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991.
We have investigated the induction of mutants resistant to 6-thioguanine (6TG) following 254 nm ultraviolet light exposure of density-inhibited cultures of human diploid fibroblasts. Phenotypic expression of 6TG resistance was maximal within 9 days and remained stable through 19 days after irradiation. In reconstruction studies, complete recovery of 6TG-resistant mutants occurred at cell densities of up to 35 000 cells per 100-mm petri dish. The induced mutation frequency increased linearly with dose over the range of 3–9 J/m2; the D0 of the survival curve was 4.2 J/m2. Delaying subculture to low density for 1.5–24 h after irradiation produced unexpected alterations in induced mutation frequencies. An increase in UV-induced mutations of approximately 3-fold was observed in cultures maintained in confluence for 3 h. This trend was reversed with longer holding times: the mutation frequency declined sharply in cultures held for 6 h compared to the 3-h value, and thereafter showed a steady and gradual diminution to background levels.
These data suggest that the repair of potentíally mutagenic damage is a complex phenomenon which can lead to an increase or decrease in mutation frequency as a function of holding time. Although the decline in mutation frequency observed following longer holding intervals is consistent with the notion of an error-free process, we hypothesize that the increased mutation frequency produced by a short holding period reflects the existence of a cell-mediated process which enhances the mutagenic potential of at least some UV-induced DNA photoproducts. 相似文献
992.
The mutagenicity of diethylstilbestrol (DES) in V79 Chinese hamster cells was examined under a variety of conditions. DES over a concentration range 0.01–10 μg/ml failed to induce any increase above the spontaneous frequency of 6-thioguanine-resistant V79 cells. The effect of varying the expression time after treatment in the mutation assay from 3 to 9 days was studied and DES was nonmutagenic at all time points, while N-methyl-N′-nitro-N-nitrosoguanidine was highly mutagenic with a peak response after a 5–7 day expression time. The mutagenicity of benzo[a]pyrene and DES, both of which induce morphological and neoplastic transformation of Syrian hamster embryo (SHE) cells, was tested by cocultivating V79 cells with SHE cells for possible metabolic activation of the chemicals. Neither compound was mutagenic to V79 cells in the absence of SHE cells. Benzo[a]pyrene, but not DES, was mutagenic to V79 cells cocultivated with SHE cells. These results support the observation that DES can induce cell transformation under conditions that do not result in any measurable gene mutations. Moreover, the ability of DES to enhance the recovery of 6-thioguanine-resistant mutations was studied by determining the ability of DES to inhibit metabolic cooperation of V79 cells. Unlike the tumor promoter 12-O-tetradecanoyl-phorbol-13-acetate, DES was a weak or inactive inhibitor of metabolic cooperation. 相似文献
993.
Summary Brink (1982) characterizes the distribution of standing crop of nectar for Delphinium nelsonii as bonanzablank, based on comparison with a Poisson. He then discusses possible effects of standing crop variability on pollinator foraging behavior. We disagree with the use of the Poisson and the resulting conclusions. The expected distribution should not be based on doling out random amounts of nectar to flowers, but based on random return times to flowers by pollinators (elapsed time=nectar accumulated). When this model is used, standing crop variance does not differ markedly from expectation. What differences do exist can be accounted for by variability in nectar production rates of individual plants. We also take issue with the use of the bonanza-blank terminology. As originally formulated this refers to nectar production differences within a plant rather than standing crop differences among plants. 相似文献
994.
995.
John R. Porter 《Physiologia plantarum》1983,57(4):579-583
Potential relationships between alfalfa ( Medicago sativa L. cv. Vernal) acetylene reduction activity (ARA) and leghemoglobin content, nodule numbers, shoot biomass, root biomass, or total plant biomass were estimated using linear regression analysis after treatment with a range of concentrations of arsenate, Cd2+ , Cu2+ , Pb2+ , Zn2+ , and F– . There were highly significant positive linear regressions between ARA and all the other plant values in the control plants, but this linear relationship was significantly altered between ARA and one or more of the other parameters in all treatments. There was also evidence for an alteration of the slope in some of the treatments when compared to control treatments even though a linear model was still applicable. 相似文献
996.
Stenosiphon linifolius is a monotypic genus of the tribe Onagreae of the Onagraceae. The species is widespread in, but restricted to, the Great Plains of the United States. Three flavonol glycosides, kaempferol 3-O-rhamnoside, quercetin 3-O-rhamnoside and myricetin 3-O-rhamnoside, were found to occur in methanolic extracts of Stenosiphon leaves. Similar compounds are found in the leaves of such related genera as Oenothera and Gaura, but in the latter genera, additional flavonols exhibiting greater substitutional variation also are found. 相似文献
997.
Peter B. Oelrichs Michael W. Hill Peter J. Vallely John K. MacLeod Tadeusz F. Molinski 《Phytochemistry》1983,22(2):531-534
Four new tetranortriterpenes, meliatoxins A1, A2, B1 and B2 have been isolated and identified as toxic constituents of the fruit of Melia azedarach L. var. australasica. Toxicity and pathological results confirm that the meliatoxins are responsible for most but not all of the symptoms resulting from the ingestion of whole fruit. 相似文献
998.
Bonnie J. Reger Maurice S.B. Ku Jeffrey W. Pottert John J. Evans 《Phytochemistry》1983,22(5):1127-1132
The ribulose-1,5-bisphosphate carboxylase/oxygenase purified from maize (a C4 monocot) to homogeneity has a MW of532 000 and sedimentation coeffici 相似文献
999.
Techniques for detecting various levels of both field and vaccine strains of infectious bronchitis virus in a deliberately contaminated Newcastle disease vaccine were compared using chick embryos, chick kidney cells, chick tracheal organ cultures and chickens with a view to determining the most appropriate method for screening vaccines for freedom from IBV contamination. Techniques examined included detection of abnormalities and deaths in embryos, cytopathic effect in chick kidney cells, ciliostasis in chick tracheal organ cultures and clinical signs and virus isolation in chickens as well as the fluorescent antibody test, negative contrast electron microscopy and serology where appropriate. Results showed that the techniques capable of detecting both strains of infectious bronchitis virus were, in order of sensitivity, the fluorescent antibody test on allantoic cells from infected embryos, ciliostasis and direct electron microscopy of allantoic fluid. One surprising feature was the poor results obtained using chickens. Some detection was achieved with tracheal virus isolation and tracheal organ cultures prepared from inoculated birds and to a lesser extent with histology and clinical signs, but no technique detected the field strain. 相似文献
1000.
Depletion of intracellular potassium arrests coated pit formation and receptor-mediated endocytosis in fibroblasts 总被引:54,自引:0,他引:54
Depletion of intracellular potassium (K+) caused a marked reduction in the rate of endocytosis of receptor-bound low density lipoprotein (LDL) and epidermal growth factor (EGF) in human fibroblasts. K+ could be depleted slowly by a 3-hr incubation of cells in isotonic K+-free buffer. Rapid K+ depletion was induced by incubation of cells for 5 min with hypotonic medium, followed by transfer to isotonic K+-free buffer. Within 30 min of this treatment, cellular K+ levels fell by more than 60%. When the K+ level fell below a threshold of 40% of normal, the number of coated pits declined by 80% and the rate of endocytosis of 125I-LDL decreased by 70 to 95% despite normal to increased receptor binding. Similar results were obtained with 125I-epidermal growth factor. Addition of KCl to the culture medium up to 2 hr after K+ depletion restored cellular K+ levels and returned endocytosis of 125I-LDL promptly to normal. RbCl was as effective as KCl, but CsCl, LiCl, and (CH3)4NCl had no effect. Restoration by KCl was blocked by ouabain, indicating that uptake via the Na+/K+ ATPase was required. These data demonstrate that depletion of intracellular K+ reversibly arrests coated pit formation and receptor-mediated endocytosis in human fibroblasts. 相似文献