Changes In Cell Membrane Fluidity Research Articles

Antineoplastic Effects of N-3 Polyunsaturated Fatty Acids in Combination With Drugs and Radiotherapy: Preventive and Therapeutic Strategies

Many data support the beneficial effect of n-3 polyunsaturated fatty acids (PUFAs) as chemopreventive and chemotherapeutic agents in the treatment of several chronic pathologies including cancer. Different molecular mechanisms have been proposed to explain their effects, including alterations in arachidonic acid oxidative metabolism and metabolic conversion of n-3 PUFAs to novel discovered bioactive derivatives; modification of oxidative stress; changes in cell membrane fluidity and structure and altered metabolism and function of membrane proteins. Considerable knowledge has been recently gathered on the possible beneficial effects of n-3 PUFAs administered in combination with different antineoplastic drugs and radiotherapy against melanoma, leukemia, neuroblastoma, and colon, breast, prostate, and lung cancer. The efficacy of these combinations has been demonstrated both in vivo and in vitro, and clinical trials have also been conducted. The aim of this review is to analyze all the n-3 PUFA combinations investigated so far, their efficacy, and the possible molecular mechanisms involved. It would be highly auspicable that the detailed analysis of the literature in this field could further support the common use of n-3 PUFAs in combination with other chemopreventive agents and warrant more clinical investigations designed to test the effectiveness of n-3 PUFA treatments coupled with conventional antineoplastic therapies.

Changes in Membrane Fluidity of Blood Platelets in Myeloid Neoplasm

In this study we evaluate the alterations in membrane fluidity of blood platelets in patients with various entities of myeloid malignancies.The clinical history of myeloproliferative and myelodysplastic disorders is often complicated by thromboembolic or hemorrhagic events.The mechanism of these major complications remains unclear. Since there is a weak correlation between the risk of these life threatening complications and the number of blood platelets, our research is focusing on qualitative defects of platelets.Membrane fluidity is an important parameter which influences many of the unique cellular functions and which is strongly correlated (among other factors) to the membrane lipid composition. We try to correlate changes in cell membrane fluidity with the clinical status of the patient disease.32 patients with various entities of myeloid neoplasm were selected (Department of Hematology, Emergency University Hospital of Bucharest). They were diagnosed according to the WHO criteria. 11 normal healthy volunteers, non smokers, drug-free, were used as controls.Membrane fluidity was assessed by fluorescence anisotropy measurements. The platelet membrane shows to be more rigid compared with controls/normal regardless of the clinical type of myeloproliferative disorder. However patients with severe clinical status due to acute myeloid leukemia have a more fluid membrane compared to the same patients found previously in a better state. Thus, the activity (or severity) of the disease correlates with the increase in membrane fluidity, as other studies revealed on lymphocytes. We consider that detection of these modifications may be useful for a better insight in cell abnormalities occurring in this pathology.

Cisplatin Cytotoxicity: DNA and Plasma Membrane Targets

Most current anticancer therapies induce tumor cell death through apoptosis where its specific involved pathways are poorly understood. For example, for many DNA-damaging agents, the specific biochemical lesions (DNA adducts) are associated with the induction of apoptosis via the mitochondria death pathway. However, several of these DNA-damaging agents like cisplatin induce apoptosis through plasma membrane disruption, triggering the Fas death receptor pathway. In this review, we focus on the role of early plasma membrane events in cisplatin-induced apoptosis. Special attention is given to changes in plasma membrane fluidity, inhibition of NHE1 exchanger, activation of acid sphingomyelinase and their consequences on the Fas death pathway in response to cisplatin.

Effects of Monoglycerides on P-Glycoprotein: Modulation of the Activity and Expression in Caco-2 Cell Monolayers

The purpose of this study was to analyze the effects of two common monoglyceride components of lipid excipients, 1-monoolein and 1-monostearin, on the activity and expression of P-glycoprotein (P-gp) in Caco-2 cells. Non-cytotoxic concentrations of 1-monoolein and 1-monostearin were determined by assessing membrane permeability and mitochondrial activity in Caco-2 cells, a human colon adenocarcinoma cell line. Concentrations of 500 and 100 microM were used to evaluate P-gp activity through Rh123 accumulation and bifunctional transport studies. The P-gp protein expression levels were quantified through the use of immunoblots. The changes in cell membrane fluidity and nuclear membrane integrity upon the addition of monoglycerides were analyzed by fluorescence anisotropy using DPH and TMA-DPH as the fluorescent labels and by using increasing salt concentrations to release the nuclear contents, respectively. The absorptive flux (apical to basolateral) in the bifunctional transport studies was not found to be statistically significant for the non-cytotoxic concentrations of 1-monoolein and 1-monostearin. However, treatments of 500 and 100 microM of 1-monoolein or 1-monostearin displayed statistically lowered efflux (basolaterial to apical, P < 0.05) compared to the controls (7.9 +/- 0.8, 12.9 +/- 2.6 x 10 (6) cm/s for 1-monoolein or 11.1 +/- 2.0, 11.4 +/- 2.3 x 10 (6) cm/s for 1-monostearin, respectively, compared to the untreated control, 21.1 +/- 2.9 x 10 (6) cm/s, n = 5). Rh123 accumulation was also found to be enhanced upon 24 h incubation with both concentrations of the monoglycerides; however, only concentrations of 500 muM of the monoglycerides were shown to significantly reduce the P-gp protein expression. The results from this study suggest that these two monoglycerides, common components in various lipid excipients, are inhibitors of P-gp.

Salt stress and plasma-membrane fluidity in selected extremophilic yeasts and yeast-like fungi

We have investigated changes in plasma-membrane fluidity in relation to NaCl concentrations in yeasts and yeast-like fungi that were isolated from either subglacial ice or hypersaline waters. In both of these natural environments, these organisms are exposed to low water activity, due to either high NaCl concentrations or low temperatures. Our data indicate that the fluidity of the plasma membrane can be used as an indicator of fitness for survival in extreme environments. Fungi that can survive in such extreme environments, such as Hortaea werneckii in the hypersaline waters of salterns, and Cryptococcus liquefaciens in subglacial environments, showed similar profiles of plasma-membrane fluidity in response to raised salinity. The same was seen for ubiquitous fungi, which are generally adapted for different types of stress, such as Aureobasidium pullulans and Rhodotorula mucilaginosa. Representatives of both of these groups modulated their plasma-membrane fluidity differently. When salinity exceeded their optimal range, the ubiquitous stress-tolerant species (A. pullulans, Rh. mucilaginosa) showed increased plasma-membrane fluidity, whereas in the dominant extremophiles (H. werneckii, Cr. liquefaciens), it decreased. On the other hand, the plasma membranes of the fungi with a narrow ecological amplitude (Arctic A. pullulans and Rhodosporium diobovatum) showed different responses.

Role of early plasma membrane events in chemotherapy-induced cell death

Most current anticancer therapies induce tumor cell death through the induction of apoptosis. However, the pathways leading to cell death are not always understood. For example, for several DNA-damaging agents the specific biochemical lesions (DNA damage) have been associated with the induction of apoptosis. However, several of these DNA-damaging agents (cisplatin, 1-beta-arabinofuranosylcytosine, daunorubicin or doxorubicin) as well as other antitumor agents, such as edelfosine or resveratrol, have been recently shown to induce apoptosis via signaling through plasma membrane lipid rafts involving the death receptor pathway. In this review we focus on the role of early plasma membrane events in chemotherapy-induced cell death. Special attention is given to changes in plasma membrane fluidity, activation of the acid sphingomyelinase and the Fas death pathway in response to chemotherapy as well as their possible interrelationships.

Alpha-tocopherol preserves endothelial cell migration in the presence of cell-oxidized low-density lipoprotein by inhibiting changes in cell membrane fluidity.

Endothelial cell (EC) migration is essential for healing areas of arterial injury and angioplasty sites. Iron or copper-oxidized low-density lipoprotein (oxLDL(Cu)) inhibits EC migration in vitro, but the effect of physiologically relevant monocyte/macrophage-oxidized LDL (oxLDL(cell)) is unknown. We postulated that oxLDL(cell) would inhibit EC migration and that this inhibition would be reversed by antioxidants. The effect of oxLDL(Cu) and oxLDL(cell) on EC migration was studied by using a razor scrape assay, and migration was assessed after 24 hours. In addition, ECs were incubated with various antioxidants, including butylated hydroxytoluene (BHT), probucol, or alpha-tocopherol, for 1 hour prior to initiation of the scrape assay and application of oxLDL. Both oxLDL(Cu) and oxLDL(cell) inhibited migration. The antioxidants did not alter the antimigratory activity of oxLDL(Cu), but alpha-tocopherol preserved EC migration in the presence of oxLDL(cell). The lack of effect of BHT or probucol suggested that the effect of alpha-tocopherol resided not in its antioxidant activity but in its membrane-stabilizing properties. To test this theory, the effect of oxLDL and alpha-tocopherol on relative cell membrane fluidity was assessed by fluorescence recovery after photobleaching. Both oxLDL(Cu) and oxLDL(cell) increased relative membrane fluidity. Preincubation with alpha-tocopherol inhibited the increase in membrane fluidity of ECs incubated in oxLDL(cell) but not in oxLDL(Cu). These studies show that alpha-tocopherol preserves EC migration in oxLDL(cell) and hastens restoration of the endothelial monolayer after injury by inhibiting changes in membrane integrity caused by oxLDL. Recent studies find that vitamin E is not efficacious in the secondary prevention of cardiovascular events, perhaps because vitamin E does not efficiently block oxidation pathways known to be operative in atherosclerotic arteries. "Non-antioxidant" properties of vitamin E, however, could be important in the primary prevention of atherosclerosis and its complications. Our in vitro studies show that alpha-tocopherol can preserve endothelial migration in the presence of cell-oxidized LDL. This effect might improve the healing of endothelial injuries at sites of arterial repair or angioplasties, especially in lipid-laden arterial walls.

Changes in plasma membrane fluidity of corn (Zea mays L.) roots after Brij 58 treatment.

The detergent Brij 58 has been introduced to reverse plasma membrane (PM) vesicles from the right-side-out to the inside-out form. The aim of the present work was to investigate the effect of Brij 58 on the formation of an ATP-dependent proton gradient and on the fluidity of the lipid phase of PM vesicles. PMs of corn (Zea mays L.) roots were isolated by phase-partitioning. The fluidity of PMs was estimated by measurement of fluorescence polarization with 1-(4-trimethylammoniumphenyl)-6-phenyl-1,3,5-hexatriene (TMA-DPH) and 1,6-diphenyl-1,3,5-hexatriene (DPH). The PMs of corn roots were relatively rigid. The hydrophobic part of the lipid bilayer was more fluid than the hydrophilic part. After intercalation of Brij 58 into the lipid bilayer the membrane fluidity changed in a concentration-dependent manner. Treatment with the detergent Brij 58 increased the degree of fluorescence polarization for TMA-DPH, while it decreased it for DPH. This effect was saturated at a detergent-to-protein ratio of 1:4 for both fluorescence probes. Although the biophysical characteristics of the membrane were changed after Brij 58 treatment, the formation of ATP-dependent proton gradients could still be measured with those vesicles. The generation of an ATP-dependent proton gradient with Brij 58-treated PM vesicles suggests that the detergent treatment indeed turned the originally right-side-out vesicles to sealed inside-out vesicles. The limits of the effect caused by Brij 58 in the context of PM enzyme activities are discussed.

Mechanisms of tamoxifen-induced apoptosis.

Tamoxifen (TAM) has been used in the treatment of breast cancer for over a decade. The observed clinical efficacy of TAM has been attributed to both growth arrest and induction of apoptosis within the breast cancer cells. Although the primary mechanism of action of TAM is believed to be through the inhibition of estrogen receptor (ER), research over the years has indicated that additional, non-ER-mediated mechanisms exist. These include modulation of signaling proteins such as protein kinase C (PKC), calmodulin, transforming growth factor-beta (TGFbeta), and the protooncogene c-myc. Recent studies, including those from our laboratory, have implicated the role of caspases and mitogen-activated protein kinases (MAPK), including c-Jun N-terminal kinase (JNK) and p38 in TAM-induced apoptotic signaling. Oxidative stress, mitochondrial permeability transition (MPT), ceramide generation as well as changes in cell membrane fluidity may also play important roles in TAM-induced apoptosis. These various signaling pathways underlying TAM-induced apoptosis will be reviewed in this article.

Cholesterol enhances platelet-derived growth factor-BB-induced [Ca2+]i and DNA synthesis in rat aortic smooth muscle cells.

In the present study, we describe possible mechanisms by which hypercholesterolemia may contribute to the development of cardiovascular diseases. Treatment of rat aortic smooth muscle cells for 20 hours with cholesterol-rich liposomes (500 micrograms/mL cholesterol, 100 micrograms/mL low-density lipoprotein) resulted in a 76 +/- 12% increase in total cholesterol content. The effects of cholesterol enrichment were examined by determination of changes in cell membrane fluidity. Fluidity of the cholesterol-enriched cell membranes was decreased at all temperatures between 15 degrees C and 40 degrees C. Changes in membrane fluidity in whole cell membranes represented changes in fluidity of microsomal membranes isolated by Percoll gradient ultracentrifugation. The basal [Ca2+]i and the maximal platelet-derived growth factor (PDGF)-BB-induced [Ca2+]i was elevated by 30% and 90% in cholesterol-enriched cells, respectively. In contrast, the resting pH, and the PDGF-BB-induced stimulation of the Na+/H+ exchange were not affected in cholesterol-enriched cells. The effect of PDGF-BB on [3H]thymidine incorporation in cholesterol-enriched cells was elevated by 40% in comparison with untreated cells. Our findings show that cellular cholesterol may be involved in the development of vascular diseases via modulation of the PDGF-induced increase in [Ca2+]i and DNA synthesis in vascular smooth muscle cells.

α- Tocopherol Binding Activity of Red Blood Cells in Smokers

Despite high plasma levels of vitamin E, red blood cell membranes contain relatively low levels of vitamin E. This suggests the existence of a selective vitamin E uptake/regeneration system in human red blood cell membranes. α-Tocopherol binding sites on human red blood cells are thought to be involved in the uptake of α-tocopherol from the plasma. To understand the role of the uptake system we have compared the α-toco-pherol content and binding activity of red blood cells from smokers and non-smokers. The specific binding of [3H]α-tocopherol to pure red blood cell preparations from smokers (n = 7, 28.4 ± 2.8 years) was 30.6 ± 3.2 fmoles per 3 × 108 red blood cells and for non-smokers (n = 17, 27.9 ± 1.3 years) was 41.7 ± 3.7 fmoles per 3 × 108 red blood cells. Thus a-tocopherol uptake activity was significantly lower in smokers (P = 0.05). Red blood cells from smokers contained less (1.8 ± 0.4 μg/gHb) α-tocopherol than non-smokers (2.8 ± 0.3 μg/gHb), (P < 0.05), despite plasma levels of α-toco-pherol being similar: 12.9 ± 0.8 μM in non-smokers vs. 12.7 ± 0.5 μM in smokers. However, adjusting plasma α-tocopherol for total plasma cholesterol plus triacyl-glycerols showed α-tocopherol levels were higher (P < 0.01) in non-smokers (2.84 ± 0.10 μmol α-tocopherol/ mmol [cholesterol+triacylglycerol]) than in smokers (2.36 ± 0.11 μmol α-tocopherol/mmol [cholesterol+triacylglycerol]). The reduced α-tocopherol levels in red blood cells from smokers may be due to impairment of α-tocopherol uptake activity. The reduced levels of α-tocopherol in smokers red blood cells was not associated with any changes in cell membrane fluidity. At present it is not known whether supplementation of smokers with vitamin E would normalise the α-tocopherol uptake activity of red blood cells.

Time-course of changes in the plasma-membrane lipid bilayer and cellular ultrastructure induced by tumour necrosis factor-alpha in K 562 and Daudi cells.

The time-course of changes in the plasma-membrane lipid bilayer induced by tumour necrosis factor-alpha (TNF) were investigated in cultured cells using spin-label electron-spin-resonance techniques. Treatment of K 562 cells, a human chronic myelocytic leukaemia cell line, in suspension culture with TNF for up to 6 h caused an initial increase in cell-membrane fluidity, which returned to the control level after 12 h of treatment. After 24 h of treatment, the cell-membrane fluidity had decreased and this decrease was maintained after 48 h of treatment. In Daudi cells, a human malignant lymphoma cell line, TNF, did not induce any changes in cell-membrane fluidity, indicating that the effect of TNF on membrane structure is cell-specific. The early and transient change in membrane fluidity in K 562 cells is probably related to signal generation, while the later, persistent change may reflect the phenotype of TNF-treated cells, in particular, changes in the plasma membrane-cytoplasmic complex. Histochemical electron microscopic studies indicated that the membrane fluidity changes induced by TNF have an ultrastructural correlate.

Changes in plasma membrane fluidity of Bryonia dioica internodes during thigmomorphogenesis

Fluidity changes in plasma membrane (PM) lipid extracts or native membranes isolated from Bryonia dioica internodes after a mechanical stimulation were monitored by steady-state fluorescence polarization with 1,6-diphenyl-1,3,5-hexatriene as a probe. The signal was shown to rapidly induce an increase in the bulk lipid fluidity. This event was closely related to a relative enrichment in some phospholipid species (PC, PG and PS) as well as a significant increase in the unsaturation index of total fatty acyl chains. Free sterols and protein content did not appear to be involved into this process. After 48 h, lipids from rubbed internodes became less fluid than PM lipids from control internodes.

EPR study of the sea anemone cytolysin, equinatoxin II, cytotoxicity on V-79 cells

Electron paramagnetic resonance (EPR) was used to study the effect of equinatoxin II (EqT II), a cytolytic protein isolated from the sea anemone Actinia equina L., on membrane fluidity and cell metabolism of V-79 cells; the reduction of the spin probe incorporated into the cell membranes as well as the oxygen consumption in the cell suspension were measured. The results were compared with the results obtained by the cell viability study. Under the influence of EqT II (less than 37.5 μg/106 cells) no significant changes in cell membrane fluidity were observed, while reduction kinetics of the spin probe and the oxygen consumption decreased when the cells were kept in Tris buffer solution. However, in the presence of 10% fetal calf serum, which prevented cell lysis, the effects of EqT II were diminished. The oxygen consumption corresponds to the cell viability changes but the reduction kinetics alterations indicate that some oxidation-reduction processes other than cell respiration are affected by EqT II in the absence of serum. The effect seems to be indirect, probably due to the formation of pores which are associated with changed permeability of plasmalemma for metabolites and ions.

Significance of Altered Fluidity of Plasma Membranes of the Liver and Kidney in Rats with Sepsis

The present study was conducted to investigate changes in plasma membrane fluidity of the liver and kidney in sepsis, which is the main cause of multiple organ failure. Male Wistar rats weighing 200-250 g were used in all experiments. Sepsis was induced by cecal ligation and puncture. As a control, a sham operation was performed. The time course of plasma membrane fluidity of the liver and the renal cortex in septicemic rats or in controls was studied. To evaluate the fluidity, fluorescence polarization was measured using 1,6- diphenyl- 1,3,5-hexatriene. The fluorescence polarization values of liver plasma membranes increased after cecal ligation and puncture: 0.183 ± 0.004 (mean ± SEM), 0.194 ± 0.008, 0.206 ± 0.003, and 0.210 ± 0.002 at 0, 24, 48, and 72 hr, respectively. Corresponding values for membranes of the renal cortex increased in a similar fashion. To determine whether factors involved in cell membrane damage exist in blood, the direct effects of lipopolysaccharide (LPS), platelet-activating factor (PAF), and serum from normal rats or from septicemic rats on membrane fluidity were studied. The fluorescence polarization of plasma membranes of the liver or renal cortex to which septicemic rat serum was added was higher than that of plasma membranes to which normal rat serum was added. The fluorescence polarization of liver plasma membranes was increased by LPS, but that of plasma membranes of the renal cortex was slightly decreased. In addition, the fluorescence polarization of liver plasma membranes was increased by PAF, but that of plasma membranes of the renal cortex was decreased. It is concluded that, in sepsis, a derangement of liver and renal plasma membrane fluidity occurs over time, presumably initiating multiple organ failure.

Temperature acclimation of marine crabs: Changes in plasma membrane fluidity and lipid composition

1. 1. Crabs were acclimated for 3 weeks to cold and warm temperatures. Plasma membranes prepared from leg muscle were analysed for fluidity and lipid composition. 2. 2. Plasma membranes from cold-acclimated crabs were more fluid and had lower cholesterol to phospholipid ratios than those from warm-acclimated crabs. The ratio saturated/unsaturated fatty acids was little altered by temperature acclimation. 3. 3. Plasma membranes from the stenothermal species ( Cancer pagurus) were slightly more fluid (at DPH probe level) and had lower cholesterol/phospholipid ratios than those from the eurythermal species ( Carcinus maenas). 4. 4. Plasma membranes from crabs acclimated in spring had more saturated and less polyunsaturated fatty acids than those from crabs acclimated in autumn.

Hydrophilic bile acids do not modify hydrophobic bile acid-induced changes in plasma membrane fluidity of rat biliary epithelium

Hydrophilic bile acids do not modify hydrophobic bile acid-induced changes in plasma membrane fluidity of rat biliary epithelium

Nongenomic regulation of chondrocyte membrane fluidity by 1,25-(OH) 2D 3 and 24,25-(OH) 2D 3 is dependent on cell maturation

1,25-(OH) 2D 3 and 24,25-(OH) 2D 3 regulate rat costochondral chondrocyte cultures in a metabolite-specific manner; 1,25-(OH) 2D 3 targets primarily growth zone cells (GC) and 24,25-(OH) 2D 3 targets primarily resting zone cells (RC). Some of the effects are nongenomic, since incubation of isolated membrane fractions with the metabolites results in regulation of enzyme activities comparable to that seen in culture. This study examined whether changes in membrane fluidity might be one mechanism involved in the nongenomic regulatory pathway. Chondrocyte cultures were incubated with the vitamin D metabolites and changes in plasma membrane fluidity monitored using the fluorophore, TMA-DPH, which is specific for membranes exposed to external fluids. Isolated matrix vesicles were also incubated directly with the metabolites and anisotropy of the membrane, as well as alkaline phosphatase-specific activity, determined. 1,25-(OH) 2D 3 caused a rapid and constant increase in alkaline phosphatase-specific activity in GC matrix vesicles; 24,25-(OH) 2D 3 caused an increase in RC matrix vesicle enzyme activity that was both dose- and time-dependent. Matrix vesicles produced by GC had a lower degree of fluidity than their parent plasma membranes or RC plasma membranes and matrix vesicles. Fluidity of the GC membrane fractions was increased by 1,25-(OH) 2D 3 in a dose- and time-dependent manner. 1,25-(OH) 2D 3 had no effect on the fluidity of the RC membranes. 24,25-(OH) 2D 3 caused a decrease in fluidity in GC at later time points. This metabolite caused an increase in fluidity of RC plasma membranes that returned to normal levels by 6 h; however, the increase induced in the matrix vesicles remained elevated throughout the 24-h experimental period. The results demonstrate that vitamin D metabolites can alter membrane fluidity in a cell-, metabolite-, and membrane-specific manner. For GC matrix vesicles, 1,25-(OH) 2D 3-dependent increased fluidity correlates with increased alkaline phosphatase activity. While 24,25-(OH) 2D 3 increases enzyme activity, the relation of this increase to changes in fluidity is less clear, supporting previous observations concerning differential specificity of response to the two metabolites in growth plate chondrocytes.

Membrane function and vascular reactivity

This communication examines the possibility that nitric oxide (NO) production by endothelial cells results from changes in cell membrane fluidity. Lysophosphatidylcholine (LPC) alters fluidity of the endothelial cell membranes causing vascular relaxation. Through membrane alterations LPC influences function of a number of membrane receptors and modulates enzyme activity. As a result of detergent action, lysophosphatidylcholine (LPC) causes activation of guanylate cyclase, stimulates sialyltransferase and regulates protein kinase C activity. It has already been demonstrated that ionic detergents, such as Triton X-100 also cause vascular relaxation, possibly induced by NO production from endothelial cells. It is postulated that production of nitric oxide results from changes in membrane viscosity; this may represent a mechanism for its regulation in biological systems.

Heat-Induced Changes in the Membrane Fluidity of Chinese Hamster Ovary Cells Measured by Flow Cytometry

Flow cytometry was used to measure the fluorescence polarization of the lipid probe trimethylammonium-diphenylhexatriene as an indicator of plasma membrane fluidity of Chinese hamster ovary (CHO) cells heated under various conditions. Fluorescence polarization was measured at room temperature about 25 min after heating. When cells were heated for 45 min at temperatures above 42 degrees C, fluorescence polarization decreased progressively, signifying an increase in plasma membrane fluidity. The fluorescence polarization of cells heated at 42 degrees C for up to 55 h was nearly the same as for unheated control populations, despite a reduction in survival. The fluorescence polarization of cells heated at 45 degrees C decreased progressively with heating time, which indicated a progressive increase in membrane fluidity. The fluorescence polarization distributions broadened and skewed toward lower polarization values for long heating times at 45 degrees C. Thermotolerant cells resisted changes in plasma membrane fluidity when challenged with subsequent 45 degrees C exposures. Heated cells were sorted on the basis of their position in the fluorescence polarization distribution and plated to determine survival. The survival of cells which were subjected to various heat treatments and then sorted from high or low tails of the fluorescence polarization histograms was not significantly different. These results show that hyperthermia causes persistent changes in the membrane fluidity of CHO cells but that membrane fluidity is not directly correlated with cell survival.