species, and overexpression of glucose-6-phosphate dehydrogenase (G6PD), a key component of the PPP, was previously shown to extend lifespan in Drosophila, we asked if this manipulation also affects sleep:wake cycles. We found that overexpression in circadian clock neurons decreases sleep in association with an increase in cellular calcium and mitochondrial oxidation, suggesting that altering PPP activity affects neuronal activity. Our findings elucidate the importance of metabolic regulation in maintaining patterns of neural activity, and thereby sleep:wake cycles.     

Uterine luminal prostaglandin F in cycling mares

Prostaglandin F was measured by radioimmunoassay in uterine flushings of cycling mares on days 4, 8, 12, 14, 16, 18 and 20 post-ovulation. Prostaglandin F was significantly (P < .05) affected by day of the estrous cycle and reached maximal levels on day 14. Least squares means for days 4, 8, 12, 14, 16, 18 and 20 were: .66, .81, 4.77, 14.31, 5.48, 3.68 and 2.97 ng/ml, respectively.     

Nutrient-driven O-GlcNAc cycling influences autophagic flux and neurodegenerative proteotoxicity

O-GlcNAcylation is an abundant post-translational modification implicated in human neurodegenerative diseases. We showed that loss-of-function of OGT (O-linked GlcNAc transferase) alleviated, while loss of OGA (O-GlcNAc selective β-N-acetyl-D-glucosaminidase) enhanced, the proteotoxicity of C. elegans neurodegenerative disease models including tauopathy, β-amyloid peptide and polyglutamine expansion. The O-GlcNAc cycling mutants act, in part, by altering insulin signaling, proteasome activity and autophagy. In mutants lacking either of these enzymes of O-GlcNAc cycling, there is a striking accumulation of GFP::LGG-1 (C. elegans homolog of Atg8 and LC3) and increased phosphatidylethanolamine (PE)-modified GFP::LGG-1 upon starvation. We speculate that O-GlcNAc cycling is a key nutrient-responsive regulator of autophagic flux acting at multiple levels including direct modification of BECN1 and BCL2.     

The evolution of nitrogen cycling

Based upon arguments concerning properties of the environment and the energetics of nitrogen transformation reactions, new hypotheses regarding their evolution are presented. These hypotheses are supported by new calculations and observations germane to understanding the evolution of the nitrogen cycle. From calculations of shock production by meteor impact, we suggest that impact produced fixed nitrogen could have resulted in the entire reservoir of Earth's N₂ being converted into fixed nitrogen at the end of accretion. We have significantly improved upon previous calculations of the abiotic fixation rate on the early earth and find a rate of fixation by lightning of 1–3 × 10¹⁶ Molecules NO/J, which is 2 to 3 times greater than previous estimates. This strengthens the suggestion, corroborated by the predominance of a single nitrogenase enzyme, that biological nitrogen fixation may have been a late evolutionary development, after the development of an aerobic atmosphere. In addition, we show for the first time that HNO, predicted to be the main product of atmospheric photochemical reactions involving NO on the primitive Earth by photochemical models, would eventually become NO₂ ^– and NO₃ ^– after reaching the Earth's surface. Based upon microbe-environment interactions on an ecological as well as a biochemical scale we suggest that denitrification arose prior to aerobic respiration and that nitrification arose after the advent of an aerobic atmosphere. We hypothesize the following evolutionary sequence for the biological transformation of nitrogen compounds: Ammonification Denitrification Nitrification Nitrogen fixation.     

Nutrient cycling strategies

This paper briefly reviews pathways by which plants can influence the nutrient cycle, and thereby the nutrient supply of themselves and of their competitors. Higher or lower internal nutrient use efficiency positively feeds back into the nutrient cycle, and helps to increase or decrease soil fertility. These tendencies are further enhanced by secondary effects (higher or lower rates of decomposition of litter and hence of nutrient mineralization) in resp. fertile and infertile soils. Such feedbacks may strongly increase the fitness of the plants involved. Plants can also influence the external inputs and outputs into the plant-soil system, by affecting the general hydrology of their environment. Sphagnum peat bogs exemplify an extreme degree of control of plants over the hydrological cycle, causing intense nutrient impoverishment by making the ecosystem dependent on nutrient supply from the atmosphere, apparently giving Sphagnum a competitive edge over other plants.     

Carbon and nitrogen stoichiometry and nitrogen cycling rates in streams

Stoichiometric analyses can be used to investigate the linkages between N and C cycles and how these linkages influence biogeochemistry at many scales, from components of individual ecosystems up to the biosphere. N-specific NH₄⁺ uptake rates were measured in eight streams using short-term ¹⁵N tracer additions, and C to N ratios (C:N) were determined from living and non-living organic matter collected from ten streams. These data were also compared to previously published data compiled from studies of lakes, ponds, wetlands, forests, and tundra. There was a significant negative relationship between C:N and N-specific uptake rate; C:N could account for 41% of the variance in N-specific uptake rate across all streams, and the relationship held in five of eight streams. Most of the variation in N-specific uptake rate was contributed by detrital and primary producer compartments with large values of C:N and small values for N-specific uptake rate. In streams, particulate materials are not as likely to move downstream as dissolved N, so if N is cycling in a particulate compartment, N retention is likely to be greater. Together, these data suggest that N retention may depend in part on C:N of living and non-living organic matter in streams. Factors that alter C:N of stream ecosystem compartments, such as removal of riparian vegetation or N fertilization, may influence the amount of retention attributed to these ecosystem compartments by causing shifts in stoichiometry. Our analysis suggests that C:N of ecosystem compartments can be used to link N-cycling models across streams.     

Retinoblastoma cell cycling

Techniques for the measurement of bromodeoxyuridine (BrdUrd) positive cells generally include either microscopic evaluation of paraffin embedded sections or measurements on cell suspensions using a fluorescent activated cell sorter. The accuracy of these measurements and their correlations can be affected by a number of technical and intrinsic tumor factors. Extrinsic parameters including degree of necrosis and tumor growth fraction are less easily analyzed in BrdUrd stained material. Retinoblastoma tumor cell cycling was prospectively studied in 11 children using in vivo and one child using in vitro BrdUrd. BrdUrd measurements were made by staining cell suspensions or sections of paraffin embedded tumor and analyzing by microscopy. Approximately 14% of viable cells were in the synthesis-phase of the cell cycle. The correlation between BrdUrd in cell suspensions and BrdUrd in paraffin embedded sections did not reach significance (r = 0.48). DNA analysis of these tumors was also performed using flow cytometry. Nine tumors were found to have a normal diploid DNA content, one had a G1 peak below the diploid control, two had a G1 peak above the diploid control, and one had two G1 peaks (a diploid and a hyperdiploid peak). There was no correlation between abnormal DNA content and the percent of cells in synthesis.     

The mathematics of breaking away and chasing in cycling

In cycling stage races a small group of riders will often form a “breakaway” and establish a lead over the main group. This paper examines the factors that affect the likelihood of success for the breakaway. A mathematical approach is used, drawing on a model of cycling previously developed and validated (Olds et al. J␣Appl Physiol 78:1596–1611, 1995). In a breakaway group, the power required to overcome air resistance is reduced because the lead can be shared, with trailing riders sheltering or drafting behind leading riders. The benefit of drafting can be quantified as a function of the distance between riders using previously obtained data. Of course, this advantage is even greater in the (larger) chasing group, so that eventually the chasing group will catch the breakaway, assuming identical bicycles and physiological characteristics. The question addressed is: what factors determine how great a lead the breakaway must have in order for the chasing group to be unable to catch the breakaway before the finish of the race? Demand-side simulations show that the critical factors are: the distance remaining in the race; the speed of the breakaway group; the number of riders in the chasing and breakaway groups; how closely riders in each group draft one another; the grade; surface roughness; as well as head- and cross-winds. When supply-side physiological factors are incorporated, the maximum sustainable speed and maximum lead time can be calculated. Accepted: 29 October 1997     

Towards a quantitative assessment of inorganic carbon cycling in photosynthetic microorganisms

Photosynthetic organisms developed various strategies to mitigate high light stress. For instance, aquatic organisms are able to spend excessive energy by exchanging dissolved CO₂ (dCO₂) and bicarbonate () with the environment. Simultaneous uptake and excretion of the two carbon species is referred to as inorganic carbon cycling. Often, inorganic carbon cycling is indicated by displacements of the extracellular dCO₂ signal from the equilibrium value after changing the light conditions. In this work, we additionally use (i) the extracellular pH signal, which requires non‐ or weakly‐buffered medium, and (ii) a dynamic model of carbonate chemistry in the aquatic environment to detect and quantitatively describe inorganic carbon cycling. Based on simulations and experiments in precisely controlled photobioreactors, we show that the magnitude of the observed dCO₂ displacement crucially depends on extracellular pH level and buffer concentration. Moreover, we find that the dCO₂ displacement can also be caused by simultaneous uptake of both dCO₂ and (no inorganic carbon cycling). In a next step, the dynamic model of carbonate chemistry allows for a quantitative assessment of cellular dCO₂, , and H⁺ exchange rates from the measured dCO₂ and pH signals. Limitations of the method are discussed.     

Identifying cycling genes by combining sequence homology and expression data

MOTIVATION: The expression of genes during the cell division process has now been studied in many different species. An important goal of these studies is to identify the set of cycling genes. To date, this was done independently for each of the species studied. Due to noise and other data analysis problems, accurately deriving a set of cycling genes from expression data is a hard problem. This is especially true for some of the multicellular organisms, including humans. RESULTS: Here we present the first algorithm that combines microarray expression data from multiple species for identifying cycling genes. Our algorithm represents genes from multiple species as nodes in a graph. Edges between genes represent sequence similarity. Starting with the measured expression values for each species we use Belief Propagation to determine a posterior score for genes. This posterior is used to determine a new set of cycling genes for each species. We applied our algorithm to improve the identification of the set of cell cycle genes in budding yeast and humans. As we show, by incorporating sequence similarity information we were able to obtain a more accurate set of genes compared to methods that rely on expression data alone. Our method was especially successful for the human dataset indicating that it can use a high quality dataset from one species to overcome noise problems in another. AVAILABILITY: C implementation is available from the supporting website: http://www.cs.cmu.edu/~lyongu/pub/cellcycle/.