Cellular and molecular consequences of reduced cell water content

Connecting the Dots: Macromolecular Crowding and Protein Aggregation

Proteins are one of the dynamic macromolecules that play a significant role in many physiologically important processes to sustain life on the earth. Proteins need to be properly folded into their active conformation to perform their function. Alteration in the protein folding process may lead to the formation of misfolded conformers. Accumulation of these misfolded conformers can result in the formation of protein aggregates which are attributed to many human pathological conditions including neurodegeneration, cataract, neuromuscular disorders, and diabetes. Living cells naturally have heterogeneous crowding environments with different concentrations of various biomolecules. Macromolecular crowding condition has been found to alter the protein conformation. Here in this review, we tried to show the relation between macromolecular crowding, protein aggregation, and its consequences.

Fertilization and development of Arbacia lixula eggs are affected by osmolality conditions

The sea urchin Arbacia lixula coexist with Paracentrotus lividus in the Mediterranean, but the two sea urchin species are quite different from each other. Concerning the female gamete, A. lixula eggs are much darker than those of P. lividus due to the characteristic pigmentation. Upon insemination, the fertilization envelope formed by A. lixula eggs is remarkably thinner than that of P. livius eggs, which implies that the cortical organization of the eggs in the two species may be quite different. In this communication, we examined the phenotypic plasticity of A. lixula eggs in the changing osmolality. The plasma membrane, cortical actin cytoskeleton and vesicles are extensively altered in the eggs exposed to 40% seawater for 15 min. When fertilized, the Ca²⁺ response in these eggs was significantly compromised and the sperm often failed to enter the eggs. Remarkably, the pattern of the Ca²⁺ response was restored when these eggs were transferring back to the natural seawater before fertilization, while the actin cytoskeleton partially reverted to the original state. Nonetheless, these eggs restored in seawater failed to regain the innate sperm receptivity that allows only one sperm to enter in natural seawater. Thus, the ability to guide monospermic fertilization is lost by water entry into the eggs, and the eggs incorporated either multiple or no sperm. On the other hand, eggs briefly exposed to hypertonic seawater exhibited no evident morphological anomaly. Nonetheless, the monospermic eggs that experienced a brief exposure (15 min) to hypertonic seawater prior to fertilization in natural seawater displayed a subtly altered sperm-induced Ca²⁺ response and morpho-functional anomaly around the pluteus stage. Our results suggest that A. lixula eggs attain only a limited extent of cytological plasticity, and that the osmolality shock affects the physical nature of the egg surface which in turn affects the developmental programming.

LENTICULE discs provide a homogenous format for external quality assessment samples: a comparison with freeze-dried samples for shellfish microbiology

AIMS

The aim was to compare the variability in Escherichia coli enumeration data and detection of Salmonella spp. between four samples of LENTICULE discs and freeze-dried samples for the Health Protection Agency's External Quality Assessment (EQA) scheme for shellfish microbiology.

METHODS AND RESULTS

Four samples of known but undisclosed microbiological content were dispatched in both freeze-dried and LENTICULE disc formats to 57 participating laboratories in 20 countries. Participants examined samples using their routine methods for the most probable number (MPN) of E. coli per 100 g and the presence/absence of Salmonella spp. There was no significant difference between the Food and Environmental Proficiency Testing Unit and participating laboratories for E. coli and Salmonella spp. results. There were significantly less outlying results using the LENTICULE discs than freeze-dried sample format and equivalent or less variance for the former for E. coli MPN. There was no significant difference between LENTICULE discs and freeze-dried samples for the presence/absence of Salmonella spp.

CONCLUSIONS

Overall the results indicated that there was equivalent or less variance in results for the LENTICULE discs than for freeze-dried samples, therefore LENTICULE discs are a homogenous and stable matrix for EQA samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides validation data for the replacement of freeze-dried samples by LENTICULE discs for the Health Protection Agency Shellfish EQA Scheme.

Resurrecting Van Leeuwenhoek's rotifers: a reappraisal of the role of disaccharides in anhydrobiosis

In 1702, Van Leeuwenhoek was the first to describe the phenomenon of anhydrobiosis in a species of bdelloid rotifer, Philodina roseola. It is the purpose of this review to examine what has been learned since then about the extreme desiccation tolerance in rotifers and how this compares with our understanding of anhydrobiosis in other organisms. Remarkably, much of what is known today about the requirements for successful anhydrobiosis, and the degree of biostability conferred by the dry state, was already determined in principle by the time of Spallanzani in the late 18th century. Most modern research on anhydrobiosis has emphasized the importance of the non-reducing disaccharides trehalose and sucrose, one or other sugar being present at high concentrations during desiccation of anhydrobiotic nematodes, brine shrimp cysts, bakers' yeast, resurrection plants and plant seeds. These sugars are proposed to act as water replacement molecules, and as thermodynamic and kinetic stabilizers of biomolecules and membranes. In apparent contradiction of the prevailing models, recent experiments from our laboratory show that bdelloid rotifers undergo anhydrobiosis without producing trehalose or any analogous molecule. This has prompted us to critically re-examine the association of disaccharides with anhydrobiosis in the literature. Surprisingly, current hypotheses are based almost entirely on in vitro data: there is very limited information which is more than simply correlative in the literature on living systems. In many species, disaccharide accumulation occurs at approximately the same time as desiccation tolerance is acquired. However, several studies indicate that these sugars are not sufficient for anhydrobiosis; furthermore, there is no conclusive evidence, through mutagenesis or functional knockout experiments, for example, that sugars are necessary for anhydrobiosis. Indeed, some plant seeds and micro-organisms, like the rotifer, exhibit excellent desiccation tolerance in the absence of high intracellular sugar concentrations. Accordingly, it seems appropriate to call for a re-evaluation of our understanding of anhydrobiosis and to embark on new experimental programmes to determine the key molecular mechanisms involved.

Intracellular trehalose improves osmotolerance but not desiccation tolerance in mammalian cells

Trehalose has been shown to play a role in osmotolerance or desiccation tolerance in some microorganisms, anhydrobiotic invertebrates and resurrection plants. To test whether trehalose could improve stress responses of higher eukaryotes, a mouse cell line was genetically engineered to express bacterial trehalose synthase genes. We report that the resulting levels of intracellular trehalose ( approximately 80 mM) are able to confer increased resistance to the partial dehydration resulting from hypertonic stress, but do not enable survival of complete desiccation due to air drying.

Physiological significance of volume-regulatory transporters

Research over the past 25 years has identified specific ion transporters and channels that are activated by acute changes in cell volume and that serve to restore steady-state volume. The mechanism by which cells sense changes in cell volume and activate the appropriate transporters remains a mystery, but recent studies are providing important clues. A curious aspect of volume regulation in mammalian cells is that it is often absent or incomplete in anisosmotic media, whereas complete volume regulation is observed with isosmotic shrinkage and swelling. The basis for this may lie in an important role of intracellular Cl- in controlling volume-regulatory transporters. This is physiologically relevant, since the principal threat to cell volume in vivo is not changes in extracellular osmolarity but rather changes in the cellular content of osmotically active molecules. Volume-regulatory transporters are also closely linked to cell growth and metabolism, producing requisite changes in cell volume that may also signal subsequent growth and metabolic events. Thus, despite the relatively constant osmolarity in mammals, volume-regulatory transporters have important roles in mammalian physiology.

Macromolecular crowding and confinement in cells exposed to hypertonicity

The nonideal properties of solutions containing high concentrations of macromolecules can result in enormous increases in the activity of the individual macromolecules. It has been proposed that molecular crowding and confinement occur in cells and are major determinants of the activity of the proteins and other intracellular macromolecules. This concept has important implications for cell volume regulation because, under crowded conditions, relatively small changes in concentration, consequent to alterations of water content, lead to large changes in macromolecular activity. This review considers several aspects of macromolecular crowding and confinement, including: 1) the physical chemical principles involved; 2) in vitro demonstrations of the effects; 3) relation to water activity; 4) estimates of the actual intracellular activity of water and macromolecules; 5) relation to osmotic regulation in various types of cells, including bacteria, red blood cells, and complex nucleated cells; and 6) the relation to inorganic ions and organic osmolytes in cells stressed by hypertonicity. We conclude that, while there is compelling evidence for important effects of molecular crowding in vitro and in red blood cells, the role of macromolecular crowding and confinement in osmotic regulation of more complex cells is an open question that deserves the extensive attention it is currently receiving.

Effects of reduced cell volume and water content on glycolysis in L-929 cells

Mouse L-929 cells were subjected to increasing concentrations of sorbitol, which remove cell water and reduce volume osmotically. The rate of lactate production from glucose was significantly higher in osmotically perturbed cells than in controls, both in monolayers and in suspensions. L cells can apparently use sorbitol as a glycolytic substrate; however, studies using other solutes (trehalose and sucrose) and permeabilized cells showed that the major effect of sorbitol on glycolysis in intact cells is mediated through a reduction in cell water content and volume. It is possible to explain some of these results by an increase in the chemical potentials of dissolved components of the glycolytic pathway caused by water loss; however, the relationship between water loss and glycolytic rate increase in not a simple linear one, suggesting that the situation is more complex than would result merely from increased concentrations of pathway components. Whatever the complete explanation might be, these studies show that glycolysis continues in an orderly fashion in cells that have lost about 85% of their original water content, suggesting that the operation of this pathway is not unduly sensitive to events taking place in the bulk aqueous phase.

Effects of osmotic manipulation of intracellular hydration of HeLa S-3 cells on their proton NMR relaxation times

Pellets of HeLa from suspension cultured cells in isotonic medium (300 mosmolar) were introduced into a Bruker CXP100 NMR spectrophotometer at 80 mHz within 5 min of the start of centrifugation. T1 and T2 times were measured within a total elapsed time of 20-25 min at 80 mHz and 37 degrees C, and averaged 1430 msec and 120 msec, respectively. Extrapolation to zero extracellular space gave a corrected T1 of 1370 msec. For cells collected after 10 min in hypotonic medium (down to 30 mosmolar) increased proton density correlated well with increased cell water content, but relaxation times did not rise in proportion to that predicted for the entry of "bulk" water (T1 of 4700 msec), except when swelling approached lysis point. Cells partially dehydrated by 10 min in hypertonic medium of up to 1500 mosmolar have also been analyzed, but once again the shortening of T1 was not proportional to the loss of "free" (bulk phase) water. At the upper limit of hypertonic treatment, lacunae or vacuoles of a watery nature separated within the cytomatrix, preventing maximum dehydration. The relationship of cell water to T1 is complex over the whole range of tonicity that HeLa S-3 cells tolerate. The data indicate, however, that hypotonically induced water probably has an average T1 time considerably lower than bulk phase water. In contrast, raising the total extracellular volume with medium had precisely the predicted effect on T1 time, further strengthening the case that water taken up by cell acquires a shorter T1 time. Cells adapting to hypotonic conditions oscillated in size and water content over 2-3 hr before returning to near their initial volume. Under these circumstances, T1 oscillated in the same way but with a reduced amplitude, consistent with the above findings.

L-929 cells under hyperosmotic conditions. Water, Na+, and K+

Changes in cell water content resulting from sorbitol addition to the environment of L-929 cells were evaluated gravimetrically using 14C-labeled polyethylene glycol as a probe of extracellular space. Reductions in cell water were proportional to sorbitol supplements up to 0.6 molal, above which no further measurable decrease occurred. No volume regulation occurred for at least 1 h but the percentage of cell water lost was quickly regained when physiological conditions were restored. The amount of cell water lost because of a given hyperosmotic exposure was found to exceed the loss of cell volume. That discrepancy could be the result of an overestimation of extracellular space and/or an underestimation of cell volume reduction as a result of in-folding of the cell surface. Na+ and K+ were also measured in cells of variable water content and volume: no significant change occurred in the amounts of these ions per cell, but large increases in total cell concentration resulted from hyperosmotic exposure. The sum of Na+ and K+ concentrations exceeds the total osmotic pressure of the medium indicating that an appreciable fraction of Na+ and K+ must be bound to fixed charges within the cells. The results are evaluated in the context of intracellular organization.

Cytoplasmic maturation of the snRNAs

The snRNAs are abundant and stable components of the interphase nucleus. Aqueous and non-aqueous cell fractionation demonstrate that the snRNAs appear transiently in the cytoplasm shortly after transcription, before returning permanently to the interphase nucleus. In pulse label and chase experiments, the newly synthesized snRNA species appear in the cytoplasm after 1 min of labeling and then return to the interphase nucleus after approximately 15 min in the cytoplasm. In order to study the maturation and intracellular transport of these particles, a battery of metabolic inhibitors and alterations in cell culture conditions were investigated for their ability to interfere with the return of the newly synthesized snRNAs to the nucleus. A wide range of inhibitors of the cytoskeleton did not interfere with this process. Only the inhibition of protein synthesis and exposure of cells to medium of at least twice the normal tonicity block the return of the snRNAs to the nucleus. Immunofluorescent staining of cells exposed to hypertonic medium identifies discrete foci in the cytoplasm that stain with the Sm antiserum, directed against proteins associated with the snRNAs. Using a detergent extraction procedure that preserves the cytoskeleton, the newly synthesized snRNAs in the cytoplasm fractionate as soluble complexes. These data are consistent with the hypothesis that the snRNAs partition into the interphase nucleus because of a preferential solubility and the existence of specific binding sites.

L-929 cells under hyperosmotic conditions: volume changes

Volume changes resulting from the addition of sorbitol to the environment of mouse L-929 cells were evaluated from cell diameter measurements. Over periods of 1 hour or less, this solute was effectively excluded from intracellular water. The reduction in cell volume was inversely related to sorbitol concentration up to levels of about 0.6 molal, above which no further significant reduction occurred. Reduced cell volumes were maintained for at least 1 hour without measurable volume regulation. The percentage of volume lost was independent of the initial cell volume and was quickly regained when physiological conditions were restored. However, cell volume was influenced strongly by cell density or by some variable related to it. L-cells store surface area when dehydrated, apparently by means of plasma membrane convolutions and microvilli, based on the rapid kinetics of reversible volume changes and on observations from scanning electron microscopy. These results are related to current views on the nature of intracellular organization.

Removal of cellular water prevents the reformation of the interphase nucleus

The mature snRNP (small nuclear ribonucleoprotein) particles are localized quantitatively in the interphase nucleus. Like many nuclear antigens, they distribute throughout the cytoplasm after the nuclear envelope breaks down during mitosis and then return to the newly formed daughter nuclei in early G1. Their abundance and stability and the availability of monoclonal antibodies that recognize them, make the snRNP particles a useful model system for studying the reformation of the nucleus at the completion of mitosis. A wide variety of metabolic inhibitors and alterations in normal culture conditions were investigated for their ability to interfere with the return of the snRNP particles to daughter nuclei after mitosis. None of the well-characterized cytoskeletal inhibitors, biosynthetic inhibitors, calcium antagonists, nor ionophores were effective in interfering with this return. However, the removal of cellular water by exposure of cells to hypertonic medium during mitosis blocked the reformation of the nucleus and trapped the snRNP particles in the cytoplasm. In medium of twice the normal tonicity, the function of the mitotic spindle and the cleavage furrow are inhibited, however, the cells reattach to the substratum as if returning to interphase. The chromatin stays condensed and does not form a normal interphase nucleus and the snRNP particles stay dispersed throughout the cytoplasm. This condition is reversible and after return to normal medium the nucleus reforms and the snRNP particles collect in the new nuclei. After gentle extraction of metaphase cells, about 30% of the snRNP particles are soluble, however, the remainder are associated with an insoluble remnant. These data are consistent with the notion that the snRNP particles accumulate in the nucleus due to both preferential solubility and specific binding sites in the interphase nucleus.

Characterization of water in unfertilized and fertilized sea urchin eggs

The water in unfertilized and fertilized sea urchin eggs was characterized with a proton nuclear magnetic resonance (NMR) titration method assuming fast proton diffusion (FPD) between water compartments. This method involves stepwise dehydration with sequential T1 relaxation time and water content determinations. The results analyzed by the FPD model give evidence of intracellular water compartments with three different correlation times: 6 X 10(-12) sec (bulk water), 1 X 10(-10) sec (structured water) and about 2 X 10(-9) sec (bound water). Fertilization is accompanied by a substantial increase in bulk water (from 111 to 414 g H2O per 100 g dry mass) and by a decrease in the water of hydration (from 128 g to 56 g per 100 g dry mass). This study shows that 54% of the water in the unfertilized sea urchin egg has motional properties different from bulk water and that this percentage decreases dramatically shortly after fertilization. Most of the change in T1 relaxation rate observed at fertilization can be accounted for by uptake of bulk water associated with elevation of the fertilization membrane.

Some structural, biochemical and biophysical characteristics of L-929 cells growing in the presence of hyperosmotic sorbitol concentrations

Mouse L-929 cells (a fibroblast-like line) were transferred from normal growth medium to one supplemented with 0.3 M sorbitol, doubling the normal external osmotic pressure. After a short lag phase and minimal cell death, the cells began to grow, and the growth rate reached that of controls after about one week. These chronically grown cells (S) have been compared to those of control cultures (C) with regard to general morphology, ability to reverse when returned to normal condition, water content, volume and selected metabolic parameters. S-cell cultures exhibited considerable heterogeneity but most contained vesicle-like cytoplasmic structures, sometimes in abundance. These structures do not appear to be completely bounded by membranes, but that is uncertain. S cells become larger and contain more water than C cells; however, the ratio of total water to total dry mass is indistinguishable from controls suggesting regulation at that level. S and C cells were found to be remarkably similar, on a per cell basis, with regard to their rate of respiration and the incorporation of glucose into metabolites and macromolecules. These results are interpreted in terms of current views on the composition and organization of the aqueous compartments of animal cells.

Respiratory metabolism of L-929 cells at different water contents and volumes

Oxygen consumption was measured in mouse L-929 cells whose volumes and water contents were reduced by adding sorbitol to the medium. The volume of water lost due to a given sorbitol supplement exceeded the loss in apparent cell volume. An explanation is given for this discrepancy. The rate of oxygen uptake in the absence of exogenous respiratory substrate was essentially the same in cells whose total volume was reduced by 45%, amounting to a loss of about 70% of the total cell water, compared to controls at 'physiological' volume and water content. Cells under these same conditions responded to added substrates (pyruvate, glucose, and glutamine) and inhibitors (iodoacetate and 2-deoxyglucose) in nearly the same way as control cells. These observations are in accord with and add to previous work showing that very large fluctuations in cell volume and water content have only modest effects on the rates and directions of a variety of metabolic processes. The results are interpreted in terms of current views on the composition and organization of the aqueous compartments of eucaryotic cells.

The effects of osmotic stress on human platelets

The effect of osmotic stress on human platelets was investigated at 0, 25, and 37 degrees C. The osmolality of the suspending plasma was decreased by adding water or increased by adding sodium chloride or sucrose. After 5 min, isotonicity was restored by dilution with an excess of isotonic phosphate-buffered saline. After centrifugation, the platelets were resuspended in autologous plasma and then incubated for 1 hr at 37 degrees C before assaying the active transport of 5-hydroxytryptamine (5-HT) and the hypotonic stress response. Anisosmotic conditions had a greater effect on the extent of volume reversal in the hypotonic stress test than on 5-HT uptake. At 25 degrees C, only moderate degrees of hypotonicity (0.25 osmol/kg) or hypertonicity (0.59 osmol/kg) were sufficient to depress the hypotonic stress response. In general, platelets tolerated departures from isotonic conditions better at 0 degree C than at the higher temperatures. Furthermore, at 0 and 25 degrees C approximately equiosmolal concentrations of sucrose and sodium chloride depressed the hypotonic stress response to similar extents, but at 37 degrees C high osmolalities (greater than 2 osmol/kg) were tolerated better when the additive was sucrose than when it was sodium chloride. Platelets shrank when subjected to hyperosmotic conditions, but their discoid shape and the peripheral band of microtubules were maintained.

Investigation of the mechanism of protein turnover in HeLa S-3 cells by incubation at elevated temperatures

An apparent paradox relating to the degradation of endogenous proteins in HeLa S-3 cells occurs at 45 degrees C, at which their proteolysis is considerably enhanced in vitro but completely inhibited in vivo. No significant differences in rates of degradation of short-lived (nascent) and long-lived ('existing') proteins synthesised at 37 degrees C were found when chased at temperatures up to 43 degrees C, but at 45 degrees C degradation of both categories was reduced to zero in vivo. Synthesis of protein was suppressed at temperatures above 41 degrees C, being reduced by up to 60% at 43 degrees C. Proteolysis in vitro proceeded 1.6-1.7 times faster at 45 degrees C than at 37 degrees C and neutral pH. Evidence is presented for the involvement of the basal system; the findings both in vivo and in vitro do not seem to implicate the lysosomal system, no firm indication being obtained of its 'induction' at elevated temperatures. The results are discussed in terms of the arrest of intracellular circulation at elevated temperatures, thereby reducing the delivery rate of proteins as substrates of the intracellular basal proteolytic enzyme system to negligible levels (i.e., to the frequency of encounters due solely to the diffusion of protein molecules with the cytoplasm).

Mini-review. On the possible importance of an intracellular circulation

Since ultrastructural, biophysical and other studies continue to demonstrate that the internum of the cell is highly structured, the question raised and discussed in this review is whether an intracellular circulatory system is essential for the maintenance of active metabolism. Although cytoplasmic streaming is evident in large animal and plant cells, it is argued that it probably occurs in all cells irrespective of size, and is of particular importance in bringing together interacting molecules fast enough for metabolic processes to occur which would otherwise be far too slow if diffusion were the only form of motion. A common intracellular system would suffice for most metabolic processes and would also help to dissipate waste products. Interruption of this internal circulation would result in the inhibition of metabolic functioning, including for example protein turnover, for which evidence is presented to substantiate this hypothesis.

Intracellular water and the cytomatrix: some methods of study and current views

The extent to which the properties of water in cells are like those of water in dilute aqueous solutions is a question of broad significance to cell biology. A detailed answer is not available at present, although evidence is accumulating that the properties of at least a large fraction of intracellular water are altered by interactions with cell ultrastructure, notably the cytomatrix. That and related evidence also suggests that the properties, composition, and activities of the "aqueous cytoplasm" of intact cells bear little resemblance to those of the "cytosol" obtained by cell fractionation. This paper will consider some of the evidence for these possibilities and some of their potential consequences with regard to cellular structure and function.

Diffusion in the aqueous compartment

Measurements of diffusion of molecules in cells can provide information about cytoplasmic viscosity and structure. In a series of studies electron-spin resonance was used to measure the diffusion of a small spin label in the aqueous cytoplasm of mammalian cells. Translational and rotational motion were determined from the same spectra. Based on measurements made in model systems, it was hypothesized that calculations of the apparent viscosity of the cytoplasm from both rotational and translational motion would distinguish between the effects of viscosity and structure on diffusion. The diffusion constant measured in several cell lines averaged 3.3 X 10(-6) cm2/s. It was greater in growing cells and in cells treated with cytochalasin B than in quiescent cells. The viscosity of the cytoplasm calculated from the translational diffusion constant or the rotational correlation time was 2.0-3.0 centipoise, about two to three times that of the spin label in water. Therefore, over the dimensions measured by the technique, 50-100 A, solvent viscosity appears to be the major determinant of particle movement in cells under physiologic conditions. However, when cells were subjected to hypertonic conditions, the translational motion of the spin label decreased threefold, whereas the rotational motion changed by less than 20%. These data suggest that the decrease in cell volume under hypertonic conditions is accompanied by an increase in cytoplasmic barriers and a decrease in the space between existing cytoplasmic components without a significant increase in viscosity in the aqueous phase. In addition, a comparison of reported diffusion values of a variety of molecules in water and in cells indicates that cytoplasmic structure plays an important role in the diffusion of proteins such as bovine serum albumin.

Diffusion of a small molecule in the cytoplasm of mammalian cells

Electron spin resonance was used to measure the diffusion of a small (Mr 170) spin label in the aqueous cytoplasm of mammalian cells. Translational and rotational motion were determined from the same spectra. Based on measurements made in model systems, it was hypothesized that calculations of the apparent viscosity from either rotational or translational motion would distinguish between the effects of cytoplasmic viscosity or cytoplasmic structure on diffusion. The diffusion coefficient calculated from spin label collision frequency, averaged 3.3 X 10(-6) cm2/sec in several cell lines. It was greater in growing cells and in cells treated with cytochalasin B than in quiescent cells. The viscosity of the cytoplasm calculated from the translational diffusion coefficient or the rotational correlation time was 2.0-3.0 centipoise (1 P = 0.1 Pa X sec), about 2-3 times that of the spin label in water. Therefore, over the dimensions measured by the technique, 50-100 A, solvent viscosity appears to be the major determinant of particle movement in cells under physiological conditions. However, when cells were subjected to hypertonic conditions, the translational motion decreased by 67%, while the rotational motion changed less than 20%. These data suggested that the decrease in cell volume under hypertonic conditions was accompanied by an increase in cytoplasmic barriers and a decrease in the spacing between existing components. In addition, a comparison of reported values for diffusion of a variety of molecules in water and in cells indicates that cytoplasmic structure plays an important role in the diffusion of proteins such as bovine serum albumin.