Density-dependent Decrease Research Articles

High-dose factor VIIa increases initial thrombin generation and mediates faster platelet activation in thrombocytopenia-like conditions in a cell-based model system.

Clinical experience has shown that high doses of recombinant factor VIIa (rFVIIa) may ensure haemostasis in thrombocytopenic patients. We have used a cell-based model system to mimic thrombocytopenia and analyse the effect of rFVIIa. Lowering the platelet density from 200 x 10(9)/l (reflecting normal conditions) to 100, 50, 20 and 10 x 10(9)/l revealed a platelet density-dependent decrease in the maximal rate of thrombin generation, a prolongation in the time to maximal thrombin activity and a lower maximal level of thrombin formed. The platelet activation, measured as the time to half-maximal P-selectin (CD62) exposure, was not significantly dependent on the platelet density in the range of 200 x 10(9)/l to 10 x 10(9)/l, although there was a tendency for slower platelet activation at 20 x 10(9) and 10 x 10(9) platelets/l than at the higher platelet densities. Addition of 50--500 nmol/l rFVIIa to samples with 20 x 10(9) or 10 x 10(9) platelets/l shortened the lag phase of thrombin generation as well as the time to half-maximal platelet activation. Our data indicate that high doses of rFVIIa may help to provide haemostasis in thrombocytopenic patients by increasing the initial thrombin generation, resulting in faster platelet activation and thereby compensating for the lower number of platelets present.

Importance of bioturbation and feeding by the polychaete Capitella sp. I in the degradation of di(2-ethylhexyl)phthalate (DEHP)

A microcosm study of the effect of Capitella sp. I (Polychaeta) population density on the mineralization of a pulse addition of the plasticizer DEHP (di[2-ethylhexyl]]phthalate) demonstrated a 2-fold increase in mineralization (cumulative 14 CO 2 production) when worms were present. An additional experiment investigated the fate of ingested particle-bound 14 C-DEHP and measured the loss of ingested 14 C-DEHP into 14 CO 2 , DO 14 C and PO 14 C pools. Less than 1% of 14 C consumed during a 1 h ingestion period was ultimately respired as 14 CO 2 , while 10% was excreted as DO 14 C and 89% as PO 14 C, Approximately 1% of ingested 14 C was retained in worm tissue 20 h after ingestion. Assuming density-independent feeding rates, worm respiration could account for 4.5% to 19.1% of the total microcosm 14 CO 2 production, suggesting that microbial respiration to 14 CO 2 was the dominant process. Pre-exposure of worms to DEHP (10 μg g -1 sediment dry wt) for 1 wk had no effect on the fate of ingested DEHP and distribution into the respective pools. Worms exerted a strong effect on ultimate DEHP degradation ( 14 CO 2 production) but the effect was manifest at the lowest worm density and did not increase with increasing population size. The lack of an increased effect at greater population densities may be due to population density-dependent factors acting to decrease such important parameters as individual worm ventilation and feeding rates. A density-dependent decrease in feeding rate is supported by the observation that measured 14 C body burdens were highest at lowest worm densities.

Thermal-Noise Temperature in GaAs-AlGaAs Heterojunctions

The theory of hot-electron thermal noise in quasi-two-dimensional systems under a strong dc electric field proposed by LEI et al. is extended to the case of multi-subband occupations. As an example, we use this extended method to calculate the thermal-noise temperature of GaAs-AlGaAs heterojunction systems at different dc electric fields including the contributions of the lowest and next lowest subbands. We find that, in comparison with the results of the one-subband theory, the inclusion of a higher subband yields an electron density-dependent decrease of the electron temperature and the thermal-noise temperature and a reduction of the cooling effect. The numerical results also confirm the fact that considering only the lowest subband of heterojunction systems is a sufficiently good approximation in the case of sufficiently low electron densities not only for common transport problems but also for thermal noise.

Regulation and control of intracellular algae (= zooxanthellae) in hard corals

To examine algal (= zooxanthellae) regulation and control, and the factors determining algal densities in hard corals, the zooxanthellae mitotic index and release rates were regularly determined in branch tips from a colony of a staghorn coral,Acropora formosa, recovering from a coral ‘bleaching’ event (the stress–related dissociation of the coral–algal symbiosis). Mathematical models based upon density–dependent decreases in the algal division frequency and increases in algal release rates during the post–bleaching recovery period accurately predict the observed recovery period (≈ 20 weeks). The models suggest that (i) the colony recovered its algal population from the division of the remaining zooxanthellae, and (ii) the continual loss of zooxanthellae significantly slowed the recovery of the coral. Possible reasons for the ‘paradoxical’ loss of healthy zooxanthellae from the bleached coral are discussed in terms of endodermal processes occurring in the recovering coral and the redistribution of newly formed zooxanthellae to aposymbiotic host cells. At a steady–state algal density of 2.1 × 106zooxanthellae cm−2at the end of the recovery period, the zooxanthellae would have to form a double layer of cells in the coral tissues, consistent with microscopic observations. Neighbouring colonies ofA. formosawith inherently higher algal densities possess proportionately smaller zooxanthellae. Results suggest that space availability and the size of the algal symbionts determines the algal densities in the coral colonies. The large increases in the algal densities reported in corals exposed to elevated nutrient concentrations (i.e between a two– and five–fold increase in the algal standing stock) are not consistent with this theory. We suggest that increases of this magnitude are a product of the experimental conditions: reasons for this statement are discussed. We propose that the stability of the coral–algal symbiosis under non–stress conditions, and the constancy of zooxanthellae densities in corals reported across growth form, depth and geographic range, are related to space availability limiting algal densities. However, at these densities, zooxanthellae have attributes consistent with nutrient limitation.

Density-dependent ant attendance and its effects on the parasitism of a honeydew-producing scale insect, Ceroplastes rubens.

The intensity of attendance by a honeydew-foraging ant, Lasuis niger, on the red wax scale insect, Ceroplastes rubens, was estimated at different manipulated densities in the field. The time that individual ants were present and the total attendance time (seconds x number of ants) of ants on scale-infested twigs significantly increased as the density of C. rubens increased, i.e. ant attendance was density dependent. To determine the effects of density dependence of ant attendance on parasitism of C. rubens by Anicetus beneficus, we measured parasitism rates in the field at different density levels of C. rubens both with ant attendance and with ants excluded. Parasitism rates were higher when ants were excluded, at each density level. Although the parasitism rate significantly deceased as scale density increased, whether or not ants attended, the difference in parasitism rate between density levels was strikingly less without ant attendance. Therefore, the density-dependent decrease of parasitism rate was more pronounced with ant attendance. Mortality not due to parasitism showed density dependence in both conditions and did not change when ants were excluded. These results indicate that attending ants reduce parasitism and that, as a consequence of the density dependence of ant attendance, the efficiency of reduction of parasitism by ants is enhanced at higher densities of C. rubens.

Population-regulating processes during the adult phase in flatfish

Flatfish support major fisheries and the study of regulatory processes are of paramount importance for evaluating the resilience of the resource to exploitation. This paper reviews the evidence for processes operating during the adult phase that may 1. generate interannual variability in recruitment; 2. contribute to population regulation through density-dependent growth, density-dependent ripening of adults and density-dependent egg production. With regard to (1), there is evidence that in the adult phase processes do occur that may generate recruitment variability through variation in size-specific fecundity, contraction of spawning season, reduction in egg quality, change in sex ratio and size composition of the adult population. However, time series of recruitment do not provide support for this hypothesis. With regard to (2), there is ample evidence that exploitation of flatfish coincides with an increase in growth, although the mechanisms involved are not always clear. The presence of density-dependent growth in the adult phase of unexploited populations appears to be the most likely explanation in some cases. From the early years of exploitation of flatfish stocks inhabiting cold waters, evidence exists that adult fish do not spawn each year. Fecundity schedules show annual variations, but the available information suggests that size-specific fecundity is stable over a broad range of population abundance and may only decrease at high population abundance. The analysis is complicated by the possibility of a trade-off between egg numbers and egg size. Nevertheless, a density-dependent decrease in growth will automatically result in a decrease in absolute fecundity because of the reduced body size. The potential contribution of these regulatory effects on population regulation is explored. Results indicate that density-dependent ripening and absolute fecundity, mediated through density-dependent growth, may control recruitment at high levels of population abundance. The effect of a density-dependent decrease in size-specific fecundity seems to play a minor role, although this role may become important at extremely high levels of population abundance.

Cell density‐dependent regulation of cell surface expression of two types of human tumor necrosis factor receptors and its effect on cellular response

Tumor necrosis factor (TNF) is a multipotential cytokine known to regulate the growth of a wide variety of normal and tumor cells. It has been shown that the density of cells in culture can modulate the growth regulatory activities of TNF, the mechanism of which, however, is not understood. In this report, we investigated the effect of cell density on the expression of TNF receptors. The receptors were examined on epithelial cells (e.g., HeLa), which primarily express the p60 form, and on myeloid cells (e.g., HL-60) known to express mainly the p80 form. We observed that binding of TNF to both cell lines decreased with increase in cell density. Scatchard analysis of binding on HeLa and HL-60 cells revealed a 4- to 5-fold reduction in the number of TNF receptors without any significant change in receptor affinity in both cell types at high density. The decrease in TNF receptor numbers at high cell density was also observed in several other epithelial and myeloid cell lines. The downmodulation at high cell density was unique to TNF receptors, since minimum change in other cell surface proteins was observed as revealed by fluorescent activated cell sorter analysis. Neutralization of binding with antibodies specific to each type of the receptors revealed that both the p60 and p80 forms of the TNF receptor were equally downmodulated. A decrease in leucine incorporation into proteins was observed with increase in cell density, suggesting a reduction in protein synthesis. Since inhibition of protein synthesis by cycloheximide also leads to a decrease in TNF receptors, it is possible that the density-dependent reduction in TNF receptor number is due to an overall decrease in protein synthesis. The density-dependent decrease in TNF receptors was accompanied by a decrease in intracellular reduced glutathione levels. A reduction in the number of receptors on TNF sensitive tumor cells induced by cell-density correlated with increase in resistance to the cytokine.

Impedance measurement during air and He-O2 breathing before and after salbutamol in normal subjects

Total respiratory resistance and reactance from 4 to 52 Hz were determined by the method of forced pseudorandom noise oscillation in 20 normal male subjects before and after inhalation of 0.200 mg salbutamol (albuterol) and before and after the subjects were equilibrated with 80% He-20% O2. During air breathing, there was a statistically significant decrease of resistance values at lower frequencies after inhalation of salbutamol. When the subject was equilibrated with 80% He-20% O2, total respiratory resistance markedly decreased at all frequencies, and a negative frequency dependence of resistance was observed between 8 and 20 Hz. Resistance values further decreased during He-O2 breathing after inhalation of salbutamol. After inhalation of salbutamol, reactance values increased during air and He-O2 breathing. The density-dependent decrease of the real part of impedance can be explained by a decrease of turbulence in the larger airways. The bronchodilating effect of salbutamol was not influenced by a change in the physical properties of the inhaled gas. During He-O2 breathing, reactance values significantly decreased, resulting in an increase of resonant frequency due to a decrease of inductive reactance. It is concluded that an increase in the capacitance of the respiratory system must be supposed to explain the increase in reactance values after inhalation of the beta-adrenergic agonist salbutamol.

Benthic Algal Community Dynamics in a Stream during and after a Spate

Changes in a benthic algal community were observed during and after a major storm. Algal abundance, mostly diatoms, increased slightly during a 3-d interval that included the storm on day 1. Positive effects of the storm on benthic algae were indicated by decreases in the proportions of diatoms that were dead, but not in the dead cell numbers, and by decreases in the proportions of sexually reproducing cells. Increases in current or nutrient supply to periphyton during the storm may have stimulated growth of live cells and retarded sexual reproduction. During two 3-d growth periods after the storm, benthic diatoms grew rapidly from 1.1× 10⁷ cells/cm² and seemed to reach a carrying capacity near 5.2× 10⁷ cells/cm² in only 8 days. Density-dependent decreases in growth rates were accompanied by increases in the proportion of diatoms that were dead or in stages of sexual reproduction, indicating nutrient limitation. Immigration was not an important mechanism of accumulation immediately after the storm. However, immigration/colonization rates increased during community development and indicated substantial turnover by cell transport during late succession. Benthic diatom communities are well adapted to storms in temperate climates in streams with gravelbed channel morphology. I hypothesize that benthic diatoms are actually affected positively by most storms and are negatively affected only by the most severe storms that overturn substrates in the stream.

DENSITY DEPENDENCE OF RESPIRATORY INPUT IMPEDANCE IN NORMAL SUBJECTS

1. The forced oscillation technique is a non-invasive and effort-independent test used to characterize the mechanical impedance of the respiratory system. Total respiratory impedance was assessed from 4 to 52 Hz in 15 normal subjects breathing air and a helium-oxygen mixture. 2. Breathing helium-oxygen reduced respiratory resistance and its frequency dependence as well as respiratory reactance very significantly. Resonant frequency during He-O2 breathing was 1.88 times higher than during air breathing. 3. It is concluded that by impedance measurement of the respiratory system in normal subjects a density-dependent decrease of respiratory resistance due to decreased turbulence in the larger airways and a density-dependent decrease in the inductive reactance can be found during breathing of low density gas mixtures.

Population dynamics of a phytophagous lady‐beetle, Henosepilachna vigintioctopunctata (Fabricius), living in spatio‐temporally heterogeneous habitats. I. Estimation of adult population parameters based on a capture‐recapture census

Summary The population processes of the lady‐beetle, Henosepilachna vigintioctopunctata (F.), were studied from the spring of 1976 to 1979 in an area (about 150,000 m2) including 17 patches of host plants. Amplitudes of yearly fluctuations of adult densities were 2.9 for overwintered adults, and 1.4 and 1.9 for the first and the second generation adults, respectively. The higher the adult density, the lower the rate of increase of the number of adult females to the next generation. However, the density dependent decrease in the reproductive rate was not caused by starvation. The patchy nature of the farmland habitats seemed to have effects on the stabilization of population numbers.

Mannose and the ‘Crowding effect’ of Hymenolepis in rats

Hymenolepis diminuta from rats infected with 10 cysticercoids and fed on a diet containing 3% (w/w) mannose for 4 weeks were found to be, on average, much heavier in terms of dry weight (46 mg) than those from rats fed on diets containing an equivalent concentration of either galactose, glucose or fructose (18 mg). Subsequently, the numbers, egg production and dry weights of worms were determined from rats which had been infected with doses varying from 0 to 160 cysticercoids per rat and fed on diets containing either 0, 1, 4, or 8% mannose (w/w). Density-dependent decreases in both worm dry weight and egg production were detected at 5 weeks post-infection, but both the number of worms recovered and their prepatent period appeared to be independent of cysticercoid dose. Differences in the distribution pattern of individual worm dry weights were observed between rats harbouring low (1–15) and high (16–135) worm burdens. Worms recovered from rats infected with 10 cysticercoids and fed on a diet containing 4% mannose (w/w) for 3 weeks were found to be, on average, more than twice as heavy (37 mg) as those from rats fed on diets containing 1, 2, or 8% mannose (w/w) for equivalent periods of time (16 mg). The results indicate that the ‘crowding effect’ cannot be explained simply in terms of inter-worm competition for carbohydrate.

The prevalence and intensity of Ascaris lumbricoides infections in Moslem children from northern Bangladesh

The results are presented of a horizontal epidemiological survey of intestinal infections of children aged between six months and 15 years in three adjacent villages in northern Bangladesh. On the basis of 203 stool sample examinations, the prevalence of Ascaris lumbricoides, Trichuris trichiura, hookworm and amoebic infections was estimated as 68, 56, 53 and 19%, respectively. Age-specific prevalence data indicated that approximately 90% of the children were harbouring patent Ascaris infections by the time they were four years old and there was some evidence to suggest differences in the pattern of age-prevalence between male and female children. The intensity of Ascaris infection was found to rise to its maximum value within the first four years of life. No significant differences were detected in the mean worm burdens of children aged between four and 15 years. Each child in this age-group harboured on average 10 worms. The frequency distribution of numbers of A. lumbricoides per host was found to be overdispersed, with a value of the negative binomial parameter, k, of 0 · 44. The degree of aggregation was found to be approximately the same for each age-class of the population between one and 15 years (0 · 26 ⩽ k ⩽ 0 · 82). No evidence was found to suggest a density-dependent reduction in the weight of either male or female Ascaris within the range one to 43 worms per host. The sex-ratio of worms recovered (male:female) from individual hosts harbouring burdens greater than 3 was 1·2 ± 0·2, with no correlation between sex-ratio and number of worms recovered. Correlation of the number of Ascaris eggs detected in stool samples with the number of worms recovered after anthelmintic treatment suggested a marked density-dependent decrease in egg output with increasing worm burden.

Influence of proliferative rates and A system substrate availability on proline transport in primary cell cultures of the R3230AC mammary tumor.

Regulation of A system amino acid transport was studied in primary cultures of the R3230AC mammary adenocarcinoma. Higher rates of carrier-mediated Na+-dependent proline transport, vc, was decreased and was attributed to a two-fold decrease in Vmax and a two-fold increase in Km. When compared to cells grown in standard media (Eagle's minimal essential medium, MEM), cells grown in media supplemented with A system substrates (alanine, serine, glycine, and proline) demonstrated adaptive decreases in proline transport; the decrease was due to two-fold reduction in Vmax, with no change in Km for proline. Even in the presence of preferred substrates for the A system, a density-dependent decrease in proline transport was manifested. Both fast- and slow-growing cultures maintained in MEM exhibited rapid increases in proline transport when switched to buffers devoid of amino acids; two-fold increases in Vmax were seen within 4 hr, but Km was unchanged. This starvation-induced adaptation was completely prevented by inclusion in the buffer of 10 mM proline, 0.1 mM alpha-(methylamino)-isobutyric acid (MetAIB) or 10 mM serine, whereas inclusion of the poorer A system substrate, phenylalanine (10 mM), had no effect. The effects of MetAIB to prevent starvation-induced increases in proline transport were dose-related, rapid, and reversible. Amino acid starvation-induced increases in proline transport were partially blocked by cycloheximide or actinomycin D. Data were obtained demonstrating a temporal relationship between increasing intracellular [proline] and decreasing vc for proline uptake. In addition, efflux of proline from preloaded cells preceded the increase in initial rates of proline entry. Taken together, we concluded that: 1) A system transport in primary cultures of this mammary adenocarcinoma is regulated by cell density as well as by availability of A system substrates, but these two types of regulation are kinetically distinct; and 2) starvation-induced enhancement of proline transport appears to be due to release from transinhibition, but may also involve a derepression-repression type of mechanism.

Life table studies on the paddy field population of the green rice leafhopper, Nephotettix cincticeps Uhler (Hemiptera: Cicadellidae), with special reference to the mechanism of population regulation

SummaryThe population growth of the green rice leafhopper, Nephotettix cincticeps, in the paddy field was analyzed based on the life table data accumulated for six years.The paddy field population, which stems from the invading adults of the first generation (G‐I), repeats two complete generations, and the hatchlings of the fourth generation (G‐IV) enter diapause and overwinter as the fourth instar nymphs in fallow paddy fields. It was clarified that the density dependent reduction in the mean longevity and oviposition rate of adult females in G‐II and III played a primary role in stabilizing the annual population densities. The annual average of the mean longevity of G‐II females (3.9 days) was much shorter than that of G‐III ones (7.7 days) and thus the density dependent reduction in the mean longevity induced a more prompt regulatory effect on the oviposition of G‐II females compared with G‐III ones. As the result, two equilibrium densities of eggs were obtained, e.g., ca 100 and 700 eggs per hill in G‐III and IV, respectively. Density dependent decrease in the proportion of mature females in the adult population was especially conspicuous in G‐II, and this was closely associated with the density dependent reduction in the mean longevity and fecundity. Thus, the density dependent dispersal (emigration) of the adult females by flight in G‐II and III was the most convincing factor in the process of population regulation. The density dependent dispersal of the adult females is effective in avoiding the deleterious effects of nymphal crowding in a breeding habitat unit (a paddy field), and may result in a more even distribution of the population over a continuous habitat units in a locality than otherwise.

Amino acid transport by membrane vesicles of virally transformed and nontransformed cells: Effects of sodium gradient and cell density

Mixed membrane vesicle populations composed of plasma membrane and endoplasmic reticulum were prepared from Balb/c 3T3 and simian virus 40-transformed Balb/c 3T3 mouse fibroblasts. The initial rates of uptake of L-leucine and alpha-aminoisobutyric acid by these vesicles were stimulated by a NaCl gradient (external greater than internal). Cation specificity for stimulation of L-leucine uptake was Na+ greater than Li+ greater than K+. NaSCN was as effective as NaCl. Stimulation of uptake of both amino acids by a NaCl gradient was twice as great in vesicles from transformed as compared to non-transformed cells. The NaCl gradient produced transient accumulation of both L-leucine and alpha-aminoisobutyric acid to twice the equilibrium level in vesicles from transformed cells. No such "overshoot" was observed in vesicles from nontransformed cells. In vesicles from the contact-inhibitable Balb/c 3T3 cells, transport of alpha-aminoisobutyric acid, but non L-leucine, exhibited a density-dependent decrease in Na+ gradient induced stimulation, from 248% for sub-confluent to 109% with confluent cells. No density-related changes in uptake were noted with vesicles from the transformed cells. These studies suggest that variation in amino acid uptake associated with viral transformation may be related, at least in part, to alterations in Na+ permeability of the surface membrane.

The Population Dynamics of Anthocoris confusus in a Laboratory Cage Ecosystem

Consideration of single aspects of predation can give only a rough outline of the way in which the predator reacts to its prey and to other predators. Previous studies of predatory insects attempted to elucidate some of the basic processes of predator-prey interactions. Generally, they concluded that there are likely to be only a few important components which govern the outcome of a predator prey interaction. For example, functional responses to prey density (Holling 1959, 1966), efficiency of capture (Dixon & Russel 1972; Evans 1976), the way in which the predator reacts to prey distribution (Banks 1957; Dixon 1958). Those studies were carried out in artificial situations and, in some cases, the results were used to augment field observations, for example, Dixon & Russel (1972) used laboratory estimates of speed of movement and range of perception to calculate the area covered by anthocorids in the field. Similar studies of insect parasites have been complemented by long term observations of laboratory ecosystems. Those studies (Hassell & Huffaker 1969; Takahashi 1961; Podoler 1974) involved interactions, over several generations, between parasites and hosts with discrete and synchronized generations. The effects of different mortality factors were quite easily measured, particularly the results of parasitism. One of the major contributors to parasite population stability was interference between parasite individuals leading to a density-dependent decrease in fecundity as parasite density increased (Hassell & Varley 1969; Hassell 1971 a,b). There is now some evidence that interference occurs in natural ecosystems. For example, a female Rhyssa persuasoria L. drives off intruding females from the same area of tree trunk (Spradbery 1970) while ignoring its cleptoparasite, Pseudorhyssa sternata Merrill (Spradbery 1969). Studies of predator-prey interactions are considerably more difficult to carry out. Prey are either removed from the system or rendered unrecognizable making it difficult to assess the degree of predation. Predator eggs can be difficult to find, particularly in a field situation, and often cannot be related to adult density. Additionally, in the field, insect predators normally have a maximum of two generations per year. Consequently data for inter-generation relationships (Hassell 1966) can take many years to accumulate. The present study was an attempt to overcome these difficulties by allowing predators and prey to interact over a number of generations within a relatively short time scale in the laboratory. The major aims were to pinpoint intraand inter-generation relationships which affect the predator's success and, if possible, to relate these to data from natural situations. Anthocoris confusus Reuter was used as the predator and Aulacorthum circumflexus Buckton as its aphid prey.

Glycolytic enzyme activities in murine sarcoma virus-transformed cultures of Balb 3T3: The role of culture density

The effects of transformation by murine sarcoma virus and of increasing cell density on the activities of several key glycolytic enzymes in Balb 3T3 cells were tested. Hexokinase levels increased with culture density in the uninfected and in the two virus-transformed (HB2 and KA31) cells. Phosphofructokinase did not increase with culture density in the uninfected cells but rose dramatically in dense cultures of virus-transformed cells. 6-Phosphogluconate dehydrogenase and glucose-6-phosphate dehydrogenase levels were high in sparse cultures of uninfected cells and decreased steadily with increased culture density. Pyruvate kinase levels increased with density only in KA31 cultures. A density-dependent decrease in the level of hexokinase type II with a concomitant increase in type I isozyme was seen in uninfected 3T3 cultures. This change was negligible in HB2 cells.