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31.
32.
Extrapolation from in vitro tests to human risk: experience with sodium fluoride clastogenicity 总被引:1,自引:0,他引:1
Genotoxic effects observed in vitro, only at high doses or high levels of cytotoxicity, will be false positives if such conditions are not achieved or cannot be tolerated in vivo. However, for such effects to be disregarded there must be a threshold dose or level of cytotoxicity below which genotoxicity is absent. Sodium fluoride (NaF) has previously been shown to be clastogenic in vitro in Syrian hamster cells and human fibroblasts. We have extended these studies in human fibroblasts and included a positive control (mitomycin C, MMC) which is clastogenic in vivo and carcinogenic, and a chemically related control (NaCl). Cytotoxicity was measured as mitotic inhibition and cell death (loss of clonogenicity). The results are used to illustrate the problems associated with quantitative extrapolation from in vitro tests to human risk, as follows. (1) There appears to be a threshold response (clastogenicity vs. dose) with NaF at around 10 micrograms/ml (48 h exposure) but a more definitive conclusion must await elucidation of the mechanisms of clastogenicity. (2) NaCl is weakly clastogenic at 1000 times the threshold dose for NaF. The mechanisms are unlikely to be similar. (3) No clastogenicity was detected with NaF below about 30% mitotic inhibition but the relationship between clastogenicity and mitotic inhibition was similar for NaF and MMC. (4) There was no obvious threshold in the relationship between clastogenicity and cell killing with NaF. MMC was less clastogenic than NaF at equitotoxic doses. Observations 3 and 4 preclude the possibility of regarding the clastogenicity of NaF as a false positive by virtue of associated cytotoxicity. 相似文献
33.
Insulin-like growth factor-II (IGF-II) receptors in primary cultures of adult rat hepatocytes were characterized and their regulation by cell density examined. In hepatocytes cultured at 5 X 10(5) cells per 3.8 cm2 plate [125I]IGF-II bound to specific, high affinity receptors (Ka = 4.4 +/- 0.5 X 10(9) l/mol). Less than 1% cross-reactivity by IGF-I and no cross-reactivity by insulin were observed. IGF-II binding increased when cells were permeabilized with 0.01% digitonin, suggesting the presence of an intracellular receptor pool. Determined by Scatchard analysis and by polyacrylamide gel electrophoresis after affinity labeling, the higher binding was due solely to an increase in binding sites present on 220 kDa type II IGF receptors. In hepatocytes cultured at low densities, the number of cell surface receptors increased markedly, from 10-20,000 receptors per cell at a culture density of 6 X 10(5) cells/well to 70-80,000 receptors per cell at 0.38 X 10(5) cells/well. The increase was not due simply to the exposure of receptors from the intracellular pool, as a density-related increase in receptors was also seen in cells permeabilized with digitonin. There was no evidence that IGF binding proteins, either secreted by hepatocytes or present in fetal calf serum, had any effect on the measurement of receptor concentration or affinity. We conclude that rat hepatocytes in primary culture contain specific IGF-II receptors and that both cell surface and intracellular receptors are regulated by cell density. 相似文献
34.
Lymphoma models for B cell activation and tolerance. VI. Reversal of anti-Ig-mediated negative signaling by T cell-derived lymphokines 总被引:1,自引:0,他引:1
D W Scott A O'Garra D Warren G G Klaus 《Journal of immunology (Baltimore, Md. : 1950)》1987,139(12):3924-3929
We have recently described three "immature" B cell lymphomas which are exquisitely sensitive to growth inhibition by anti-Ig reagents and may serve as models for tolerance induction in normal B cells. These cells are inhibited from cell cycle progression into S after receiving a negative signal in early G1. In this paper, we demonstrate that the growth inhibition by anti-Ig can be prevented and reversed by the addition of supernatants from T cell lines. One such line, called Tova, produces factors which restore normal levels of DNA synthesis in the presence of concentrations of anti-Fab or anti-kappa immunoglobulins which cause up to a 90% inhibition of thymidine incorporation in a 2- to 3-day culture period. This factor is at least partially effective when added up to 24 hr after anti-Ig to unsynchronized lymphoma cells and it does not alter the growth of control cultures. Studies using synchronized lymphoma cells indicated that the T cell factor permitted cycle progression into S when added during the early G1 exposure to anti-kappa and was less effective when added late in G1. Preliminary characterization suggests that both B cell growth factor II (interleukin 5) and B cell stimulatory factor 1 (interleukin 4) have additive activity in this system, although another unidentified lymphokine may also be involved. The relevance of T cell reversal of Ig receptor-mediated negative signaling to neonatal B cell tolerance is emphasized. 相似文献
35.
The isolation of a fetal rat liver glutathione S-transferase isoenzyme with high glutathione peroxidase activity 总被引:3,自引:0,他引:3
A previously uncharacterized glutathione S-transferase isoenzyme which is absent from normal adult rat livers has been isolated from fetal rat livers. The enzyme was purified using a combination of affinity chromatography, CM-cellulose column chromatography and chromatofocusing. It is composed of two non-identical subunits, namely, subunit Yc (Mr 28,000) and a subunit (Mr 25,500) recently reported by us to be uniquely present in fetal rat livers and which we now refer to as subunit 'Yfetus'. The enzyme which we term glutathione S-transferase YcYfetus has an isoelectric point of approx. 8.65 and has glutathione S-transferase activity towards a number of substrates. The most significant property of the fetal isozyme is its high glutathione peroxidase activity towards the model substrate cumene hydroperoxide. We suggest that this isozyme serves a specific function in protecting fetuses against the possible teratogenic effects of organic peroxides. 相似文献
36.
M J Cole E A Shaffer R B Scott 《Canadian journal of physiology and pharmacology》1987,65(10):2124-2130
The gallbladder has both storage and contractile properties, but is pressure-volume characteristics have not been fully described. We studied gallbladder pressure, compliance, stress relaxation, and the work performed during infusion-withdrawal of fixed volumes of bile at increments of base-line volume under halothane anesthesia in 13 dogs. Two cannulae were inserted in the gallbladder fundus: one for cyclic infusion-withdrawal of bile, and one for pressure monitoring. Following ligation of the cystic duct, the gallbladder was fully aspirated and then filled to successive predetermined base-line volumes (15, 20, 25, 30, or 35 mL). At each base-line volume, 5 mL of bile was infused and withdrawn at 2.47 mL/min. Studies were performed under basal conditions and with cholecystokinin octapeptide (CCK) infusion (10 or 30 ng.kg-1.h-1iv). Pressure was measured at base-line, after infusion, and after withdrawal. Compliance during infusion (delta V/delta P) was calculated for each cycle. Stress relaxation was defined as the difference between base-line pressure and any reduction in pressure after withdrawal. Hysteresis, the difference between work of infusion and work of withdrawal, was calculated. The results were such that base-line, end-infusion, and end-withdrawal pressures; work of infusion, work of withdrawal, and hysteresis; and stress relaxation all increased significantly with increases in the predetermined base-line volumes (p less than 0.001). Compliance decreased significantly (p less than 0.001) with increasing base-line volume. CCK at 10 ng.kg-1.h-1iv had no effect, but infusion of CCK at 30 ng.kg-1.h-1 significantly (p less than 0.05) increased pressure and work, decreased compliance, and increased stress relaxation compared with controls.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
37.
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39.
Expression and function of the homoeotic genes Antennapedia and Sex combs reduced in the embryonic midgut of Drosophila 总被引:7,自引:0,他引:7
Drosophila homoeotic genes control the formation of external morphological features of the embryo and adult, and in addition affect differentiation of the nervous system. Here we describe the morphogenetic events in the midgut that are controlled by the homoeotic genes Sex combs reduced (Scr) and Antennapedia (Antp). The midgut is composed of two cell layers, an inner endoderm and an outer visceral mesoderm that surround the yolk. Scr and Antp are expressed in the visceral mesoderm but not in the endoderm. The two genes are required for different aspects of the midgut morphogenesis. In Scr null mutant embryos the gastric caeca fail to form. Scr is expressed in the visceral mesoderm cells posterior to the primordia of the gastric caeca and appears to be indirectly required for the formation of the caeca. Antp is expressed in visceral mesoderm cells that overlie a part of the midgut where a constriction will form, and Antp null mutant embryos fail to form this constriction. An ultrastructural analysis of the midgut reveals that the visceral mesoderm imposes the constriction on the endoderm and the yolk. The mesodermal tissue contracts within the constriction and thereby penetrates the layer of the midgut endoderm. Microtubules participate in the morphological changes of the visceral mesoderm cells. The analysis of the expression of Scr in Antp mutant embryos revealed a case of tissue-specific regulation of Scr expression by Antp. In the epidermis, Antp has been shown to negatively regulate Scr, but it positively regulates Scr in the visceral mesoderm. 相似文献
40.