Magnitude Of Turnover Research Articles

Inducibility of crassulacean acid metabolism (CAM) in Clusia species; physiological/biochemical characterisation and intercellular localization of carboxylation and decarboxylation processes in three species which exhibit different degrees of CAM

The biochemical basis for photosynthetic plasticity in tropical trees of the genus Clusia was investigated in three species that were from contrasting habitats and showed marked differences in their capacity for crassulacean acid metabolism (CAM). Physiological, anatomical and biochemical measurements were used to relate changes in the activities/amounts of key enzymes of C3 and C4 carboxylation to physiological performance under severe drought stress. On the basis of gas-exchange measurements and day/night patterns of organic acid turnover, the species were categorised as weak CAM-inducible (C.aripoensis Britt.), C3-CAM intermediate (C. minor L.) and constitutive CAM (C.␣rosea Jacq. 9.). The categories reflect genotypic differences in physiological response to drought stress in terms of net carbon gain; in C. aripoensis net carbon gain was reduced by over 80% in drought-stressed plants whilst carbon gain was relatively unaffected after 10 d without water in C. rosea. In turn, genotypic differences in the capacity for CAM appeared to be directly related to the capacities/amounts of phosphoenolpyruvate carboxylase (PEPCase) and phosphoenolpyruvate carboxykinase (PEPCK) which increased in response to drought in both young and mature leaves. Whilst measured activities of PEPCase and PEPCK in well-watered plants of the C3-CAM intermediate C. minor were 5–10 times in excess of that required to support the magnitude of organic acid turnover induced by drought, close correlations were observed between malate accumulation/PEPCase capacity and citrate decarboxylation/PEPCK capacity in all the species. Drought stress did not affect the amount of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein in any of the species but Rubisco activity was reduced by 35% in the weak CAM-inducible C. aripoensis. Similar amounts of glycine decarboxylase (GDC) protein were present in all three species regardless of the magnitude of CAM expression. Thus, the constitutive CAM species C. rosea did not appear to show reduced activity of this key enzyme of the photorespiratory pathway, which, in turn, may be related to the low internal conductance to CO2 in this succulent species. Immuno-histochemical techniques showed that PEPCase, PEPCK and Rubisco were present in cells of the palisade and spongy parenchyma in leaves of species performing CAM. However, in leaves from well-watered plants of C. aripoensis which only performed C3 photosynthesis, PEPCK was localized around latex-producing ducts. Differences in leaf anatomy between the species suggest that the association between mesophyll succulence and the capacity for CAM in these hemi-epiphytic stranglers has been selected for in arid environments.

Gross flows vs. net flows in the labor market: What is there to be learned?

Gross flows of firms, jobs and workers are very large in all phases of the business cycle, implying a great deal of heterogeneity. The order of magnitude of gross job turnover is between 16% and 23% per year, surprisingly similar across countries that differ in their institutions and net employment growth. Worker turnover is also impressive: it is found to be somewhat more variable across countries than job turnover. Relations between job flows and worker flows are established by the vacancy chain model.

Rapid plasma virus and CD4+T-cell turnover in HIV-1 infection

To analyse the short-term kinetics of viral plasma RNA and CD4+ T cells numbers in patients with different initial CD4+ T-cell counts treated with different antiretroviral regimens. In 10 HIV-1 positive patients, in vivo kinetics of plasma HIV RNA and CD4+ T cells were studied during antiretroviral treatment. Lymphocyte subpopulation analysis, quantitative polymerase chain reaction (PCR), p24 antigen enzyme immunoassay (EIA) and beta 2-microglobulin EIA were performed at days 0, 3, 7, 10, 14, 21 and 28 of treatment. One additional patient served as a control. The resulting curves were fitted. Half-lives were calculated using the time constant T of decrease or increase [T1/2 = In(2) x T]. Calculations of virus and CD4+ T-cell turnover were multiplied by the total blood volume. Viral plasma RNA half-life ranged from 1.1 to 5.1 days, independent of prior or actual treatment and initial CD4+ T-cell count. The calculated peripheral blood viral plasma RNA turnover varied between 0.02 and 55.8 x 10(8) copies/ml/day and showed some negative correlation with initial CD4+ T-cell counts. CD4+ T-cell turnover estimates ranged from 0.01 to 7.5 x 10(8) cells/day. Most patients showed an immediate reincrease of virus load after the nadir. Changes in HIV p24 antigen paralleled HIV plasma RNA in p24 antigen-positive patients. beta 2-microglobulin decreased until day 7-15 in all but one case and rapidly reincreased to pretreatment values. The kinetics of virus and CD4+ T-cell turnover are uniformly rapid throughout a wide range of initial CD4+ T-cell counts. The magnitude of virus turnover varies considerably among individuals and appears to be inversely related to the initial CD4+ T-cell count. These data also argue for a rapid resumption of virus production and lymphocyte turnover during treatment.

Preservational, paleoecological and evolutionary patterns in the Paleogene of Wyoming-Montana and the Neogene of Pakistan

Abstract Contributions to this special issue have compared the tectonic settings, fluvial systems, paleoclimates, paleofloras and faunas, vertebrate taphonomy, and mammalian paleoecology and evolution from two long, continental records of Cenozoic ecosystems. In this concluding paper, we summarize highlights of earlier papers to provide an overview of similarities and differences between these Paleogene and Neogene records. The relative influences of tectonic, climatic, and fluvial processes on lithofacies, environments of fossil preservation, and the productivity of the fossil record are compared for each sequence. Tectonism was the primary control on sediment accumulation rate and the distribution of major depositional environments in each basin. Fossil productivity, species richness, and the distribution of fossil localities among fluvial environments change across formation boundaries in each sequence. The taxonomic, temporal, and spatial resolution of vertebrate assemblages varies among fluvial environments and differs between the two sequences. As a result, the Paleogene record is better suited for detailed study of evolutionary change within local lineages and the Neogene record for detailed paleocommunity reconstruction. Several aspects of biotic change are considered. (1) The timing and magnitude of mammalian faunal turnover in relation to climatic change are evaluated in terms of three models of evolutionary change within ecosystems: Van Valen's “Red Queen model,” Stenseth and Maynard Smith's “stationary model,” and Vrba's “turnover-pulse model.” In both records, climatic change was accompanied by change in faunal composition and ecological structure, but both pulsed and diachronous biotic change, at a resolution of about 0.5 m.y., also occurred over periods when no significant climatic change was discernible. This pattern best matches the predictions of the Red Queen model. (2) In both records, the rate of mammalian faunal turnover (first and last appearances of taxa) is not highly correlated with standing richness, suggesting that paleocommunities were not a equilibrium richness on these time scales. (3) In the Paleogene record, plant species richness declined while mammalian species richness increased. (4) In each sequence, some episodes of mammalian faunal turnover were in step with changes in size or trophic structure, while other changes in faunal composition entailed no corresponding change in ecological structure. While both records would benefit from improved paleoclimatic, temporal, species-level, and ecomorphological resolution, this initial synthesis suggests that both physical and biotic factors were important influences on faunal composition, the timing and rate of turnover, and ecological structure in these paleocommunities.

Patterns of faunal turnover and diversity in the Neogene Siwaliks of Northern Pakistan

The fluvial Neogene Siwalik formations of northern Pakistan contain a long and richly fossiliferous sequence of terrestrial vertebrate faunas in which patterns of faunal turnover and changes in diversity can be documented and analyzed for intervals having durations of 0.5 m.y. The complete sequence extends from circa 18.5 to 1 Ma, but the part between 18.5 and 5.5 Ma is best sampled, and most intervals within it are well represented. Thirteen orders of Siwalik mammals have been identified, with well-sampled intervals having 50 or more species. Most Siwalik mammals, however, are either rodents or artiodactyls. Bovids are the most common and most speciose of the larger mammals, while murid and “cricetid” rodents dominate the small mammal assemblages. Between 18.5 and 5.5 Ma species diversity varied considerably. Among artiodactyls and rodents the number of species first increased between 15 and 13 Ma and then fell. Data on stratigraphic ranges of rodents and artiodactyls show that faunal change in the Siwaliks was episodic, occurring during short intervals with high turnover followed by longer periods with considerably less change. Maxima of first appearances occurred at approximately 13.5 and 8.5 Ma, while maxima of last occurrences were at 12.5 and 8.0 Ma. Some of the observed faunal events can be correlated to climatic and environmental changes. The Middle Miocene diversification occurred during a period of global cooling, while the latest Miocene decline in diversity and increased turnover accompanied oxygen and carbon isotopic changes that correlate to globally increasing seasonality and aridity. Other correlations are ambiguous. The marked decrease in diversity and the major turnover events between 13 and 8 Ma do not correspond to known local or global events. The Neogene Siwaliks and Paleogene Bighorn-Crazy Mountains sequence in Wyoming and Montana share many similarities. They have equivalent levels of temporal resolution and similar levels of completeness of their fossil records. Siwalik ordinal abundance and diversity patterns differ markedly from those of the Paleogene, but generic, and probably species, diversity was approximately the same, although the Siwalik faunas may have been slightly less diverse. Over time, changes in diversity were of comparable magnitude, with monotonic trends persisting for more than 5 million years. The magnitude of faunal turnover was also similar, ranging from less than half to 3.5 times that expected. In both sequences faunal change appears to have been episodic, with strong pulses between intervals of low turnover. The Siwaliks, in contrast to the Paleogene sequence, may have had more distinct pulses and longer intervals between pulses. Neither sequence has peaks of first occurrences coinciding with peaks of last occurrences.

Why are establishments so heterogeneous?

Changes in the size of establishments — as opposed to the turnover of workers across a given set of jobs — account for a non-negligible component of gross workers flows. Based on a rich database of establishment-level employment records in western Germany, this paper analyses the magnitude and determinants of job turnover and its relation with labour turnover. Consistently with findings from other countries, it is shown that job turnover is mainly the byproduct of the dispersion of establishment-level outcomes within any industry rather than of job reallocation across different sectors. The role played by disturbances to consumer preferences over differentiated products is also assessed in an attempt to shed some light on the determinants of this tremendous heterogeneity of establishment-level employment changes within each industry.

Influence of leaf traits on the spatial distribution of insect herbivores associated with an overstorey rainforest tree.

The spatial distribution of insect herbivores associated with the Australian rainforest treeArgyrodendron actinophyllum (Sterculiaceae) was investigated by restricted canopy fogging. The foliage of this species was low in nitrogen and water content, and high in fibre content. Herbivore abundance was positively correlated with the amount of young foliage present within the samples and in adjacent samples, and with the nitrogen content of young leaves. In particular, the occurrence of phloem-feeders was correlated with the magnitude of translocation within the samples. The influence of leaf water content upon herbivore distribution was marginal, presumably because this factor is not limiting in rain-forest environments during the wet season, which usually coincides with the season of leaf-flush. Specific leaf weight, leaf size and foliage compactness had little or no apparent effect on herbivore distribution. Since the magnitude of leaf turnover affected both the quantity and the quality, as exemplified by translocation effects, of young foliage available, this factor may be critical to herbivores associated with evergreen rainforest trees which are particularly low in foliar nutrients, such asA. actinophyllum.

Employee Turnover among Full-Time Public Librarians

Analysis of employee turnover is common in business and industry. Understanding the cause and magnitude of turnover can help assess basic aspects of the organization such as the adequacy of pay, hiring practices and evaluation procedures, fairness of supervision, effectiveness of communication, and quality of leadership. Unfortunately, turnover analysis in librarianship is rare. The purpose of this study is to examine employee turnover in thirty-one public libraries in the Midwest. Particular emphasis is placed on establishing base-line turnover rates and examining the relationship of gender to turnover behavior. Among the findings are that (1) turnover rates are low compared to other occupations and (2) turnover rates of males and females are similar, but females are more likely to leave their jobs for family and personal reasons.

Regulation of nucleosomal core histone variant levels in differentiating murine erythroleukemia cells.

During hexamethylenebis(acetamide)-induced terminal differentiation of murine erythroleukemia (MEL) cells in vitro, the histone variant proportions undergo changes similar to those observed in vivo in terminally differentiating cells of the young mouse. Thus, there is a rapid increase in the relative amounts of the variants H2A.1 and H2B.2 in parallel with the increase in the number of hemoglobin-producing cells and the sharp decrease in the growth rate. We show that the changes in variant proportions are not associated with slower growth per se but are most likely due to differential changes in the rates of variant synthesis as a result of commitment to terminal differentiation. In addition, we observed an inducer-specific increase in the rate of synthesis and the relative amount of the minor H2A variant 4, well before hemoglobin accumulation. We also present evidence that H2A and H2B histones are synthesized and incorporated into chromatin at a significant rate even when DNA synthesis is inhibited, suggesting turnover of these histones. H2A and H2B turnover can be detected directly even in exponentially growing cells. H2A.1 and H2B.2 have higher turnover rates than H2A.2 and H2B.1, respectively, in exponentially growing cells, a difference which is even more pronounced in induced cells. The magnitude of the differential turnover is not sufficient to account for the changes in the histone variant proportions in the short life of induced MEL cells but could explain the slow accumulation of H2A.2, H2B.1, and H3.3 in nondividing adult tissues of the mouse.

An algae - Desert snail food chain: Energy flow and soil turnover.

The feeding ecology of adult desert snails (Sphincterochila zonata), inhabiting a loess plain in the Negev Desert, was studied during a 7-year period. Energy flow and soil turnover determinations were made at the individual and population level on the basis of field observations and data derived from laboratory simulations. Sphincterochila zonata were only active on 8-27 winter days annually, otherwise they were dormant. The snails fed exclusively on algae that grew on the soil surface following rain. The mean annual dry-weight biomass of algae was 197.4(±118.1) gr·m-2. The dry weight biomass of snails ranged from 0.02-0.14 gr·m-2. Annual production in the food chain varied substantially from year to year (1-1539 Kj·m-2), but the net annual energy balance of the snails was always positive. Soil crust turnover, resulting from grazing of snails on algae, was estimated at 142 kg·hectare-1 during the study period.The annual magnitude of energy flow and soil turnover is determined by the soil surface moisture regime which in turn, is a function of rainfall patterns. Sphincterochila zonata may serve an important role in Negev ecosystems by dispersing soil algae and altering soil structure. The snails are not subject to substanding predation by rodents but may occasionally serve as an important food source for migrating birds.

The actions of parathyroid hormone on bone: Relation to bone remodeling and turnover, calcium homeostasis, and metabolic bone disease: Part I of IV parts: Mechanisms of calcium transfer between blood and bone and their cellular basis: Morphological and kinetic approaches to bone turnover

The supracellular organization of living bone enables the study of isolated cellular and subcellular systems to be related to the study of the whole organism. Bone is formed by osteoblasts in successive stages, separated in both time and space, of matrix formation and primary mineralization. Osteoblasts are joined by tight junctions and largely cover the osteoid seam which separates them from mineralized bone. Secondary mineralization is not completed for several months and is not regulated by the osteoblast. Bone is resorbed by osteoclasts which simultaneously accomplish mineral dissolution and matrix digestion. Active osteoblasts occupy about 5% of the free bone surface, osteoid seams with less active osteoblasts about 10%, active osteoclasts about 0.5%, and Howship's lacunae at which bone remodeling is either quiescent or arrested about 5%. The remaining 80% of the free bone surface is covered by a leaky envelope of thin flattened cells, termed surface osteocytes. Some osteoblasts become permanently buried in the bone as deep osteocytes, around which a specialized and metabolically active perilacunar bone is formed. This bone is less highly mineralized and can temporarily lose or gain calcium in accordance with homeostatic needs. Deep osteocytes maintain contact with each other and with the surface osteocytes, their cell processes within canaliculi being joined by gap junctions. Remodeling of cortical bone proceeds with the excavation by osteoclasts of a longitudinal tunnel which is refilled by osteoblasts to form a new osteon. The anatomically discrete longitudinally oriented structure consisting of a cutting cone of osteoclasts in front and a closing cone of osteoblasts behind is termed a cortical remodeling unit. The events of centrifugal resorption and centripetal formation which occur in a single cross section is termed a cortical remodeling cycle. Normally each new cycle is slightly out of phase with its predecessor. The quantities which characterize cortical remodeling are the birth rate of new remodeling cycles or activation frequency (mu), and the durations of the resorptive period (sigma r), the quiescent interval (sigma q) and the formation period (sigma f). The average distances traveled by the osteoclast and osteoblast are indicated respectively by the mean cement line diameter and mean wall thickness of completed osteons. These quantities show little interindividual variation. Because of this constancy the magnitude of bone turnover (the bone formation rate) is almost entirely a function of mu, the activation frequency of new remodeling cycles. Variations in the velocity of advance of osteoclasts (the linear resorption rate) or of osteoblasts (the appositional rate) alter inversely both the extent of surface engaged in resorption or formation and the time taken to replace a particular moiety of bone, but in a steady state do not influence the rate of turnover of the skeleton as a whole...

Magnitude and significance of NAD turnover in human cell line D98/AH2

IN the oxidation–reduction metabolism of a cell the pyridine nucleotide NAD is used catalytically; any NAD that is reduced is reoxidised. But NAD is consumed in certain metabolic reactions in which it serves as a substrate. In eukaryotic cells, the most intriguing of these reactions is the cleavage of NAD to form nicotinamide and a unique polymer, poly adenosine diphosphoribose (poly ADPR) (Fig. 1); the reaction is catalysed by the enzyme poly ADPR synthetase1–4. Other well studied reactions involving the destruction of NAD include the cleavage of NAD to form AMP and NMN by bacterial DNA ligases5,6 and the breakdown of NAD (with the concomitant inactivation of protein synthesis) by diphtheria toxin7,8.

Cholesterol Turnover in Man

Publisher Summary This chapter provides an overview of cholesterol turnover in man. The magnitude of cholesterol turnover and the factors that regulate it differ among different tissues; some of these factors are unique for a specific tissue yet are not identical in the same tissue in all mammalian species. The heterogeneous nature of cholesterol metabolism emphasizes the oversimplification that is inherent in attempts to measure and quantitate cholesterol turnover in an entire organism. The modes of action of a number of cholesterol-lowering drugs have been studied in man by measuring cholesterol turnover. The various techniques for the measurement of cholesterol turnover are ideally suited for the definition of the overall modes of action of these drugs in the entire living organism. Techniques that merely require measurement of plasma cholesterol specific activity are simpler to perform, but they do not answer the questions that can be solved by direct quantitation of fecal steroids or by a combination of the two methods. The following are the different techniques and the information that they have provided: (1) plasma specific activity-time curve analysis, (2) isotope sterol balance methods, (3) chromatographic techniques, and (4) turnover of bile acids.

RNA synthesis in Taper hepatoma and mouse-liver cells

The RNA synthesized in normal mouse liver and the Taper hepatoma were compared by DNA/RNA hybridization experiments. The hepatoma cells appear to contain a more diverse selection of molecules than are present in normal liver. The tumor cell nuclei contain sequences of rapidly labeled RNA which are not transferred to the cytoplasm, although the magnitude of this intranuclear turnover appears to be lower than in normal liver.

Ultrastructural Differences Between Thymic and Lymph Node Small Lymphocytes of Mice : Nucleolar Size and Cytoplasmic Volume

Abstract Ultrastructural differences between small lymphocytes of the thymic cortex and those located in the cortical zone of lymph nodes of mice were examined by electron microscopy and cytometric methods. The particle size analyzer TGZ 3 (Zeiss) was used to measure the size-frequency distribution of nucleolar cross-section diameters, and the resulting data were computed mathematically and statistically to determine the corresponding mean volumina. It could be shown that thymic, as compared to lymph node cortical small lymphocytes, (1) have a smaller mean nucleolar volume (0.11µ3 versus 0.46µ3); (2) represent a more uniform population with regard to the frequency distribution of the nucleolar cross section diameters; and (3) are of smaller size due to less cytoplasm, while the mean nuclear volume does not differ significantly from that of small lymphocytes located in the cortex of lymph nodes. These findings are discussed in relation to qualitative ultrastructural peculiarities and probable differences in the magnitude of RNA turnover as estimated by the use of tritiated cytidine. The possible significance of cytometrically defined populations of small lymphocytes with regard to sensitized and nonsensitized cells is considered.

EFFECT OF AGE AND HORMONAL STATE ON THE NUMBERS OF DEOXYRIBONUCLEIC ACID SYNTHESIZING NUCLEI IN RAT ADRENAL CORTEX.

THE use of tritiated thymidine to label nuclei synthesizing DNA (S-phase) prior to mitosis has provided much new information on the pattern and magnitude of cell turnover in many tissues and organs. Less is known, however, about the relative frequency of cell turn-over in the adrenal gland. Accordingly the investigation described here was undertaken to determine whether the number of nuclei in DNA synthesis in the different zones of the rat adrenal cortex varied with age and with experimental procedures designed to alter its hormonal environment.

The serum lipoprotein and cholesterol concentrations of Central and North Americans with different dietary habits

1. 1. The total cholesterol and beta-lipoprotein measurements of the blood sera of a group of rural Central American subjects subsisting on a largely vegetarian and very low fat diet have been compared with those of urban Guatemalan and North American subjects who habitually eat large amounts of fat. 2. 2. The rural Central American subjects, both male and female, were shown to have lower mean cholesterol levels at all ages studied than did the other groups. There was little evidence of the typical North American increase of serum cholesterol level with age among these Central Americans. There was no sex-determined difference of mean cholesterol levels by decade among the rural Central American subjects. The cholesterol levels of urban Guatemalans living in a good economic status were similar to those of North Americans. 3. 3. The serum lipoprotein differences observed among the groups were variable. They were only slightly and irregularly lower in the rural Central American males than in the North Americans and the beta-lipoproteins of the females of this group were frequently at higher levels than those of the North Americans. The urban Guatemalans showed lipoprotein levels as high or higher than those of the North American group. 4. 4. These cholesterol and lipoprotein level differences could not be explained by the greater leanness of the Central Americans although this factor could be shown to have a small effect. 5. 5. The greater leanness of the rural Guatemalan group taken with higher caloric consumption of that group suggests that the serum lipoprotein levels may be dependent upon the magnitude of energy turnover whereas the serum cholesterol levels are increased by energy accretion or fat deposition. 6. 6. The differences in dietary fat intake among the groups do not serve to explain the serum lipid differences because they do not permit an explanation of the dissociation of cholesterol and lipoprotein measurements which is unique in the rural Guatemalans. 7. 7. These Guatemalan populations appear to be a useful place to evaluate the relative importance of serum beta-lipoprotein and cholesterol levels in atherogenesis.