Cell volume regulation in the nervous system

There is good evidence that the three main compartments of the brain, i.e. extracellular space, neurones and glial cells, change their volume during physiological and pathophysiological neuronal activity. However, there is strikingly little knowledge about the mechanisms underlying such alterations in cell volume. For this purpose, a better understanding of the electrophysiological behavior of the neurones and glial cells during volume changes is necessary. Examples are discussed for which changes in cell volume can be derived from the underlying changes in membrane permeabilities. Volume regulatory mechanisms in the brain have not been described under isotonic conditions. However, a rapid volume regulatory decrease is occurring in cultured glial cells during exposure to hypotonic solutions. In contrast, in these cells no volume regulatory increase was found during superfusion with hypertonic media. On the other hand, the entire brain is able to compensate chronic hypertonic perturbations within hours to days. Interestingly, not only ion fluxes induce cellular volume changes but, in turn, water movements can also influence ion fluxes in both neurones and glial cells. With respect to this it should be considered that volume regulatory membrane processes might not exclusively be activated by changes in transmembranal ion gradient, but also by changes of membrane surface shape. Future studies on cellular mechanisms of volume regulation in the brain should imply a combined use of recent techniques such as computerized video-imaging, radiotracer flux measurements and ion-sensitive microelectrodes in defined cell cultures. Optical monitoring and ion-sensitive microelectrodes should enable measurements of volume changes in identified cellular elements of intact nervous structures such as brain slices.     

低浓度戊二醛对红细胞膜力学特性的影响

采用显微动态图像分析技术无扰、实时、在位地研究了低于0．005％浓度的戊二醛对年轻、老龄红细胞膜变形能力的作用．实验结果表明随着戊二醛浓度的增加,年轻、老龄红细胞膜的弯曲弹性模量明显变大,而老龄红细胞增加幅度更大;且随着戊二醛固定时间的延长,年轻、老龄红细胞膜变形能力的差距越来越大．说明作为免疫分析中常用固定剂的戊二醛对年轻、老龄红细胞细胞骨架的影响差异较大。研究结果可为适当选取在免疫分析测定中,以及以年轻红细胞作为药物载体时需采用的戊二醛浓度提供依据。     

Utilization of membranous lipid substrates by membranous enzymes: activation of the latent sphingomyelinase of hen erythrocyte membrane

Previous studies demonstrated that hen erythrocytes have an inoperative, latent sphingomyelinase which is activated when the cells are hemolyzed in a hypotonic medium. Within minutes after hemolysis about 60-80% of the sphingomyelin (SPM) of the RBC "ghost" membrane was hydrolyzed. In this paper, expression of sphingomyelinase activity was further investigated. The percentage of total SPM hydrolyzed depended on the volume of the hypotonic hemolyzing buffer. Thus, suspending the erythrocytes in 4 vol of the buffer resulted in clumping of the hemolyzed "ghosts" and no hydrolysis of SPM. In comparison, suspension in 19 vol of the hypotonic buffer showed no clumping and sphingomyelinase activity was fully expressed. But centrifugation of the latter or, alternatively, addition of concanavalin A induced clumping and elimination of sphingomyelinase activity. Hen RBC could also be hemolyzed in an isotonic medium in the presence of Triton X-100, mellitin, halothane, and phospholipase C. Activation of the latent sphingomyelinase occurred at concentrations of these reagents which caused cell lysis. Hen RBC were dispersed in an isotonic medium containing glutaraldehyde (0.1%) or formaldehyde (10%). This rendered the cells resistant to hemolysis, even when subsequently dispersed in a hypotonic medium or water. But incubation of the "fixed" cells in a hypotonic or isotonic medium activated the enzyme, resulting in hydrolysis of 60% of the cellular SPM. In contrast, when glutaraldehyde was included in the hypotonic buffer, hemolysis occurred but sphingomyelinase activity was eliminated.     

Cell volume regulation in liver

The maintenance of liver cell volume in isotonic extracellular fluid requires the continuous supply of energy: sodium is extruded in exchange for potassium by the sodium/potassium ATPase, conductive potassium efflux creates a cell-negative membrane potential, which expelles chloride through conductive pathways. Thus, the various organic substances accumulated within the cell are osmotically counterbalanced in large part by the large difference of chloride concentration across the cell membrane. Impairment of energy supply leads to dissipation of ion gradients, depolarization and cell swelling. However, even in the presence of ouabain the liver cell can extrude ions by furosemide-sensitive transport in intracellular vesicles and subsequent exocytosis. In isotonic extracellular fluid cell swelling may follow an increase in extracellular potassium concentration, which impairs potassium efflux and depolarizes the cell membrane leading to chloride accumulation. Replacement of extracellular chloride with impermeable anions leads to cell shrinkage. During excessive sodium-coupled entry of amino acids and subsequent stimulation of sodium/potassium-ATPase by increase in intracellular sodium activity, an increase in cell volume is blunted by activation of potassium channels, which maintain cell membrane potential and allow for loss of cellular potassium. Cell swelling induced by exposure of liver cells to hypotonic extracellular fluid is followed by regulatory volume decrease (RVD), cell shrinkage induced by reexposure to isotonic perfusate is followed by regulatory volume increase (RVI). Available evidence suggests that RVD is accomplished by activation of potassium channels, hyperpolarization and subsequent extrusion of chloride along with potassium, and that RVI depends on the activation of sodium hydrogen ion exchange with subsequent activation of sodium/potassium-ATPase leading to the respective accumulation of potassium and bicarbonate. In addition, exposure of liver to anisotonic perfusates alters glycogen degradation, glycolysis and probably urea formation, which are enhanced by exposure to hypertonic perfusates and depressed by hypotonic perfusates.     

The shape of red blood cells as a function of membrane potential and temperature

Summary It is well known that a pH shift of the outside medium from 5 to 9 produces a shape transformation of washed human red blood cells from stomatocytes to echinocytes in isotonic salt solutions. In addition, a stomatocytogenic effect is demonstrated here due to solutions of low ionic strength (below 70mm). An analysis of the true cell state in these situations, proved by measurements of predicted volume changes, indicates a good correlation between transmembrane potential and cell shape. The fact that amphotericin B acts as echinocytogenic agent in low ionic strength medium at pH 7.4 but not at pH 5.1 underlines this explanation. Therefore, a transmembrane potential positive inside produces stomatocytes, slightly negative inside (below–10 mV), normocytes, and strongly negative, echinocytes. The temperature dependence of this process underlines the rigidity-pattern hypothesis of red blood cell shape (Glaser & Leitmannová, 1975, 1977).     

Mechanical behavior of closed lamellar membranes as a possible common mechanism for the establishment of developmental shapes.

The mechanical behavior of a closed membrane composed of two layers in contact is described as it is obtained by finding the minimum of the membrane bending energy at constant membrane area, constant difference between the areas of the two layers, and constant enclosed volume. It is shown that the membrane bending energy is a discontinuous function of the volume (v) and difference between the layer areas (delta a) defined relative to the volume and area difference of a sphere with the same membrane area, respectively. However, for different classes of shapes it is possible to obtain regions in the v/delta a diagram within which the shapes change continuously with v and delta a. These regions are shown for the egg, dumbbell and cup shape classes, respectively. The results of the shape analysis are used in the discussion of cell polarity, cytokinesis and gastrulation. Cell polarity is related to the decrease in symmetry during the transition from the radially symmetrical spherical shape to the asymmetrical shapes of the egg class. It is proposed that symmetrical cytokinesis occurs within the dumbbell class and that asymmetrical cytokinesis occurs within the egg class. Gastrulation is described as shape transformations within the class of cup shapes.     

Effects of freezing on the structure of chloroplast membranes

Electron spin resonance (ESR) spectra of stearic acid spin labels incorporated into spinach thylakoids can be used to monitor membrane changes during freezing. Changes in the ESR parameters can be directly correlated to the extent of functional freeze damage. Freeze-induced changes in the ESR parameters strongly depend on the osmotic conditions of the incubation medium. Similar changes as on freezing can be observed by transferring thylakoids from an isotonic to a hypotonic medium, i.e., by swelling osmotically flattened thylakoids. This and computer simulations of spin label ESR spectra, which allow for variation of vesicle shape, lead to the conclusion that freeze-induced ESR spectral changes are due to swelling of the thylakoids. Indeed, van't Hoff plots of thylakoid packed volume indicate a freeze-induced increase in the apparent number of osmotically active molecules within the intrathylakoid lumen. During freezing, salt and/or sugar leak into the lumen. Simultaneously, proton channels are irreversibly opened. As the structural alterations obtained upon freezing are not accompanied by a change in bulk fluidity, these data are interpreted in terms of a local action of cryotoxic agents on critical microstructures, possibly at the rims of the thylakoid membranes.     

Osmolar Concentration and Fixation of Mycoplasmas

Broth cultures of Acholeplasma laidlawii were fixed with various concentrations of cacodylate-buffered glutaraldehyde. The shape and ultrastructure of the organisms varied with the osmolar concentration of the fixative. When the fixation mixture was hypertonic to the culture medium, ultrathin sections suggested that the cells had shrunk. Phosphate buffer, sodium chloride, or sucrose at comparable osmolaities had the same effect as sodium cacodylate. Glutaraldehyde itself also contributed to the osmotic effects of the fixation mixture but to a lesser extent than salts or sucrose, to which the cell membrane is impermeable. The osmolar concentration of the fixation mixture seemed of greater importance than pH in determining morphology. The mycoplasma was still susceptible to damage by high concentrations of cacodylate after fixation with 2.5% glutaraldehyde. The best procedure was to fix and wash the organism under conditions isotonic with the growth medium. These conditions were also satisfactory for a filamentous mycoplasma, Mycoplasma orale.     

Quantitative measurement of fusion between human immunodeficiency virus and cultured cells using membrane fluorescence dequenching

Human immunodeficiency virus (HIV) was purified by sucrose gradient centrifugation and labeled with octadecylrhodamine B-chloride (R-18) under conditions resulting in 90% quenching of the fluorescence label. Incubation of R-18-labeled HIV (R-18/HIV) with CD4-positive CEM and HUT-102 cells, but not with CD4-negative MLA-144 cells, resulted in fluorescence dequenching (DQ, increase in fluorescence) of 20-25%. Similar level of DQ was observed upon incubation of CEM cells with R-18-labeled Sendai virus. DQ was observed when R-18/HIV was incubated with CD4+ cells at 37 degrees C, but not at 4 degrees C. Most of the increase in fluorescence occurred within 5 min of incubation at 37 degrees C and was independent of medium pH over the range of pH 5-8. Preincubation of cells with the lysosomotropic agent NH4Cl had no inhibitory effect on DQ. Complete inhibition was observed when target cells were fixed with glutaraldehyde prior to R-18/HIV addition. Our results demonstrate application of membrane fluorescence dequenching method to a quantitative measurement of fusion between HIV and target cell membranes. As determined by DQ, HIV penetrates into target cells by a rapid, pH-independent, receptor-mediated and specific process of fusion between viral envelope and cell plasma membrane, quite similar to that observed with Sendai virus.     

Heterogeneity in dog red blood cells: sodium and potassium transport

After incubation in isotonic KCl, dog red blood cells can be separated by centrifugation into subgroups which assume different cell volumes and possess different transport characteristics. Those red cells which swell in isotonic KCl exhibit a higher permeability to K and possess a greater volume dependence for transport of K than those red cells which shrink. A high Na permeability characterizes cells which shrink in isotonic KCl and these cells exhibit a larger volume-dependent Na flux than those red cells which swell. These two subgroups of red cells do not seem to represent two cell populations of different age. The results indicate that the population of normal cells is evidently heterogeneous in that the volume-dependent changes in Na and K permeability are distributed between differnt cell types rather than representing a single cell type which reciprocally changes its selectivity to Na and K.     

Characterization of volume-sensitive, calcium-permeating pathways in the osteosarcoma cell line UMR-106-01

Measurements of cell volume changes, free cytosolic Ca2+ concentration [( Ca2+]i) with Fura 2 and cell membrane potential with 3,3'-dipropylthiodicarbocyanine iodide were used to study the effect of cell volume change on Ca2+ influx and the membrane potential of the osteoblastic osteosarcoma cell line, UMR-106-01. Swelling the cells by hypo-osmotic stress was followed by reduction in cell volume which was markedly impaired by removal of medium Ca2+. Accordingly, cell swelling resulted in [Ca2+]i increase only in the presence of medium Ca2+. The cell swelling-activated Ca2+ entry pathway was active at resting membrane potentials, and Ca2+ influx through this pathway markedly increased upon cell hyperpolarization. A linear relationship between Ca2+ entry and the potential across the plasma membrane was observed. Thus, the volume-activated Ca2+ permeating pathway in UMR-106-01 cells has conductive properties. These pathways do not spontaneously inactivate with time when the cells are not allowed to volume regulate. The pathway can be blocked by micromolar concentrations of nicardipine and La3+ but display very low sensitivity to diltiazem and verapamil. Activation of the volume-sensitive, Ca2+ permeating pathway was not dependent on an increase in [Ca2+]i. Likewise, activation of the pathway was independent of a change in membrane potential between -85 and -3 mV. The increase in [Ca2+]i resulted in hyperpolarization of the cells, probably due to activation of Ca2+-activated K+ channels. The volume-sensitive pathways were partially active under isotonic conditions. Their activity was inhibited by cell shrinkage and increased by cell swelling. The pathways were sensitive to small changes in cell volume, particularly around a medium osmolarity of 310 mosM.     

Ultrastructural studies of beef heart mitochondria. 3. The inequality of gross morphological change and oxidative phosphorylation

The relationships between membranes and intramembrane compartments of isolated heart mitochondria are inadequately defined to express the induced morphological changes associated with the structural organization. The inner membrane and matrix are the major structural entities which undergo transformation upon alteration of metabolism or incubation conditions. To better express these morphological changes within a mitochondrion, two inner membranes plus enclosed matrix are defined as an inmerix (plural inmerices). Three general morphological forms of mitochondria can be distinguished by the size and shape of inmerices. These are distended, condensed, and coalesced inmerixal configurations. Hypotonic conditions and P_i in isotonic sucrose generate distended configurations. This P_i distention is apparently dependent on utilization of energy. It does not occur under anaerobic conditions. Oxidizable substrates generate condensed configurations. ADP and dADP generate coalesced configurations and stop formation of condensed and distended inmerixal configurations in the absence of inhibitors. ADP coalescence is apparently not dependent on an energy input. It occurs under aerobic and anaerobic conditions, and in isotonic and hypotonic media. Atractyloside completely inhibits the effects of ADP on inmerixal membranes whereas oligomycin does not. Distention by P_i is unaffected by the two inhibitors. Distended inmerices, without added P_i (12 mM and 62 mM sucrose), are coalesced by ADP. These studies indicate that coalescence of inmerixal membranes probably reflects the consequences of specific stoichiometric binding or translocation of adenine nucleotides.     

Hypotonic Challenge of Endothelial Cells Increases Membrane Stiffness with No Effect on Tether Force

Regulation of cell volume is a fundamental property of all mammalian cells. Multiple signaling pathways are known to be activated by cell swelling and to contribute to cell volume homeostasis. Although cell mechanics and membrane tension have been proposed to couple cell swelling to signaling pathways, the impact of swelling on cellular biomechanics and membrane tension have yet to be fully elucidated. In this study, we use atomic force microscopy under isotonic and hypotonic conditions to measure mechanical properties of endothelial membranes including membrane stiffness, which reflects the stiffness of the submembrane cytoskeleton complex, and the force required for membrane tether formation, reflecting membrane tension and membrane-cytoskeleton attachment. We find that hypotonic swelling results in significant stiffening of the endothelial membrane without a change in membrane tension/membrane-cytoskeleton attachment. Furthermore, depolymerization of F-actin, which, as expected, results in a dramatic decrease in the cellular elastic modulus of both the membrane and the deeper cytoskeleton, indicating a collapse of the cytoskeleton scaffold, does not abrogate swelling-induced stiffening of the membrane. Instead, this swelling-induced stiffening of the membrane is enhanced. We propose that the membrane stiffening should be attributed to an increase in hydrostatic pressure that results from an influx of solutes and water into the cells. Most importantly, our results suggest that increased hydrostatic pressure, rather than changes in membrane tension, could be responsible for activating volume-sensitive mechanisms in hypotonically swollen cells.     

Two-dimensional impedance studies of BSA buoyant density separated human erythrocytes

Combined DC (Coulter Volume) and radio frequency impedance studies were performed on human erythrocytes which had been separated by buoyant density in linear, neutral, isotonic bovine serum albumin gradients. The individual buoyant density fractions showed no reproducible shift in volume with buoyant density but did show a shift with opacity, radio frequency impedance divided by dc impedance. This new electronic parameter of opacity can be related to cell age, since both it and cell age are directly related to buoyant density. This increase in opacity with buoyant density is correlated with a change in shape.     

Cytoplasmic pH and human erythrocyte shape.

Altered external pH transforms human erythrocytes from discocytes to stomatocytes (low pH) or echinocytes (high pH). The mechanism of this transformation is unknown. The preceding companion study (Gedde and Huestis) demonstrated that these shape changes are not mediated by changes in membrane potential, as has been reported. The aim of this study was to identify the physiological properties that mediate this shape change. Red cells were placed in a wide range of physiological states by manipulation of buffer pH, chloride concentration, and osmolality. Morphology and four potential predictor properties (cell pH, membrane potential, cell water, and cell chloride concentration) were assayed. Analysis of the data set by stratification and nonlinear multivariate modeling showed that change in neither cell water nor cell chloride altered the morphology of normal pH cells. In contrast, change in cell pH caused shape change in normal-range membrane potential and cell water cells. The results show that change in cytoplasmic pH is both necessary and sufficient for the shape changes of human erythrocytes equilibrated in altered pH environments.     

Regulation by cell volume of Na+-K+-2Cl− cotransport in vascular endothelial cells: role of protein phosphorylation

Summary Na⁺-K⁺-2Cl^– cotransport in aortic endothelial cells is activated by cell shrinkage, inhibited by cell swelling, and is responsible for recovery of cell volume. The role of protein phosphorylation in the regulation of cotransport was examined with two inhibitors of protein phosphatases, okadaic acid and calyculin, and a protein kinase inhibitor, K252a. Both phosphatase inhibitors stimulated cotransport in isotonic medium, with calyculin, a more potent inhibitor of protein phosphatase I, being 50-fold more potent. Neither agent stimulated cotransport in hypertonic medium. Stimulation by calyculin was immediate and was complete by 5 min, with no change in cell Na + K content, indicating that the stimulation of cotransport was not secondary to cell shrinkage. The time required for calyculin to activate cotransport was longer in swollen cells than in normal cells, indicating that the phosphorylation step is affected by cell volume. Activation of cotransport when cells in isotonic medium were placed in hypertonic medium was more rapid than the inactivation of cotransport when cells in hypertonic medium were placed in isotonic medium, which is consistent with a shrinkage-activated kinase rather than a shrinkage-inhibited phosphatase. K252a, a nonspecific protein kinase inhibitor, reduced cotransport in both isotonic and hypertonic media. The rate of inactivation was the same in either medium, indicating that dephosphorylation is not regulated by cell volume. These results demonstrate that Na⁺-K⁺-2Cl^– cotransport is activated by protein phosphorylation and is inactivated by a Type I protein phosphatase. The regulation of cotransport by cell volume is due to changes in the rate of phosphorylation rather than dephosphorylation, suggesting the existence of a volume-sensitive protein kinase. Both the kinase and the phosphatase are constitutively active, perhaps to allow for rapid changes in cotransport activity.This work was supported by a Clinical Investigator Award DK01643 (to W.C.O) and a Grant-in-Aid from the American Heart Association of Georgia.     

Network thermodynamic analysis and stimulation of isotonic solute-coupled volume flow in leaky epithelia: an example of the use of network theory to provide the qualitative aspects of a complex system and its verification by stimulation

A network thermodynamic model was developed to provide insights into the nature of isotonic solute-coupled volume flow in "leaky" epithelia, where the transepithelial volume flow is assumed to be primarily through the cellular pathway. The coupled flows of solute and volume at each membrane in this four membrane model are described by the practical phenomenological equations as developed by Kedem & Katchalsky (1958). The model contains one permeable non-electrolyte solute (s) and a fixed amount of an impermeable non-electrolyte (i) inside the cell. The cell is assumed to be capable of volume regulation under the steady-state experimental conditions simulated. A solute-pump, located in the basolateral membrane, uses feedback regulation to adjust Cs in the cell in order to maintain cell volume at or near control levels in all simulations. Model behavior is, in general, very consistent with experimental observations with respect to tonicity and magnitude of volume flow over a wide range of experimental conditions. Examination of the parameter space suggests the following important features when isotonic solute-coupled volume flow moves primarily through the cellular pathway: (1) the apical membrane reflection coefficient must be less than that of the basolateral membrane; (2) the basement membrane reflection coefficient must be small; (3) the apical membrane solute permeability and reflection coefficient are the two most "sensitive" parameters and need to vary in an inverse manner in order to maintain isotonicity when both solute and volume flows increase; and (4) relationships (1) and (3) above imply the need for at least two separate solute pathways in the apical membrane, one that is shared with volume flow and one that is not.     

Spectrin phosphorylation and shape change of human erythrocyte ghosts

Human erthrocyte membranes in isotonic medium change shape from crenated spheres to biconcave disks and cup-forms when incubated at 37 degrees C in the presence of MgATP (M. P. Sheetz and S. J. Singer, 1977, J. Cell Biol. 73:638-646). The postulated relationship between spectrin phosphorylation and shape change (W. Birchmeier and S. J. Singer, 1977, J. Cell Biol. 73:647-659) is examined in this report. Salt extraction of white ghosts reduced spectrin phosphorylation during shape changes by 85-95%. Salt extraction did not alter crenation, rate of MgATP-dependent shape change, or the fraction (greater than 80%) ultimately converted to disks and cup-forms after 1 h. Spectrin was partially dephosphorylated in intact cells by subjection to metabolic depletion in vitro. Membranes from depleted cells exhibited normal shape-change behavior. Shape-change behavior was influenced by the hemolysis buffer and temperature and by the time required for membrane preparation. Tris and phosphate ghosts lost the capacity to change shape after standing for 1-2 h at 0 degrees C. Hemolysis in HEPES or N- tris(hydroxymethyl)methyl-2-aminoethanesulfonic acid yielded ghosts that were converted rapidly to disks in the absence of ATP and did not undergo further conversion to cup-forms. These effects could not be attributed to differential dephsphorylation of spectrin, because dephosphorylation during ghost preparation and incubation was negligible. These results suggest that spectrin phosphorylation is not required for MgATP-dependent shape change. It is proposed that other biochemical events induce membrane curvature changes and that the role of spectrin is passive.     

Ionic dependence of the extracellular ATP-induced permeabilization of transformed mouse fibroblasts: role of plasma membrane activities that regulate cell volume

Extracellular ATP rendered the plasma membrane of transformed mouse fibroblasts permeable to normally impermeant molecules. This permeability change was prevented by increasing the ionic strength of the isotonic medium with NaCl. Conversely, the cells exhibited increased sensitivity to ATP when the NaCl concentration was decreased below isotonicity, when the KCl concentration was increased above 5 mM while maintaining isotonicity, and when the pH of the medium was raised above 7.0. These conditions as well as the addition of ATP itself caused cell swelling. However, the effect of ATP was independent of cell volume and dependent upon the ionic strength and not the osmolarity of the medium since 1) addition of sucrose to isotonic medium did not prevent permeabilization although media made hypertonic with either sucrose or NaCl caused a decrease in cell volume; and 2) addition of sucrose or NaCl to hypotonic media caused a decrease in cell volume, but only NaCl addition decreased the response to ATP. Conditions that have been shown to inhibit plasma membrane proteins that play a reciprocal role in cell volume regulation had reciprocal effects on the permeabilization process, even though the effect of ATP was independent of cell volume. For example, inhibition of the Na+,K+-ATPase by ouabain increased sensitivity of cells to ATP while conditions which inhibit Na+,K+,Cl- -cotransporter activity, such as treatment of the cells with the diuretics furosemide or bumetanide or replacement of sodium chloride in the medium with sodium nitrate or thiocyanate, inhibited permeabilization. The furosemide concentration that inhibited permeabilization was greater than the concentration that inhibited Na+,K+,Cl- -cotransporter-mediated 86Rb+ (K+) uptake, suggesting that the effect of furosemide on the permeabilization process may not be specific for the Na+,K+,Cl- -cotransporter.     

Cell Surface Area Regulation and Membrane Tension

The beautifully orchestrated regulation of cell shape and volume are central themes in cell biology and physiology. Though it is less well recognized, cell surface area regulation also constitutes a distinct task for cells. Maintaining an appropriate surface area is no automatic side effect of volume regulation or shape change. The issue of surface area regulation (SAR) would be moot if all cells resembled mammalian erythrocytes in being constrained to change shape and volume using existing surface membrane. But these enucleate cells are anomalies, possessing no endomembrane. Most cells use endomembrane to continually rework their plasma membrane, even while maintaining a given size or shape. This membrane traffic is intensively studied, generally with the emphasis on targeting and turnover of proteins and delivery of vesicle contents. But surface area (SA) homeostasis, including the controlled increase or decrease of SA, is another of the outcomes of trafficking. Our principal aims, then, are to highlight SAR as a discrete cellular task and to survey evidence for the idea that membrane tension is central to the task. Cells cannot directly ``measure' their volume or SA, yet must regulate both. We posit that a homeostatic relationship exists between plasma membrane tension and plasma membrane area, which implies that cells detect and respond to deviations around a membrane tension set point. Maintenance of membrane strength during membrane turnover, a seldom-addressed aspect of SA dynamics, we examine in the context of SAR. SAR occurs in both animal and plant cells. The review shows the latter to be a continuing source of groundbreaking work on tension-sensitive SAR, but is principally slanted to animal cells. Received: 1 May 2000/Revised: 14 August 2000