The role of cellular hydration in the regulation of cell function

Hepatic glutamine transport and metabolism

Although the liver was long known to play a major role in the uptake, synthesis, and disposition of glutamine, metabolite balance studies across the whole liver yielded apparently contradictory findings suggesting that little or no net turnover of glutamine occurred in this organ. Efforts to understand the unique regulatory properties of hepatic glutaminase culminated in the conceptual reformulation of the pathway for glutamine synthesis and turnover, especially as regards the role of sub-acinar distribution of glutamine synthetase and glutaminase. This chapter describes these processes as well as the role of glutamine in hepatocellular hydration, a process that is the consequence of cumulative, osmotically active uptake of glutamine into cells. This topic is also examined in terms of the effects of cell swelling on the selective stimulation or inhibition of other far-ranging cellular processes. The pathophysiology of the intercellular glutamine cycle in cirrhosis is also considered.

Hyperosmotic induction of the mitogen-activated protein kinase phosphatase MKP-1 in H4IIE rat hepatoma cells

The action of hyperosmotic stress on the MAP kinase phosphatase MKP-1 mRNA expression was studied in H4IIE rat hepatoma cells. Hyperosmotic (405 mosmol/L) challenge of the cells led to a transient expression of MKP-1 mRNA, which was maximal after 6-8 h and disappeared completely after 24 h. Hyperosmotic MKP-1 mRNA induction was preceded by a transient activation of the MAP kinases Erk-1, Erk-2, and JNK-2, which were not prerequisite for MKP-1 mRNA accumulation. However, the hyperosmolarity-induced MKP-1 mRNA expression was sensitive to antioxidants and to inhibition of p38 by SB203580. A reduced sensitivity of Erk-1/Erk-2 to other stimuli was found after prolonged hyperosmotic exposure. The data are consistent with a hyperosmolarity-induced MKP-1 expression via reactive oxygen intermediates and p38, which may participate in the termination of MAP kinase activation and contribute to desensitization of the MAP kinases after prolonged hyperosmotic exposure of the cells.

Differential osmosensing signalling pathways and G-protein involvement in human cervical cells with different tumour potential

Previous studies show that the regulatory volume decrease (RVD) in human cervical cells with different tumour potential may be mediated by different ion channels. The signalling events involved in regulating these channel activities are not clear. To screen the possible mechanisms involved in cell volume regulation in these cells, we examine intracellular mechanisms and second messengers listed as follows: phospholipase C (PLC), phospholipase A2 (PLA2), tyrosine kinase (TK), protein kinase C (PKC), protein kinase A (PKA), and cAMP. The involvement of G-protein was also studied. Our results showed that PLC signalling with downstream activation of PKC was involved in the cell volume regulation of cervical cancer cells. On the other hand, different PKC isoforms that were not related to upstream PLC regulation were involved in the RVD of human papillomavirus (HPV)-immortalised and normal cervical epithelia. Furthermore, GTP-gamma S facilitated the process of RVD in cervical cancer cells, while pertussis toxin retarded this process. In contrast, neither GTP-gamma S nor pertussis toxin showed effect on the RVD responses of HPV-immortalised and normal cervical cells.

Functional significance of cell volume regulatory mechanisms

To survive, cells have to avoid excessive alterations of cell volume that jeopardize structural integrity and constancy of intracellular milieu. The function of cellular proteins seems specifically sensitive to dilution and concentration, determining the extent of macromolecular crowding. Even at constant extracellular osmolarity, volume constancy of any mammalian cell is permanently challenged by transport of osmotically active substances across the cell membrane and formation or disappearance of cellular osmolarity by metabolism. Thus cell volume constancy requires the continued operation of cell volume regulatory mechanisms, including ion transport across the cell membrane as well as accumulation or disposal of organic osmolytes and metabolites. The various cell volume regulatory mechanisms are triggered by a multitude of intracellular signaling events including alterations of cell membrane potential and of intracellular ion composition, various second messenger cascades, phosphorylation of diverse target proteins, and altered gene expression. Hormones and mediators have been shown to exploit the volume regulatory machinery to exert their effects. Thus cell volume may be considered a second message in the transmission of hormonal signals. Accordingly, alterations of cell volume and volume regulatory mechanisms participate in a wide variety of cellular functions including epithelial transport, metabolism, excitation, hormone release, migration, cell proliferation, and cell death.

Modulation of tryptophan environment in membrane-bound melittin by negatively charged phospholipids: implications in membrane organization and function

Melittin is a cationic hemolytic peptide isolated from the European honey bee, Apis mellifera. Since the association of the peptide in the membrane is linked with its physiological effects, a detailed understanding of the interaction of melittin with membranes is crucial. We have investigated the interaction of melittin with membranes of varying surface charge in the context of recent studies which show that the presence of negatively charged lipids in the membrane inhibits membrane lysis by melittin. The sole tryptophan residue in melittin has previously been shown to be critical for its hemolytic activity. The organization and dynamics of the tryptophan residue thus become important to understand the peptide activity in membranes of different charge types. Wavelength-selective fluorescence was utilized to monitor the tryptophan environment of membrane-bound melittin. Melittin exhibits a red edge excitation shift (REES) of 5 nm when bound to zwitterionic membranes while in negatively charged membranes, the magnitude of REES is reduced to 2-3 nm. Further, wavelength dependence of fluorescence polarization and near-UV circular dichroism spectra reveal characteristic differences in the tryptophan environment for melittin bound to zwitterionic and anionic membranes. These studies are supported by time-resolved fluorescence measurements of membrane-bound melittin. Tryptophan penetration depths for melittin bound to zwitterionic and anionic membranes were analyzed by the parallax method [Chattopadhyay, A., and London, E. (1987) Biochemistry 26, 39-45] utilizing differential fluorescence quenching obtained with phospholipids spin-labeled at two different depths. Our results provide further insight into molecular details of membrane lysis by melittin and the modulation of lytic activity by negatively charged lipids.

Involvement of integrins and the cytoskeleton in modulation of skeletal muscle glycogen synthesis by changes in cell volume

Muscle glycogen synthesis is modulated by physiologically relevant changes in cell volume. We have investigated the possible involvement of integrin-extracellular matrix interactions in this process using primary cultures of rat skeletal muscle subject to hypo- or hyper-osmotic exposure with integrin binding peptide GRGDTP to disrupt integrin actions and the inactive analogue GRGESP as control. Osmotically induced increases (77%) and decreases (34%) in glycogen synthesis (D-[14C]glucose incorporation into glycogen) were prevented by GRGDTP (but not GRGESP) without affecting glucose transport. Cytoskeletal disruption with cytochalasin D or colchicine had similar effects to GRGDTP. Osmotically induced modulation of muscle glycogen synthesis involves integrin-extracellular matrix interactions and cytoskeletal elements, possibly as components of a cell-volume 'sensing' mechanism.

Identification of a novel Ca2+-stimulated S6-kinase in rat liver

Extracellular calcium addition transiently stimulated two S6 peptide kinase activities in isolated rat hepatocytes. Mono Q chromatography revealed that the activities eluting at 0.15 M NaCl and 0.18 M NaCl were stimulated 4-fold and 2-fold, respectively. The kinase stimulated by calcium was a 40000-Mr S6 peptide kinase, as demonstrated by partial purification from whole liver. The protein kinase did not crossreact with antibodies directed against the N- or C-terminal part of p70 ribosomal S6 kinase (p70(S6K)) and the C-terminal part of p90 ribosomal S6 kinase (p90(rsk)). Following digestion of 40000-Mr S6 peptide kinase with trypsin, six peptides were sequenced. There was no similarity with the sequences of p70(S6K) and p90(rsk). Moreover, the obtained sequences could not be identified in the SwissProt or EMBL-genebank databases, suggesting that 40000-Mr S6 peptide kinase probably represents a novel protein kinase.

The heat-shock transcription factor HSF1 is rapidly activated by either hyper- or hypo-osmotic stress in mammalian cells

Osmoregulation, the cellular response to environmental changes of osmolarity and ionic strength, is important for the survival of living organisms. We have demonstrated previously that an exposure of mammalian cells to hypo-osmotic stress, either in growth medium (30% growth medium and 70% water) or in binary solution containing sorbitol and water, prominently induced the DNA-binding activity of the heat-shock transcription factor (HSF1) [Huang, Caruccio, Liu and Chen (1995) Biochem. J. 307, 347-352]. Since hyperosmotic and hypo-osmotic stress usually elicit opposite biological responses, we wondered what would be the effect of hyperosmotic stress on HSF activation. In this study we have examined the HSF DNA-binding activity in HeLa cells maintained in the sorbitol/water binary solution over a wide concentration range (0.1-0.9 M) and in Dulbecco's medium supplemented with sorbitol or NaCl. We found that HSF-binding activity could be induced prominently under both hypo-osmotic (0.1-0.25 M) and hyperosmotic conditions (0.50-0.90 M). In both cases, HSF activation was observed within 5 min after changing the osmotic pressure. The activation was accompanied by both HSF trimerization and nuclear translocation, and appeared to be independent of protein synthesis. The effects of hypo- or hyper-osmotic stress on HSF activation could be reversed once the cells were returned to iso-osmotic conditions (0.30M) with a half-life (t12) of 25 min or less. This rapid turnover of the osmotic-stress-induced HSF-binding activity was inhibited by cycloheximide, a potent inhibitor of protein synthesis. Unlike heat shock, activation of HSF by either hypo- or hyper-osmotic stress did not lead to an accumulation of heat-shock protein 70 (HSP70) mRNA in HeLa cells. We propose that HSF activation during osmotic stress may serve physiological functions independent of the synthesis of heat-shock proteins.

Expression of naked plasmid DNA injected into the afferent and efferent vessels of rodent and dog livers

A variety of reporter genes within plasmid constructs were injected into the afferent and efferent vessels of the liver in mice, rats, and dogs. Efficient plasmid expression was obtained following delivery via the portal vein, the hepatic vein, and the bile duct. The use of hyperosmotic injection solutions and occlusion of the blood outflow from the liver substantially increased the expression levels. Combining these surgical approaches with improved plasmid vectors enabled uncommonly high levels of foreign gene expression in which over 15 microg of luciferase protein/liver was produced in mice and over 50 microg in rats. Equally high levels of beta-galactosidase (beta-Gal) expression were obtained, in that over 5% of the hepatocytes had intense blue staining. Expression of luciferase or beta-Gal was evenly distributed in hepatocytes throughout the entire liver when either of the three routes were injected. Peri-acinar hepatocytes were preferentially transfected when the portal vein was injected in rats. These levels of foreign gene expression are among the highest levels obtained with nonviral vectors. Repetitive plasmid administration through the bile duct led to successive events of foreign gene expression. The integration of these findings into laboratory and clinical protocols is discussed.

Insulin-like growth factor 1 stimulates protein synthesis and inhibits protein breakdown in muscle from burned rats

BACKGROUND

Burn injury is associated with substantial whole-body protein loss, reflecting mainly a catabolic response in skeletal muscle. Recent studies suggest that treatment with insulin-like growth factor 1 (IGF-1) may reverse the catabolic response to burn injury, but the effects of IGF-1 on muscle protein synthesis and breakdown rates after burn injury are not known. We tested the hypothesis that IGF-1 blunts the catabolic response in skeletal muscle after burn injury by stimulating protein synthesis and inhibiting protein breakdown and that this effect of IGF-1 is caused by a direct effect on muscle tissue.

METHODS

Intact extensor digitorum longus muscles from burned, sham-burned, and untreated rats were incubated in the absence or presence of different concentrations of IGF-1. Total and myofibrillar protein breakdown rates were measured as net release of tyrosine and 3-methylhistidine, respectively. Protein synthesis rates were determined by measuring the incorporation of (U-14C)-phenylalanine into protein.

RESULTS

IGF-1 stimulated protein synthesis and inhibited protein breakdown in a dose-dependent fashion in muscles from burned and unburned rats. The maximal effect of IGF-1 on protein synthesis was seen at a hormone concentration of 100 ng/mL, whereas protein breakdown was further inhibited when the hormone concentration was increased to 1 microgram/mL. Ubiquitin messenger RNA (mRNA) levels were reduced by IGF-1 in incubated muscles, suggesting that IGF-1 may inhibit ubiquitin-dependent protein breakdown.

CONCLUSIONS

These results suggest that the anabolic effects of IGF-1 after burn may reflect inhibited protein breakdown and stimulated protein synthesis in skeletal muscle and that this response may be caused by a direct effect of IGF-1 on muscle tissue.

N-methyl-D-aspartate evokes rapid net depolymerization of filamentous actin in cultured rat cerebellar granule cells

Filamentous actin (F-actin) was measured in cultured rat cerebellum granule neurons with the use of fluorescently labeled phallotoxin as a site-specific probe for F-actin, and fluorescence microscopy. The averaged apparent intensity of soma-associated F-actin-derived fluorescence (F(app)) was measured from fixed cells after incubation in either 1) normal Krebs solution containing 2 mM extracellular calcium ([Ca2+]ex) or 2) normal Krebs solution plus N-methyl-D-aspartate (NMDA) for 2 min immediately before fixation. NMDA (10, 50, and 100 microM) decreased F(app) to 63 +/- 5% (mean +/- SE), 53 +/- 4%, and 47 +/- 2%, respectively, of that measured from control cells. This effect was mimicked by treatment of cells with ionomycin. The ability of NMDA to reduce the F(app) in the presence of [Ca2+]ex was abolished when cells were maintained in [Ca2+]ex-free medium. Cells first treated with NMDA for 2 min and then left in normal medium for 30 min before fixation gave F(app) fluorescence similar to control values (91 +/- 12%). However, if the F-actin polymerization inhibitor cytochalasin D was added to cells immediately after NMDA was removed, the F(app) did not recover with time (36 +/- 3%). Cells treated for 30 min with cytochalasin D alone showed a small reduction in staining (approximately 20%). It is concluded that the actin polymerization state of rat cerebellar granule neurons is sensitive to changes in intracellular calcium, and that NMDA receptor activation evokes an initial rapid depolymerization of F-actin.

Swelling-activated efflux of taurine and other organic osmolytes in endothelial cells

We used a combined biochemical, pharmacological, and electrophysiological approach to study the effects of hyposmotic swelling on organic osmolyte efflux in endothelial cells (EC). In [3H]taurine-loaded monolayers of calf pulmonary artery EC (CPAEC), hyposmolality activated time- and dose-dependent effluxes of [3H]taurine. Swelling-activated [3H]taurine efflux (Jtau swell)in CPAEC was inhibited by the anion channel blockers tamoxifen, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB), 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS), fenamates, and also quinine (in a pH-dependent manner), ATP, and the phospholipase A2 inhibitor 4-bromophenacyl bromide. In contrast, Jtau swell was partly or totally insensitive to bumetanide, forskolin, phorbol 12-myristate 13-acetate, and staurosporine. Swelling also activated myo-[3H]inositol efflux that was blocked by tamoxifen, NPPB, DIDS, and niflumic acid. Moreover, the cellular content of taurine and other amino acids was significantly reduced in osmotically activated CPAEC. Finally, in whole cell patch-clamp experiments, taurine, glycine, aspartate, and glutamate exhibited significant permeability for swelling-activated anion channels. In conclusion, hyposmotic swelling activates efflux of taurine and other organic osmolytes in EC. In addition, our results suggest that anion channels may provide a pathway for swelling-activated efflux of organic osmolytes in EC.

Shrinkage-induced protein tyrosine phosphorylation in Chinese hamster ovary cells

To investigate the signal transduction of osmotic stress, we examined hypertonicity-induced tyrosine phosphorylations in Chinese hamster ovary cells. Hyperosmosis elicited characteristic phosphotyrosine accumulation in at least 3 proteins (approximately 42, approximately 85, and approximately 120 kDa). The most prominent response occurred in the 85-kDa band (p85) whose phosphorylation was rapid, sustained, apparent already at mild hypertonicity (350 mosM), proportional to the extracellular osmotic concentration, and reversible. Hyperosmotic environment could not induce tyrosine phosphorylation if cell shrinkage was prevented by nystatin and appropriately composed media. Conversely, isotonic shrinkage caused strong tyrosine phosphorylation. Thus, the initial signal is a decrease in cell volume and not an increase in the intra- or extracellular osmotic concentration, or a rise in cytosolic K+ and Cl- levels. Tyrosine phosphorylation of p85 was not due to the hypertonicity-induced protein kinase C-dependent stimulation of the extracellular signal-regulated protein kinase, nor to the activation of stress-activated protein kinases. Tonicity-responsive proteins interacted with Grb2-glutathione S-transferase fusion proteins: the 120-kDa protein complexed with the SH2 and both SH3 domains, whereas p85 associated with the SH2 and the N-terminal SH3 domains of the adapter. Tyrosine phosphorylation of p85 is a sensitive indicator of reduced intracellular hydration and might signify a hitherto unrecognized, early volume-dependent signaling event.

Taurine is an osmolyte in rat liver macrophages (Kupffer cells)

BACKGROUND/AIMS

The availability of betaine as an osmolyte was recently shown to interfere strongly with important cell functions of liver macrophages (Kupffer cells), such as eicosanoid and tumor necrosis factor-alpha production or phagocytosis. We therefore investigated whether taurine is also used as an osmolyte by Kupffer cells and whether it is involved in the control of Kupffer cell functions.

METHODS/RESULTS

Hyperosmotic (hypoosmotic) exposure of cultured rat liver macrophages (Kupffer cells) for 6-12 h led to an increase (decrease) in the mRNA levels of the taurine transporter (TAUT) and an increase (decrease) in taurine transport into the cells. The hyperosmolarity-induced increase in TAUT-mRNA levels was diminished by 37+/-10% upon addition of taurine, but not upon addition of betaine. When Kupffer cells were preloaded with taurine, hypoosmotic exposure led to a rapid efflux of taurine from the cells, which was significantly delayed in the presence of the anion exchanger inhibitor 4,4'-diisothiocyanostilbene-2,2'-disulfonic acid (DIDS). Taurine efflux was also stimulated during phagocytosis of Latex particles; however, Latex was without effect on the hyperosmolarity-induced increase of TAUT mRNA levels. Lipopolysaccharide (LPS) led to an induction of cyclooxygenase-2, which was markedly enhanced during hyperosmotic conditions. Taurine diminished the induction of cyclooxygenase-2 and inhibited the LPS/hyperosmolarity-induced stimulation of prostaglandin E2 formation.

CONCLUSIONS

The data suggest that, in addition to betaine, taurine also acts as an osmolyte in Kupffer cells, and that taurine availability may be an important modulater of Kupffer cell functions such as eicosanoid synthesis.

Autophagic proteolysis: control and specificity

The rate of proteolysis is an important determinant of the intracellular protein content. Part of the degradation of intracellular proteins occurs in the lysosomes and is mediated by macroautophagy. In liver, macroautophagy is very active and almost completely accounts for starvation-induced proteolysis. Factors inhibiting this process include amino acids, cell swelling and insulin. In the mechanisms controlling macroautophagy, protein phosphorylation plays an important role. Activation of a signal transduction pathway, ultimately leading to phosphorylation of ribosomal protein S6, accompanies inhibition of macroautophagy. Components of this pathway may include a heterotrimeric Gi3-protein, phosphatidylinositol 3-kinase and p70S6 kinase. Recent evidence indicates that lysosomal protein degradation can be selective and occurs via ubiquitin-dependent and -independent pathways.

Heat changes in lipid membranes under sudden osmotic stress

Closed lipid vesicles act as osmometers increasing or decreasing their volume under the influence of osmotic gradients. The enthalpy changes accompanying membrane compression or expansion have not been measured yet, and first results obtained with high-sensitivity titration calorimetry are reported here. Phospholipid vesicles suspended in and in equilibrium with an electrolyte or nonelectrolyte with a defined initial concentration of c(i), were injected into a solution with a final concentration of c(f), and the heat changes were monitored with a titration microcalorimeter. Osmotic compression (delta c = c(f) - c(i) > 0) produced an exothermic heat change with deltaH approximately -500 +/- 100 cal/mol and osmotic expansion (delta c < 0) an endothermic heat change with deltaH approximately 1000 +/- 200 cal/mol; both results normalized to a concentration gradient of delta c = 1 M NaCl. The heats of compression and expansion varied linearly with the lipid content and the size of the osmotic gradient but were independent of the vesicle size. The cubic thermal expansion coefficient alpha(v) which equals (1/V)(deltaV/deltaT)p could be derived and was found to be 1.25 x 10(-3) and 2.5 x 10(-3) K(-1) for the compressed and expanded bilayer vesicles, respectively. The entropy changes associated with compression and expansion could be estimated. Compression of the membrane led to a negative entropy change and increased the hydrocarbon chain order. Expansion of the membrane was accompanied by a positive entropy change which can be explained, in part, by more disordered hydrocarbon chains. Vesicle expansion and compression thus appear to be asymmetric as far as the thermodynamic driving force is concerned.

Hypotonicity and thrombin activate taurine efflux in BC3H1 and C2C12 myoblasts that is down regulated during differentiation

The efflux of organic osmolytes such as taurine is an important mechanism by which cells regulate their volume. The effects of hypotonicity and thrombin on taurine efflux were studied in BC3H1 and C2C12 cells, two mouse myoblastic cell lines that can be induced to differentiate with serum deprivation. In proliferating cultures of both cell types preloaded with [3H]taurine, exposure to 27% hypotonicity activated a 10- to 20-fold increase in [3H]taurine efflux (Jtau). This effect was blocked by the C1- channel inhibitors NPPB and flufenamic acid. Thrombin and the thrombin receptor agonist SFLLRN also activated Jtau that was abolished by NPPB and flufenamic acid. Together, hypotonicity and thrombin synergistically activated Jtau. In differentiated myocytes, the effect of thrombin was abolished, while that of hypotonicity was significantly reduced. These results suggest that (i) hypotonicity and thrombin activate taurine-permeable anion channels in BC3H1 and C2C12 cells, and (ii) these anion channels may be involved in cell proliferation.

Fcgamma receptor I activation triggers a novel Ca2+-activated current selective for monovalent cations in the human monocytic cell line, U937

Previous reports have suggested that receptors for immunoglobulin G (IgG), FcgammaRs, directly activate a nonselective cation channel (Young, J. D.-E., Unkeless, J. C., Young, T. M., Mauro, A., and Cohn, Z. A. (1983) Nature 306, 186-189; Nelson, D. J., Jacobs, E. R., Tang, J. M., Zeller, J. M., and Bone, R. C. (1985) J. Clin. Invest. 76, 500-507). To investigate the mechanisms underlying membrane conductance changes following human high affinity (FcgammaRI) receptor activation, we have used the human monocytic cell line U937 and combined conventional whole cell patch-clamp recordings with single cell fura-2 Ca2+ measurements. Using a K+-free internal solution, antibody cross-linking of IgG-occupied FcgammaRI activated an inward current at negative potentials, whose amplitude and time course mirrored the concomitant rise in intracellular Ca2+. Current-voltage relationships, obtained under different ionic conditions, revealed a monovalent cation-selective conductance that, under physiological conditions, would result in Na+ influx. Noise analysis of current recordings indicated a single channel conductance of 18 picosiemens and a mean opening time of 4.5 ms. This current was also activated by rises in intracellular Ca2+ induced by ionomycin (3 microM) or thapsigargin (1 microM). Addition of the Ca2+ chelator 1,2-bis(2-aminophenoxy)ethane-N,N,N', N'-tetraacetic acid to the intracellular medium abolished any channel activation by ionomycin, FcgammaRI, or the low affinity receptor, FcgammaRII. These results demonstrate that FcgammaRI activation triggers a novel Ca2+-activated channel selective for monovalent cations and that neither FcgammaRI nor FcgammaRII can directly activate a channel.

Signal transduction pathways in macroautophagy

Macroautophagy is a major cellular catabolic pathway involved in the regulation of cell homeostasis. It is initiated by the sequestration of intracellular material by a wrapping membrane and terminates with the fusion of autophagic vacuoles with the lysosomal compartment. Macroautophagy has been extensively studied at the morphological level and in terms of environmental responses (nutrient deprivation, hormones). Recently a burst of data has emerged concerning the intracellular molecular events involved in the control of macroautophagic sequestration. It is becoming clear that the initial sequestration step of macroautophagy is under the control of different signalling pathways.

Mammary protein synthesis is acutely regulated by the cellular hydration state

The effect of cell-volume pertubations on mammary tissue protein synthesis has been examined. Cell-swelling, induced by a hyposmotic shock, increased the rate of incorporation of radiolabelled leucine and methionine into trichloroacetic acid precipitable material. The incorporation of radiolabel under both isosmotic and hyposmotic conditions was inhibited by cycloheximide. The increases in mammary protein synthesis as a result of cell-swelling may be attributable to an increase in casein synthesis. Conversely, cell-shrinking, as a consequence of a hyperosmotic challenge, almost abolished mammary protein (casein) synthesis. The finding that cell-volume pertubations had no significant effect on steady-state casein mRNA levels suggests that the regulation, within the time course of the experiments, is at the level of translation. The results strongly suggest that mammary cell volume may be an important cellular signal in the control of mammary protein synthesis in general and casein synthesis in particular.

Volume-sensitive taurine efflux from mammary tissue is not obliged to utilize volume-activated anion channels

Cell-swelling, induced by a hyposmotic shock, activates the release of taurine from lactating rat mammary tissue explants. The degree of stimulation of taurine efflux was dependent upon the extent of cell-swelling. Volume-sensitive taurine release was attenuated by the anion transport inhibitors NPPB, DIOA, DIDS, niflumate, flufenamate, mefenamate and diiodosalicylate but not by salicylate. Cell-swelling, following a hyposmotic challenge, did not increase the unidirectional efflux of radiolabelled I- or D-asparate from mammary tissue explants. The results suggest that although mammary tissue expresses a volume-sensitive amino acid transport system which is inhibited by anion transport blockers the pathway has no identity with volume-activated anion channels.

Modulation of glycogen synthesis in rat skeletal muscle by changes in cell volume

1. The hypothesis that cellular hydration state modulates muscle glycogen synthesis was tested by measuring the incorporation of [14C]glucose into glycogen (glycogen synthesis) in primary rat myotubes after experimentally induced volume changes. 2. Glycogen synthesis in myotubes increased (by 75%, P < 0.01) after swelling induced by 60 min exposure to hyposmotic media (170 mosmol kg-1) relative to isosmotic control (300 mosmol kg-1) values, it decreased (by 31%, P < 0.05) after shrinkage induced by 60 min exposure to hyperosmotic (430 mosmol kg-1) media. Myotube 2-deoxy-D-glucose (0.05 mM) uptake was unaffected by changes in external osmolality. 3. Wortmannin (100 nM; 60 min), a phosphatidylinositol 3-kinase inhibitor, decreased basal glycogen synthesis by 28% whereas rapamycin (100 nM; 60 min), which blocks the activation of p70 S6 kinase, had no effect. Both wortmannin (100 nM; 60 min) and rapamycin (100 nM; 60 min) blocked the changes in glycogen synthesis resulting from hypo- and hyperosmotic exposure. 4. Myotube glycogen synthesis is modulated by volume changes independently of changes in glucose uptake. The phenomenon may be physiologically important in promoting glycogen storage during circumstances of myofibrillar swelling, e.g. after feeding or exercise.

Cell volume and the metabolic actions of insulin

Insulin increases the volume of isolated hepatocytes and cells in perfused livers, but effects of the hormone on the volume of fat or muscle cells have not been demonstrated. Exogenous amino acids may stimulate swelling of liver cells and induce insulin-like effects on hepatic protein metabolism; however, swelling of liver cells can be induced by some treatment that do not induce insulin-like metabolic responses. Exogenous amino acids also influence protein metabolism of fat and muscle cells, but no relationship with cell volume has been established and no corresponding effects on metabolism of carbohydrates or lipids have been observed. Three families of mitogen-activated protein kinases are activated after changes in extracellular osmolarity but they appear to play little or no role in the metabolic actions of insulin. Direct evidence against a metabolic role for the extracellular signal-regulated kinases ERK-1 and ERK-2 is discussed. The c-Jun N-terminal kinases (also called stress-activated protein kinases) and the mammalian homologs of the yeast Hog protein kinase are strongly activated by environmental stresses associated with catabolic metabolism. We conclude that cell volume and protein metabolism may be correlated in liver but there is no compelling evidence that the effects of insulin on metabolism of liver, fat, or muscle cells can be accounted for by changes in cell volume. The effects of insulin on cell volume may represent a discrete aspect of the complete physiological response rather than an obligatory intermediate step in metabolic signalling.

[Regulation of cell function by level of hydration]

The hydration state of mammalian cells is dynamic and changes within minutes under the influence of nutrients, hormones and oxidative stress. Such changes in cell hydration act as an independent signal which modulates cellular metabolism and gene expression by activating intracellular signalling systems. Although the structures which sense hydration changes are unknown, this creates a novel and elegant mechanism for adaptation of cell function and gene expression to environmental challenges and provides new aspects for clinical medicine.