In vitro toxicology of fumonisins and the mechanistic implications

The effects of fumonisins B₁FB₁, B₂(FB{2}), and the backbone of fumonisin B₁ remaining after hydrolysis of the tricarballylic groups with base (HFB₁) on sphingolipid biosynthesis were studied in both primary rat hepatocytes and pig kidney epithelial cells (LLC-PK₁). Fumonisins were potent inhibitors of sphingolipid biosynthesis in hepatocytes (IC₅₀ of FB₁=0.1 M), but overt toxicity was not observed. In renal cells, fumonisins also inhibited sphingosine biosynthesis (IC₅₀ for FB₁=35 M), and caused decreased cell proliferation as well. Higher doses (70 M) killed renal cells after exposure for 3 days. The inhibition of de novo sphingolipid biosynthesis was specific, and appeared to be at the site of ceramide synthase, which catalyzes the formation of dihydroceramide or ceramide by the addition of the amide-linked fatty acid to sphinganine or sphingosine. These results may account for the ability of fumonisins to cause equine leucoencephalomalacia and to promote tumor formation.     

Ammonia regulation of phosphate-activated glutaminase displays regional variation and impairment in the brain of aged rats

The regulation of PAG by ammonia in whole brain (Sprague-Dawley) and regional (Fischer-344) synaptosomal preparations from adult and aged animals was assessed. Whole brain synaptosomal preparations from both age groups displayed a significant decrease in PAG activity with increasing ammonium chloride concentrations, however, the aged rats exhibited a significant attenuation in ammonia-induced PAG inhibition. PAG activity measured in synaptosomes prepared from the striatum (STR), temporal cortex (TCX) and hippocampus (HIPP) was also inhibited by ammonium chloride. The STR showed the greatest degree of ammonia-induced PAG inhibition (55%) followed by the HIPP (30–35%) and the TCX (25–30%). This reduction in PAG activity was significantly attenuated in STR from aged rats at ammonium chloride concentrations greater than 50 M and in the TCX, PAG activity was significantly attenuated in the aged rats at ammonia concentrations of 0.5 and 1.0 mM. Ammonia regulation of PAG activity in the HIPP appeared to be unaffected by age. Ammonium chloride concentrations up to 5 mM had no effect on GLU release from cortical slices, although GLN efflux was significantly enhanced. These findings suggest that isozymes of PAG may exist in different brain regions based on their differential sensitivity to ammonia. The attenuation of ammonia-induced PAG inhibition seen in aged rats may have deleterious effects in the aged brain.Abbreviations PAG phosphate-activated glutaminase: L-glutamine amidohydrolase; EC 3.5.1.2 - STR striatum - TCX temporal cortex - HIPP hippocampus     

Human adenovirus type 9-induced rat mammary tumors.

Following subcutaneous inoculation of newborn Wistar-Furth rats with human adenovirus type 9 (Ad9), 16 of 16 female and 0 of 11 male rats developed mammary tumors. Tumor-positive animals usually developed tumors in multiple glands. Histopathological analyses indicated that three general categories of tumor could be identified. Mammary fibroadenomas were the most common tumor type encountered, but phyllodeslike tumors and solid sarcomas were also frequently found. In situ hybridization and immunohistochemical techniques established that benign fibroadenomas were derived from mammary fibroblasts (collagen type I- and vimentin-positive cells) and that malignant tumors were derived from myoepithelial cells (collagen type IV-, vimentin-, and muscle-specific actin-positive cells). The fact that mammary tumors were limited to female rats suggested that female hormones are essential for tumor growth and development. In this regard, ovariectomy of Ad9-infected female rats prevented tumor development, while subsequent diethylstilbestrol (DES) treatment elicited tumor formation. In addition, Ad9-infected and castrated male rats which received DES also developed mammary tumors. Established male mammary tumors regressed when DES treatment was stopped and reappeared after DES treatment was resumed. Together, these results indicate that estrogen is required for both initiation and maintenance of Ad9-induced mammary tumors. Southern blot analysis of high-molecular-weight tumor DNA showed that mammary tumor cells contained single or multiple integrated copies of the entire Ad9 genome. RNase protection experiments established that estrogen receptor as well as Ad9 E1a and E4 mRNAs were expressed in mammary tumors, but Ad9 E3 and, surprisingly, E1b mRNAs were not expressed at detectable levels.     

Stochastic simulation of clonal growth in the tall goldenrod,Solidago altissima

Summary As clonal plants grow they move through space. The movement patterns that result can be complex and difficult to interpret without the aid of models. We developed a stochastic simulation model of clonal growth in the tall goldenrod, Solidago altissima. Our model was calibrated with field data on the clonal expansion of both seedlings and established clones, and model assumptions were verified by statistical analyses.When simulations were based on empirical distributions with long rhizome lengths, there was greater dispersal, less leaf overlap, and less spatial aggregation than when simulations were based on distributions with comparatively short rhizome lengths. For the field data that we utilized, variation in rhizome lengths had a greater effect than variation for either branching angles or rhizome initiation points (see text). We also found that observed patterns of clonal growth in S. altissima did not cause the formation of fairy rings. However, simulations with an artificial distribution of branching angles demonstrate that fairy rings can result solely from a plant's clonal morphology.Stochastic simulation models that incorporated variation in rhizome lengths, branching angles, and rhizome initiation points produced greater dispersal and less leaf overlap than deterministic models. Thus, variation for clonal growth parameters may increase the efficiency of substrate exploration by increasing the area covered and by decreasing the potential for intraclonal competition. We also demonstrated that ramet displacements were slightly, but consistently lower in stochastic simulation models than in random-walk models. This difference was due to the incorporation of details on rhizome bud initiation into stochastic simulation models, but not random-walk models. We discuss the advantages and disadvantages of deterministic, stochastic simulation, and random-walk models of clonal growth.     

Effects of rapid changes in oxygen concentration on the respiration of carrot roots

Rates of CO₂ production and O₂ consumption from aged disks of carrot ( Daucus carota L.) root tissues were measured for 4 h after they were transferred from 21% to 0, 1, 2, 4 or 8% O₂ in gas mixtures. A transient peak in the rate of CO₂ production started 5 to 7 min after transfer to 2% or lower O₂ mixtures and peaked at 50 min. After the peaks in CO₂ production from the 0, 1 and 2% O₂ treatments and after the stable production from the 4 and 8% O₂ treatments, the rate of CO₂ production from all low O₂ treatments started to decline at 50 min, reaching stable rates by 160 to 240 min. Concentrations of lactate and ethanol that were significantly higher than the 21% O₂ controls had started to accumulate in disks between 10 and 50 min after exposure to atmospheres containing 2% or less O₂. Production of CO₂ started to increase 5 to 7 min after transfer to 0, 1 and 2% O₂, while the initial decline and then rise in pH and the accumulation of ethanol did not occur until 30 min after the change in atmosphere. Ethanol accumulation paralleled the increase in pH; first at 0.4 μmol g⁻¹ h⁻¹ from 30 to 60 min as the pH shifted from 5.97 to 6.11, and then at 0.08 μmol g⁻¹ h⁻¹ from 60 to 100 min as the pH stablized around 6.12. The peak at 50 min in CO₂ production roughly coincided with the shift from the rapid to the slow change in pH and ethanol accumulation.     

Aluminium,boron and molybdenum availability and uptake by rice in acid sulfate soils

Although Al toxicity is believed to be a problem in acid sulfate soils cropped to rice (Oryza, sativa L.), little is known about the behavior of other trace metals such as B and Mo in these soils. The objectives of this study were to measure the availability of Al, B, and Mo in these soils, to determine what governs the availability of these metals and to investigate the relationships between metal availability and uptake by rice. Metal availability and uptake by rice were evaluated in 134 flooded acid sulfate soils in the Central Plains region of Thailand and in a growth chamber study using 50 of the same soils. Soil and plant metal analyses were conducted at the panicle differentiation stage of growth in both studies and in the soil prior to transplanting in the growth chamber study. Metal activities were determined with GEOCHEM. The mineral phases believed to be governing Al³⁺ activities were jurbanite under low pH conditions and amorphous Al(OH)₃ at high pH. The Al chemistry is believed to be intimately linked to the redox-pH cycle, which is driven by the monsoonal climate. Mortality of rice associated with Al toxicity was observed under field and growth chamber conditions. Interference in P uptake and/or assimilation was believed to be the mechanism of Al toxicity. Activities of B(OH) ₄ ^– and B(OH) ₃ ⁰ were found to be highly correlated to pH and ionic strength, respectively, with the latter being the dominant B ion found in these soils. Activities of MoO ₄ ^2– were positively correlated to pH and appeared to be controlled by wulfenite. Leaf Mo contents were found to be positively correlated with MoO ₄ ^2– activity.