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Measurements of the transepithelial electrical resistance correlated with freeze-fracture observations have been used to study the process of tight junction formation under various experimental conditions in monolayers of the canine kidney epithelial cell line MDCK. Cells derived from previously confluent cultures and plated immediately after trypsin- EDTA dissociation develop a resistance that reaches its maximum value of several hundred ohms-cm(2) after approximately 24 h and falls to a steady-state value of 80-150 ohms- cm(2) by 48 h. The rise in resistance and the development of tight junctions can be completely and reversibly prevented by the addition of 10 μg/ml cycloheximide at the time of plating, but not when this inhibitor is added more than 10 h after planting. Thus tight junction formation consists of separable synthetic and assembly phases. These two phases can also be dissociated and the requirement for protein synthesis after plating eliminated if, following trypsinization, the cells are maintained in spinner culture for 24 h before plating. The requirement for protein synthesis is restored, however, if cells maintained in spinner culture are treated with trypsin before plating. Actinomycin D prevents development of resistance only in monolayers formed from cells derived from sparse rather than confluent cultures, but new mRNA synthesis is not required if cells obtained from sparse cultures are maintained for 24 h in spinner culture before plating. Once a steady-state resistance has been reached, its maintenance does not require either mRNA or protein synthesis; in fact, inhibition of protein synthesis causes a rise in the resistance over a 30-h period. Following treatments that disrupt the junctions in steady- state monolayers recovery of resistance also does not require protein synthesis. These observations suggest that proteins are involved in tight junction formation. Such proteins, which do not turn over rapidly under steady-state conditions, are destroyed by trypsinization and can be resynthesized in the absence of stable cell-cell or cell-substratum contact. Messenger RNA coding for proteins involved in tight junction formation is stable except when cells are sparsely plated, and can also be synthesized without intercellular contacts or cell-substratum attachment.  相似文献   
48.
Non-uniform stress and strain fields are prevalent in many tissues in vivo, and often exacerbated by disease or injury. These mechanical gradients potentially play a role in contributing to pathological conditions, presenting a need for experimental tools to allow investigation of cell behavior within non-uniformly stimulated environments. Herein, we employ two in vitro cell-stretching devices (one previously published; one newly presented) capable of subjecting cells to cyclic, non-uniform stretches upon the surface of either a circular elastomeric membrane or a cylindrical PDMS tube. After 24 hours of cyclic stretch, 10T1/2 cells on both devices showed marked changes in long-axis orientation, with tendencies to align parallel to the direction of minimal deformation. The degree of this response varied depending on location within the stretch gradients. These results demonstrated the feasibility of conducting cell mechanobiology investigations with the two novel devices, while also highlighting the experimental capabilities of non-uniform mechanical environments for these types of studies. Such capabilities include robust data collection for developing mechanobiological dose-response curves, signal threshold identification, and potential spatial targeting for drug delivery.  相似文献   
49.
Interaction of cremophor EL with human plasma   总被引:2,自引:0,他引:2  
1. Interaction of cremophor EL (CRM) with human plasma lipoproteins and nonlipoproteins has been investigated by ultracentrifugation. 2. VLDL has only a low or negligible capacity to bind CRM, i.e. there is little or no change in the optical absorption at 280 nm of VLDL when CRM is added. 3. A low density subfraction of low density lipoproteins seems to associate substantially with CRM at relatively low CRM concentrations (1-3 mg/ml), but such association is not evident for CRM concentrations in the region 12-116 mg/ml. 4. Low density lipoproteins (LDL) may act as a carrier for CRM-emulsions, yet there seems to be no concomitant change in the 280 nm optical absorption of the proteins of LDL. 5. The position in the gradient (i.e. in the centrifugation tube after centrifugation) of high density lipoproteins (HDL) is shifted towards lower density in the presence of 1-4 mg CRM/ml. For higher concentrations of CRM, a destruction of HDL can be observed: the HDL distribution is converted into a bimodal distribution of respectively lighter and heavier "HDL"-particles than the normal ones; the densities at the peaks of these distributions are approximately 1.07 g/ml (light), 1.20 g/ml (heavy) and 1.11 g/ml (normal HDL). The optical extinction coefficient is apparently the same for the proteins of normal--and modified HDL. 6. Even high CRM concentrations (less than or equal to 116 mg/ml) have no perceptible effect on the gradient positions and profile of human serum albumin (HSA) and/or other heavy proteins. 7. The possible biological significance of these findings is briefly touched upon.  相似文献   
50.
Hematoporphyrin ethers--I. Generalized synthesis and chemical properties   总被引:1,自引:0,他引:1  
A series of hematoporphyrin di-ethers, from methyl to hexyl, has been prepared by a generalized procedure based on reaction of the selected carbinol with the HBr adduct of protoporphyrin, followed by hydrolysis of ester functions and chromatographic purification. Spectroscopic and other properties are reported. They crystallize well. HPLC retention time increases linearly with the number of carbon atoms in the alcohol employed. In previous work we have shown that the more hydrophobic ethers are very efficient photosensitizers of malignant cells.  相似文献   
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