Polyamine starvation prolongs the S and G2 phases of polyamine-dependent (arginase-deficient) CHO cells.

This study analyzes the effects of polyamine starvation on cell cycle traverse of an arginase-deficient CHO cell variant (CHO-A7). These cells grow well in serum-free medium, provided that it contains ornithine or polyamines or both. In the absence of ornithine or polyamines or both, the CHO-A7 cells develop severe polyamine deficiency and, as a consequence, grow more slowly. When grown to a stationary phase in the presence of ornithine or putrescine or both, the CHO-A7 cells became arrested in G0/early G1. However, when starved for ornithine and polyamines, they accumulated in the S and G2 phases. Ornithine and polyamine starvation of CHO-A7 cells causes an increase in ornithine decarboxylase activity. When this increase was prevented by treatment with DL-alpha-difluoromethylornithine, an enzyme-activated irreversible inhibitor of ornithine decarboxylase, growth was further suppressed, and a greater fraction of cells were found in the S and G2 phases of the cell cycle.     

Rapid, landscape scale responses in riparian tundra vegetation to exclusion of small and large mammalian herbivores

Productive tundra plant communities composed of a variety of fast growing herbaceous and woody plants are likely to attract mammalian herbivores. Such vegetation is likely to respond to different-sized herbivores more rapidly than currently acknowledged from the tundra. Accentuated by currently changing populations of arctic mammals there is a need to understand impacts of different-sized herbivores on the dynamics of productive tundra plant communities. Here we assess the differential effects of ungulate (reindeer) and small rodent herbivores (voles and lemmings) on high productive tundra vegetation. A spatially extensive exclosure experiment was run for three years on river sediment plains along two river catchments in low-arctic Norway. The river catchments were similar in species pools but differed in species abundance composition of both plants and vertebrate herbivores. Biomass of forbs, deciduous shrubs and silica-poor grasses increased by 40–50% in response to release from herbivory, whereas biomass of silica-rich grasses decreased by 50–75%. Hence both additive and compensatory effects of small rodents and reindeer exclusion caused these significant changes in abundance composition of the plant communities. Changes were also rapid, evident after only one growing season, and are among the fastest and strongest ever documented in Arctic vegetation. The rate of changes indicates a tight link between the dynamics of productive tundra vegetation and both small and large herbivores. Responses were however not spatially consistent, being highly different between the catchments. We conclude that despite similar species pools, variation in plant species abundance and herbivore species dynamics give different prerequisites for change.     

Collapsing population cycles

During the past two decades population cycles in voles, grouse and insects have been fading out in Europe. Here, we discuss the cause and implication of these changes. Several lines of evidence now point to climate forcing as the general underlying cause. However, how climate interacts with demography to induce regime shifts in population dynamics is likely to differ among species and ecosystems. Herbivores with high-amplitude population cycles, such as voles, lemmings, snowshoe hares and forest Lepidoptera, form the heart of terrestrial food web dynamics. Thus, collapses of these cycles are also expected to imply collapses of important ecosystem functions, such as the pulsed flows of resources and disturbances.     

Ehrlich ascites tumour cells become refractory to alpha-difluoromethylornithine at a certain stage of growth

When Ehrlich ascites tumour cells are induced to proliferate by serum stimulation, the ornithine decarboxylase (ODC) activity increases rapidly and reaches two to three peaks during the first 24 h. Inhibition of the first peak in ODC activity (occurring at 4 h) by adding alpha-difluoromethylornithine (DFMO) within 2 h of serum stimulation, results in maximal growth inhibition. Under these conditions, similar degrees of polyamine depletion are achieved. When DFMO is added 3 h after seeding, however, enough polyamines have already accumulated during the initial burst in ODC activity to reduce the antiproliferative effect of the drug. The antiproliferative effect is further reduced when DFMO is added 6 h after seeding. When DFMO is added 23 h after seeding, i.e. after maximal accumulation of polyamines, there is no inhibition of cell proliferation. These findings are important to consider both when designing experimental as well as clinical regimens for this drug.     

The importance of marine vs. human-induced subsidies in the maintenance of an expanding mesocarnivore in the arctic tundra

1. Most studies addressing the causes of the recent increases and expansions of mesopredators in many ecosystems have focused on the top-down, releasing effect of extinctions of large apex predators. However, in the case of the northward expansion of the red fox into the arctic tundra, a bottom-up effect of increased resource availability has been proposed, an effect that can counteract prey shortage in the low phase of the multi-annual rodent cycle. Resource subsidies both with marine and with terrestrial origins could potentially be involved. 2. During different phases of a multi-annual rodent cycle, we investigated the seasonal dynamics and spatial pattern of resource use by red foxes across a coast to inland low arctic tundra gradient, Varanger Peninsula, Norway. We employed two complementary methods of diet analyses: stomach contents and stable isotope analysis. 3. We found that inland red foxes primarily subsisted on reindeer carrions during the low phase of a small rodent population cycle. Lemmings became the most important food item towards the peak phase of the rodent cycle, despite being less abundant than sympatric voles. Isotopic signatures of tissue from both predator and prey also revealed that red foxes near the coast used marine-derived subsidies in the winter, but these allochthonous resources did not spillover to adult foxes living beyond 20-25 km from the coast. 4. Although more needs to be learned about the link between increasing primary productivity due to climatic warming and trophic dynamics in tundra ecosystems, we suggest that changes in reindeer management through a bottom-up effect, at least regionally, may have paved the way towards the establishment of a new mesopredator in the tundra biome.     

Endozoochory varies with ecological scale and context

Several studies on endozoochory have established large herbivores as important for seed dispersal, yet no studies have evaluated how endozoochory is dependent on ecological scale and context. Here we address effects of reindeer density on endozoochory in a hierarchical, multi-scale study, encompassing several ecological contexts.
We found reindeer density effects on endozoochory to vary with spatial scale. Higher reindeer densities at the level of landscape areas, as indexed through faeces abundance, were related to both less species and lower abundance of emerging plants from faeces. In contrast, there was no effect of higher reindeer densities at the level of herding districts (i.e. large scale assemblages of landscapes). Lack of consistency between scales reflects ecological hierarchy, indicating that reindeer density effects on endozoochory best matches at the scale of landscapes.
Pasture seed plant composition was only partly an important ecological factor. That is, ericoid species, the dominating plants in the pastures, were also the most abundant seed plants found to emerge from the faeces. However, most herbaceous seed plant species in the pastures were not emerging from the faeces and the few that emerged were positively related to the site fertility and altitude of the pasture.
Studies addressing endozoochory of ruminants are typically concerned with seed plants, whereas in this study we also found indications of that diaspores of ferns are viable after passing the digestive tract of large herbivores. Vascular spore plants were even more abundant in the faeces than were vascular seed plants.
Results from this study demonstrate that reindeer may counteract a potentially negative impact on seed limitation from their grazing by returning viable seeds in their faeces. However, in Finnmark, northern Norway, this effect is only marginal, relates only to a very few species and individuals and shows ecological scale and context dependence.     

Ornithine decarboxylase inhibitors increase the cellular content of the enzyme: implications for translational regulation

Ehrlich ascites tumor cells grown in the presence of inhibitors of ornithine decarboxylase (EC 4.1.1.17) exhibited an elevated content of this enzyme. The increase could not solely be explained by a decrease in the degradation rate of the enzyme. Instead a stimulation of enzyme synthesis, probably mediated via the polyamine-depleting properties of the inhibitors, is suggested. The enhancement of cellular ornithine decarboxylase content was not accompanied by any significant changes in the amount of ornithine decarboxylase mRNA, indicating a regulation at the level of translation.     

Irradiation influences the expression of substance P and enkephalin in the rat larynx

The effects of radiotherapy on neuropeptide expression in the rat larynx were studied. Irradiation was given for five days, 6 or 8 Gray daily. Ten days after the end of irradiation, the larynx, the laryngeal nerves and different ganglia related to the larynx were dissected out from irradiated and control animals and processed for neuropeptide immunohistochemistry. There was an increased immunolabelling for two of the neuropeptides tested, substance P and enkephalin, in the innervation of the subglottic glands and in the acetylcholinesterase-positive ganglionic cells of the local ganglia. These cells were interpreted as representing postganglionic parasympathetic ganglionic cells. The changes seen in the subglottic glands were interpreted as most likely being related to the changing pattern of staining seen in the local ganglia. No changes in substance P-and enkephalin expression were observed in other laryngeal structures, the nodose ganglia, superior cervical ganglia or laryngeal nerve paraganglia. Thus, in certain respects neuropeptide expression in the larynx is modulated by radiotherapy. Since neuropeptides have both neurotransmitter and/or neuromodulator effects in airway tissue and since they show effects as growth factors, the occurrence of this plasticity in neuropeptide expression should be taken into consideration in future studies examining the effects of irradiation on normal/diseased airway tissues.     

Brain Amino Acids Measured by Intracerebral Dialysis in Portacaval Shunted Rats

Changes in brain amino acid uptake and metabolism have been proposed as a possible etiological factor in hepatic encephalopathy. By use of a brain dialysis technique (a thin tube implanted in the brain of the living animal), the extracellular amino acid concentrations in the striatum of portacaval (PC)-shunted and sham-operated rats were measured. Leucine, phenylalanine, methionine, and glutamine were increased two- to sixfold in the PC-shunted rats, whilst no changes were seen for GABA, valine, glutamate, or isoleucine, confirming previous reports. Aspartate levels were 350% higher in the PC-shunted rats, and this rise, as well as that of phenylalanine, was significantly correlated with the lower motor activity observed in the PC-shunted rats, suggesting a possible importance of these amino acids in the etiology of hepatic encephalopathy. The amino acid concentrations measured in whole blood demonstrated the well-known pattern of low levels of branched-chain amino acids and increased concentrations of phenylalanine, glutamine, and histidine.