Cell volume in the regulation of hepatic function: a mechanism for metabolic control

Analysis of enzyme reactions in situ

Estimations of metabolic rates in cells and tissues and their regulation on the basis of kinetic properties of enzymes in diluted solutions may not be applicable to intact living cells or tissues. Enzymes often behave differently in living cells because of the high cellular protein content that can lead to homologous and heterologous associations of protein molecules. These associations often change the kinetics of enzymes as part of post-translational regulation mechanisms. An overview is given of these interactions between enzyme molecules or between enzyme molecules and structural elements in the cell, such as the cytoskeleton. Biochemical and histochemical methods are discussed that have been developed for in vivo and in situ analyses of enzyme reactions, particularly for the study of effects of molecular interactions. Quantitative (histochemical) analysis of local enzyme reactions or fluxes of metabolites has become increasingly important. At present, it is possible to calculate local concentrations of substrates in cells or tissue compartments and to express local kinetic parameters in units that are directly comparable with those obtained by biochemical assays of enzymes in suspensions. In situ analysis of the activities of a number of enzymes have revealed variations in their kinetic properties (Km and Vmax) in different tissue compartments. This stresses the importance of in vivo or in situ analyses of cellular metabolism. Finally, histochemical determinations of enzyme activity in parallel with immunohistochemistry for the detection of the total number of enzyme molecules and in situ hybridization of its messenger RNA allow the analysis of regulation mechanisms at all levels between transcription of the gene and post-translational activity modulation.

Changes in amino acid levels in rat plasma, cisternal cerebrospinal fluid, and brain tissue induced by intravenously infused arginine-vasopressin

Circulating arginine-vasopressin (AVP) is known to reduce the blood-to-brain transfer of large neutral amino acids (AA). As a first step to examine whether the reduced uptake by brain endothelial cells is reflected in changes in large neutral amino acid levels of the extracellular fluid environment of cells within the nervous tissue, we measured the concentrations of amino acids in plasma, cerebrospinal fluid (CSF), and hippocampal tissue of rats before and after infusion of AVP (34 and 68 ng/min/kg, respectively) over the time period of 60 min. AA levels changed in all compartments investigated during both saline and AVP infusions. Whereas in the saline-infused controls changes in CSF AA levels paralleled those in plasma, this correlation was abolished by raising AVP concentrations. The effect of AVP was found to be i) dependent on the AA, ii) different with respect to direction and iii) magnitude of changes in AA levels, and iv) in some cases dose dependent. In summary, AVP infusion increased plasma levels of 10 AA, but decreased all 15 AA measured by some 30% in CSF. In contrast to CSF, levels of AA were slightly enhanced in the hippocampal tissue. The results are not solely explicable by a reduced blood-to-brain transfer of AA. We conclude that further mechanisms by which AVP affects the availability of AA to the brain may exist. The physiological significance of the findings might be related to brain osmoregulation, especially in situations of stress.

[Role of new nitrogen substrates during peri-operative artificial nutrition in adults]

Some amino acids and their derivatives, such as arginine (ARG), glutamine (GLN) in free form or as dipeptides, and ornithine-ketoglutarate (OKG), have specific pharmacological properties concerning namely immunomodulation, control of protein turn-over, maintenance of gut trophicity. In the context of the postoperative nutrition in the adult, supplementation of enteral nutrition with ARG and of parental nutrition with GLN and OKG has improved nutritional and biochemical markers such as nitrogen balance, muscle protein synthesis and glutamine content. However, only few studies have tried to demonstrate a clinical benefit with such a supplementation. At present only the beneficial effect of OKG on postoperative wound healing has been recognized.

Membrane mechanisms and intracellular signalling in cell volume regulation

Recent work on selected aspects of the cellular and molecular physiology of cell volume regulation is reviewed. First, the physiological significance of the regulation of cell volume is discussed. Membrane transporters involved in cell volume regulation are reviewed, including volume-sensitive K+ and Cl- channels, K+, Cl- and Na+, K+, 2Cl- cotransporters, and the Na+, H+, Cl-, HCO3-, and K+, H+ exchangers. The role of amino acids, particularly taurine, as cellular osmolytes is discussed. Possible mechanisms by which cells sense their volumes, along with the sensors of these signals, are discussed. The signals are mechanical changes in the membrane and changes in macromolecular crowding. Sensors of these signals include stretch-activated channels, the cytoskeleton, and specific membrane or cytoplasmic enzymes. Mechanisms for transduction of the signal from sensors to transporters are reviewed. These include the Ca(2+)-calmodulin system, phospholipases, polyphosphoinositide metabolism, eicosanoid metabolism, and protein kinases and phosphatases. A detailed model is presented for the swelling-initiated signal transduction pathway in Ehrlich ascites tumor cells. Finally, the coordinated control of volume-regulatory transport processes and changes in the expression of organic osmolyte transporters with long-term adaptation to osmotic stress are reviewed briefly.

Subpopulations of rat hepatocytes separated by Percoll density-gradient centrifugation show characteristics consistent with different acinar locations

Freshly isolated viable rat hepatocytes were separated into five subpopulations on shallow discontinuous Percoll density gradients. The periportal marker enzymes alanine aminotransferase (ALT), malate dehydrogenase (MDH) and lactate dehydrogenase (LDH) showed gradients of increasing activity from the subpopulation of least density (band 1, rho = 1.07 g/ml) to the subpopulation of greatest density (band 5, rho = 1.09 g/ml). The perivenous marker enzymes pyruvate kinase (PK) and glutamate dehydrogenase (GDH) showed gradients of decreasing activity from band-1 cells to band-5 cells. Glutamine synthetase (GS), which is confined to the two or three cell layers around the hepatic venule, was almost entirely restricted to band-1 hepatocytes. Band-5: band-1 ratios of enzyme activity were as follows: ALT, 8.0; LDH, 2.1; MDH, 1.6; GDH, 0.7; PK, 0.2; GS, 0.01. Band-5:band-1 ratios for ALT, LDH, PK and GS were maintained after culture of subpopulations in identical conditions for up to 72 h, whereas the ratios for MDH and GDH decreased and increased respectively towards unity. Band-1 hepatocytes exhibited greater cytotoxicity than band-5 cells after incubation with carbon tetrachloride or paracetamol. These perivenous-selective toxins produced greater decreases in cell viability and greater release of ALT and LDH from band-1 hepatocytes than from band-5 hepatocytes. Conversely, band-5 hepatocytes were more susceptible than band-1 hepatocytes to the cytotoxic effects of 1-naphthylisothiocyanate and methotrexate (known periportal-selective toxins). It is concluded that band-5 hepatocytes are enriched in periportal cells, whereas band-1 hepatocytes are enriched in perivenous cells. Isolation of hepatocyte subpopulations by Percoll density-gradient centrifugation has the considerable advantage that periportal and perivenous cells can be obtained from the same liver.

Modulation of phosphoenolpyruvate carboxykinase mRNA levels by the hepatocellular hydration state

Exposure of isolated perfused rat livers to hypo-osmotic (225 mosmol/l) perfusion media for 3 h led to a decrease of about 60% in mRNA levels for phosphoenolpyruvate carboxy-kinase (PEPCK) compared with normo-osmotic (305 mosmol/l) perfusions. Conversely, PEPCK mRNA levels increased about 3-fold during hyperosmotic (385 mosmol/l) perfusions. The anisotonicity effects were not explained by changes in the intracellular cyclic AMP (cAMP) concentration or by changes of the extracellular Na+ or Cl- activity. Similar effects of aniso-osmolarity on PEPCK mRNA levels were found in cultured rat hepatoma H4IIE.C3 cells, the experimental system used for further characterization of the effect. Whereas during the first hour of anisotonic exposure no effects on PEPCK mRNA levels were detectable, near-maximal aniso-osmolarity effects were observed within the next 2-3 h. PEPCK mRNA levels increased sigmoidally with the osmolarity of the medium, and the anisotonicity effects were most pronounced upon modulation of osmolarity between 250 and 350 mosmol/l. The aniso-osmolarity effects on PEPCK mRNA were not affected in presence of Gö 6850, protein kinase C inhibitor. cAMP increased the PEPCK mRNA levels about 2.3-fold in normo-osmotic media, whereas insulin lowered the PEPCK mRNA levels to about 8%. The effects of cAMP and insulin were also observed during hypo-osmotic and hyperosmotic exposure, respectively, but the anisotonicity effects were not abolished in presence of the hormones. The data suggest that hepatocellular hydration affects hepatic carbohydrate metabolism also over a longer term by modulating PEPCK mRNA levels. This is apparently unrelated to protein kinase C or alterations of cAMP levels. The data strengthen the view that cellular hydration is an important determinant for cell metabolic function by extending its regulatory role in carbohydrate metabolism to the level of mRNA.

Ca(2+)-mobilizing hormones potentiate hypotonicity-induced activation of ionic conductances in Intestine 407 cells

Human Intestine 407 cells respond to hyposmotic stimulation by activating the conductive efflux of both Cl- and K+ (regulatory volume decrease) through pathways involving protein tyrosine phosphorylation (Tilly, B. C., N. van den Berghe, L. G. J. Tertoolen, M. J. Edixhoven, and H. R. de Jonge. J. Biol. Chem. 268: 19919-19922, 1993). Stimulation of the cells with hormones linked to the phospholipase C signaling cascade (e.g., bradykinin, histamine, or thrombin) or with the phosphotyrosine phosphatase inhibitor vanadate, potentiated the osmosensitive anion efflux by two- to threefold but did not affect anion efflux under isotonic conditions. No substantial increase in intracellular Ca2+ concentration ([Ca2+]i) was observed on mild hypotonicity-induced cell swelling. In addition, loading the cells with the intracellular Ca2+ chelator 1,2-bis(2-amino-phenoxy)ethane- N,N,N',N',-tetraacetic acid acetoxymethyl ester (BAPTA-AM) caused a partial reduction of the osmoshock-induced 125I- efflux but did not affect its potentiation by vanadate. In contrast, bradykinin transiently elevated [Ca2+]i, and its potentiation of the osmosensitive anion efflux was completely inhibited after BAPTA-AM loading. Both the Ca(2+)-mobilizing hormones as well as osmotic cell swelling rapidly triggered the phosphorylation of several proteins on tyrosine residues. However, the effects of the hormones, but not the effect of hypotonicity, on protein tyrosine phosphorylation was largely abolished in BAPTA-loaded cells. Taken together the results indicate a novel role for Ca(2+)-mobilizing hormones, although elevation of [Ca2+]i, in potentiating volume-sensitive ionic efflux even in cell types lacking the expression of Ca(2+)-activated Cl- channels in their plasma membrane.

Increased intracellular potassium and water contents in rat heart after alpha-1-adrenoceptor stimulation

Potassium accumulation in rat heart after alpha-1-adrenoceptor stimulation has previously been reported from indirect measurements. Here we present data on intracellular potassium content measured directly in the heart. Isolated rat hearts perfused in a non-recirculating system were exposed to alpha-1-adrenoceptor stimulation (5 x 10(-5) mol/l phenylephrine in the presence of 10(-6) mol/l timolol). 14C-Sucrose was used to estimate the extracellular space. From heart homogenates intracellular potassium, magnesium and cellular water contents were determined and the ion concentrations calculated accordingly. The intracellular magnesium content remained unchanged during all experimental conditions. alpha-1-Adrenoceptor stimulation evoked an increase in potassium content by 9% (4, 14; 95% confidence interval (CI), P = 0.0006). Due to an observed increase in intracellular water by 17% (9, 26; 95% CI, P = 0.0006), the potassium concentration apparently decreased by 8% (0.3, 15; 95% CI, P = 0.04). During partial inhibition of the Na+/K(+)-ATPase by 10(-5) mol/l ouabain, there was an increase in potassium content by 5% (1, 9; 95% CI, P = 0.008). There was, however, no significant increase in intracellular water in this situation. Calculated intracellular potassium concentration showed accordingly a slight increase. The effects upon potassium and water both in the absence and presence of ouabain were eliminated by the alpha-1-adrenoceptor blocker prazosin (10(-6) mol/l). alpha-1-Adrenoceptor stimulation apparently increased cellular dry weight by 10% (2, 18; 95% CI, P = 0.02). Changes in translocation of potassium and water must be considered as part of the alpha-1-adrenergic heart effects.

Effects of aniso-osmolarity and hydroperoxides on intracellular pH in isolated rat hepatocytes as assessed by (2',7')-bis(carboxyethyl)-5(6)-carboxyfluorescein and fluorescein isothiocyanate-dextran fluorescence

Freshly isolated rat hepatocytes were plated for 4-6 h and either loaded with (2',7)-bis(carboxyethyl)-5(6)-carboxyfluorescein (BCECF) or allowed to endocytose fluorescein isothiocyanate (FITC)-coupled dextran in order to study the effects of aniso-osmotic exposure and oxidative stress on cytosolic (pHcyt) and apparent vesicular pH (pHves) by single-cell fluorescence recordings. In the presence of normo-osmotic (305 mosmol/l) medium pHcyt was 7.23 +/- 0.03 (n = 108), whereas an apparent pH of 6.07 +/- 0.02 (n = 156) was found in the vesicular compartment accessible to endocytosed FITC-dextran. Substitution of 60 mM NaCl against 120 mM raffinose had no effect on pHcyt or apparent pHves, whereas addition of NH4Cl increased both pHcyt and apparent pHves. Hypo-osmotic cell swelling lowered pHcyt, whereas simultaneously apparent pHves increased. These effects were rapidly reversible upon re-institution of normo-osmotic media. Similarly, an increase of apparent pHves was observed when cell swelling was induced by Ba2+, glutamine or histidine. Conversely, hyperosmotic cell shrinkage due to addition of NaCl or raffinose led to a cytosolic alkalinization and a vesicular acidification. Both, H2O2 (0.2 mmol/l) and t-butyl-hydroperoxide (0.2 mmol/l) were without effect on pHcyt, but lowered apparent pHves by about 0.2 pH units. Ba2+ (1 mmol/l) diminished the acidifying effect of the hydroperoxides by about 50%. Pretreatment of the cells with colchicine, but not with lumicolchicine, largely abolished the effects of aniso-osmolarity and hydroperoxides on pHves. The data suggest that hepatocellular hydration affects the proton gradients built up across the membranes of endocytotic FITC-dextran-accessible compartments in a microtubule-dependent way. They further suggest that hydroperoxides induce vesicular acidification in a colchicine- and Ba(2+)-sensitive way. Because hydroperoxides induce Ba(2+)-sensitive cell shrinkage [Hallbrucker, Ritter, Lang, Gerok and Häussinger (1992) Eur. J. Biochem. 211, 449-458], the results are compatible with the view that hydroperoxide-induced cell shrinkage mediates vesicular acidification. It is concluded that modulation of vesicular pH by the hepatocellular hydration state may play a role in triggering some metabolic changes in response to cell swelling/shrinkage.

Effects of urea on K+ fluxes and cell volume in perfused rat liver

Exposure of the perfused rat liver to a perfusate made hyperosmotic by the presence of 200 mmol l-1 glucose led, as expected, to marked, transient hepatocellular shrinkage followed by volume-regulatory net K+ uptake. However, even after this volume-regulatory K+ uptake had ceased, the liver cells are still slightly shrunken. Withdrawal of glucose from the perfusate resulted in marked transient cell swelling, net K+ release from the liver and restoration of cell volume. However, when the Krebs-Henseleit perfusate was made hyperosmotic by the presence of urea (20-300 mM), there was no immediate decrease in liver mass, yet a slight and persistent cell shrinkage developing 2 min after the onset of exposure to urea. Surprisingly, urea induced concentration-dependent net K+ efflux from the liver and removal of urea net K+ reuptake from the inflowing perfusate. The urea (200 mM)-induced net K+ release resembled that observed following a lowering of the influent [NaCl]: making the perfusate hypoosmotic (245 mosmol l-1, by reducing influent [NaCl] by 30 mM) gave roughly the same K+ response as hyperosmotic exposure (505 mosmol/l) resulting from the presence of 200 mM urea. The urea-induced K+ efflux was not inhibited in the presence of ouabain (1 mM), or in Ca(++)-free perfusion, but was modified in the presence of quinidine (1 mM) or Ba++ (1 mM). The direction in which the liver was perfused had no effect on the urea-induced net K+ release.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence for a regulatory protein involved in the increased activity of system A for neutral amino acid transport in osmotically stressed mammalian cells

System A for neutral amino acid transport is increased by hypertonic shock in NBL-1 cells previously induced to express system A activity by amino acid starvation. The hypertonicity-mediated effect can be blocked by cycloheximide but is insensitive to tunicamycin. The activity induced may be inactivated irreversibly by the addition of system A substrates, by a rapid mechanism insensitive to cycloheximide. In CHO-K1 cells, hypertonicity increases system A activity, as has been shown in NBL-1 cells. This effect is additive to the activity produced by derepression of system A by amino acid starvation and is insensitive to tunicamycin. Furthermore, the alanine-resistant mutant CHO-K1 alar4, which bears a mutation affecting the regulatory gene R1, involved in the derepression of system A activity after amino acid starvation, is still able to respond to the hypertonic shock by increasing system A activity to a level similar to that described in hypertonicity-induced derepressed CHO-K1 (wild type) cells. These results suggest (i) that the hypertonicity-mediated increase of system A activity occurs through a mechanism other than that involved in system A derepression and (ii) that a regulatory protein coded by an osmotically sensitive gene is responsible for further activation of preexisting A carriers.

Involvement of microtubules in the link between cell volume and pH of acidic cellular compartments in rat and human hepatocytes

Cell swelling is shown to induce an increase in acridine orange fluorescence intensity, an effect pointing to the alkalinization of acidic vesicles. Since autophagic hepatic proteolysis is accomplished by pH-sensitive proteinases within acidic lysosomes, this effect may contribute to the well-known inhibitory effect of cell swelling on proteolysis. In the present study, the role of microtubules in volume-dependent alterations of pH in acidic vesicles of rat and human hepatocytes was studied. Colcemid and colchicine were used to depolymerize microtubules and vesicular pH was monitored using two different fluorescent dyes, fluorescein isothiocyanate conjugated-dextran and acridine orange. Colcemid and colchicine, but not the inactive stereoisomer gamma-lumicolchicine, blunted the increase of pH during osmotic cell swelling. The alkalinization of acidic vesicles by NH4Cl was not significantly modified by colcemid or colchicine, indicating that the vesicles were still sensitive to alkalinizing procedures other than cell swelling. Further, colchicine, but not gamma-lumicolchicine, inhibited the antiproteolytic action of osmotic cell swelling. The present observations point to an involvement of the microtubule network in the link of cell volume, lysosomal pH, and proteolysis.

Lipid dynamics and peripheral interactions of proteins with membrane surfaces

A large body of evidence strongly indicates biomembranes to be organized into compositionally and functionally specialized domains, supramolecular assemblies, existing on different time and length scales. For these domains and intimate coupling between their chemical composition, physical state, organization, and functions has been postulated. One important constituent of biomembranes are peripheral proteins whose activity can be controlled by non-covalent binding to lipids. Importantly, the physical chemistry of the lipid interface allows for a rapid and reversible control of peripheral interactions. In this review examples are provided on how membrane lipid (i) composition (i.e., specific lipid structures), (ii) organization, and (iii) physical state can each regulate peripheral binding of proteins to the lipid surface. In addition, a novel and efficient mechanism for the control of the lipid surface association of peripheral proteins by [Ca2+], lipid composition, and phase state is proposed. The phase state is, in turn, also dependent on factors such as temperature, lateral packing, presence of ions, metabolites and drugs. Confining reactions to interfaces allows for facile and cooperative large scale integration and control of metabolic pathways due to mechanisms which are not possible in bulk systems.

Glutamine metabolism and utilization: relevance to major problems in health care

Glutamine plays an important role in normal and pathophysiological states. In this review we describe the biochemical synthesis and degradation pathways of glutamine, as well as its utilization by the immune system and in rapidly dividing cells. Also discussed are glutamine behaviour in catabolic states and the therapeutic implications of this amino acid in total parenteral nutrition, digestive diseases and cancer.

Changes in the lipid dynamics of liposomal membranes induced by poly(ethylene glycol): free volume alterations revealed by inter- and intramolecular excimer-forming phospholipid analogs

Influence of osmotic shrinkage, swelling, and dehydration on large unilamellar liposomes (LUVs) of 1,2-dioleoylsn-glycero-3-phosphocholine (DOPC) was investigated using the fluorescent lipid probes 1-palmitoyl-2-[10-(pyren-1-yl)]-decanoyl-sn-glycero-3-phosphocholi ne (PPDPC) and 1,2-bis[10-(pyren-1-yl)]decanoyl-sn-glycero-3-phosphocholine (bisPDPC). Increasing concentrations of poly(ethylene glycol) (PEG, average molecular weight of 6000) producing osmotic gradients delta omega up to 250 mOsm/kg were first added to the outside of LUV labeled with 0.1 mol% of either of the above fluorescent phospholipids. The resulting osmotic shrinkage was accompanied by a progressive reduction in the lateral diffusion of the membrane-incorporated PPDPC, evident as a decrease in the rate of its intermolecular excimer formation. In contrast, under the same conditions the rate of intramolecular excimer formation by bisPDPC increased. Notably, signals opposite to those described above were observed for both of the fluorescent probes upon osmotic swelling of DOPC liposomes with encapsulated PEG. The lateral diffusion of PPDPC became progressively reduced upon membrane dehydration due to increasing concentrations of symmetrically distributed PEG (with equal polymer concentrations inside and outside of the liposomes) when neither shrinkage nor swelling occurs while enhanced excimer formation by bisPDPC was evident. The later results were interpreted in terms of osmotically induced changes in the hydration of lipids. In brief, the removal of water from the phospholipid hydration shell diminishes the effective size of the polar headgroup, which subsequently allows for an enhanced lateral packing of the phospholipid acyl chains. Our findings are readily compatible with membrane free volume Vf changes due to osmotic forces under three different kinds of stress (shrinkage, swelling, and dehydration) applied on the lipid bilayers.

Up-regulation of liver system A for neutral amino acid transport in euglycemic hyperinsulinemic rats

To determine the role of insulin on the in vivo modulation of liver system A activity, we used the euglycemic hyperinsulinemic clamp coupled to the measurement of solute uptakes into plasma membrane vesicles partially purified from livers of hyperinsulinemic rats and their saline-infused controls. The clamp was performed in chronically catheterized rats, either in the fasted state, 24 h after surgery (Group I), or after 3 days of recovery (Group II). System A activity, measured as the MeAIB-inhibitable L-alanine uptake, was selectively induced by hyperinsulinemia, although the effect was much greater in Group II than in Group I rats (137% vs. 24% over the basal values, respectively). This might be explained by the higher basal levels found in those liver plasma membrane vesicles from Group I fasted animals. Hyperinsulinemia also decreased blood amino acids but to a similar extent in both experimental groups. This suggests that amino acid depletion by itself may not cause up-regulation of system A. Other transport activities involved in neutral amino acid transport (Systems ASC, N and L) were not modified by the clamp. The induction of system A cannot be explained by changes in the dissipation rate of the Na+ transmembrane gradient, because the differences between insulin- and saline-infused rats remained even when the electrochemical Na+ gradient was disrupted in the presence of monensin. Thus, hyperinsulinemia might induce an increase in the number of transporters inserted into the plasma membrane.

Inhibition of carnitine palmitoyltransferase I by hepatocyte swelling

Incubation of hepatocytes under conditions known to increase their volume, i.e. with amino acids (glutamine, proline) or in hypo-osmotic medium, decreased carnitine palmitoyl-transferase I (CPT-I) activity. This effect of hepatocyte swelling was antagonized by okadaic acid and dibutyryl-cAMP. Physiological concentrations of glutamate inhibited CPT-I activity in digitonin-permeabilized hepatocytes but not in isolated mitochondria. Results suggest that the amino acid-induced inhibition of CPT-I shares a common mechanism with the amino acid-induced stimulation of acetyl-CoA carboxylase and glycogen synthase [(1993) Eur. J. Biochem. 217, 1083-1089].

Role of cell volume variations in Na(+)-K(+)-ATPase recruitment and/or activation in cortical collecting duct

The aim of this study was to examine whether cell volume variations could play a role in the previously reported Na(+)-K(+)-ATPase pump recruitment and/or activation induced by an increase in intracellular Na concentration (Nai) in cortical collecting ducts (CCD). Isolated CCD from kidneys of aldosterone-repleted mice were incubated in hyper-, hypo-, or isosmotic solutions with and without Na to modify Nai and cell volume independently. Nai, cell volume, and the number of basolateral pumps were measured using 22Na, image analysis, and specific [3H]ouabain binding, respectively. Ouabain-sensitive 86Rb uptake was also measured. In CCD with high Nai, pump recruitment and/or activation was observed only when an increase in tubular volume was associated with Na load. Pump recruitment and/or activation was also induced by cell swelling in the absence of Na load. Recruited and/or activated pumps display an affinity for ouabain and a specific activity (ouabain-sensitive Rb uptake per pump unit) similar to basal pumps. We conclude that 1) cell swelling is implied in the process of Nai-dependent pump recruitment and/or activation, 2) cell swelling can promote pump recruitment and/or activation independently of Na load, 3) basal and recruited and/or activated pumps probably correspond to the same Na(+)-K(+)-ATPase isoform.

Imidazol-1-ylalkanoic acids as extrinsic 1H NMR probes for the determination of intracellular pH, extracellular pH and cell volume

The synthesis and biological evaluation of a novel series of extrinsic probes for intracellular pH (pHi), extracellular pH (pHo) and cell volume determination by 1H NMR is described. Imidazol-1-ylacetate, malonate, 3-glutarate and 2-succinate esters were synthesized by reaction of imidazole with alpha-bromo esters or with alpha, beta-unsaturated esters. The corresponding acids were prepared by hydrolysis. Rat erythrocytes were readily permeable to methyl imidazol-1-ylacetate, moderately permeable to diethyl 2-imidazol-1-ylsuccinate and impermeable to diethyl 3-imidazol-1-yl-glutarate esters. Imidazol-1-ylacetic acid was the only acid derivative which penetrated the erythrocyte interior when added directly to the incubation medium. Transport of the permeable compounds to the erythrocyte interior was non-saturable up to 200 mM added compound. Addition of methyl imidazol-1-ylacetate or diethyl 2-imidazol-1-ylsuccinate esters to erythrocyte suspensions, resulted in hydrolysis to imidazol-1-ylacetic acid and 2-imidazol-1-ylsuccinic acid mono-ethyl ester in the intracellular and extracellular spaces, respectively. pHi and pHo were determined from the different chemical shifts of the H-2 proton of the acid derivatives in the intracellular (H-2i) and extracellular (H-2o) compartments. In addition, the relative intracellular and extracellular volumes could be calculated from the areas of the intracellular and extracellular H-2 resonances.

Differing patterns of carbohydrate metabolism in liver and muscle

The effect of a period of starvation followed by refeeding on skeletal muscle glycogen was investigated by the use of double-labelled radioactive glucose precursors in rats. Skeletal muscle glycogen, which is not depleted to anything like the extent of liver glycogen, shows a remarkable stability with respect to its overall molecular size distribution during starvation and subsequent refeeding. The experiments also indicate that there is a control mechanism in muscle tissue enabling the synthesis of lysosomal glycogen to be switched off during the initial part of the refeeding process. The results emphasise the inadequacy of the Cori cycle and a modified version is proposed.

Increase of c-jun mRNA upon hypo-osmotic cell swelling of rat hepatoma cells

c-jun mRNA levels were increased in rat hepatoma cells (H4-II-E-C3) when exposed to hypotonic medium (205 mosmol/l) with a maximal induction observed after 1 h of hypotonic exposure. At this time point an approximate 5-fold increase in c-jun expression could be detected in relation to normotonic control incubations (305 mosmol/l). Hypertonic exposure (405 mosmol/l) had only a slight effect on c-jun expression. In contrast to the increased c-jun mRNA levels under hypotonic conditions, expression of the c-fos proto-oncogene was unaffected by changes in the osmolarity. The hypotonicity-induced increase in c-jun expression was also detectable in the presence of a protein kinase C (PKC) inhibitor. This indicates that PKC is not involved in the signal transduction pathway leading to c-jun expression upon hypotonic cell swelling in these cells.

Endogenous hydroperoxide formation, cell volume and cellular K+ balance in perfused rat liver

Addition of benzylamine (0.5 mM) to isolated perfused rat liver led to a net release of K+ of 10.5 +/- 0.3 mumol/g, which was accompanied by a decrease in liver mass by 9.3 +/- 0.4% and a decrease of the intracellular water space by 13.7 +/- 0.6%, suggestive of hepatocellular shrinkage. Benzylamine had no effect on the perfusion pressure, and there was a close relationship between benzylamine-induced net K+ release and the accompanying decrease in liver mass. Benzylamine-induced net K+ release was sensitive to inhibition of monoamine oxidase by pargyline and increased with benzylamine flux through monoamine oxidase, suggesting its dependence on intracellular H2O2 formation. In line with this, infusion of H2O2 (but not of benzaldehyde, the other product of benzylamine metabolism) stimulated net K+ release from the liver. However, at a given H2O2 load K+ release was about 2-3-fold higher when H2O2 was generated intracellularly during the oxidation of benzylamine, as compared with exogenously delivered H2O2. Inhibition of catalase by 3-amino-1,2,4-triazole (0.2 mM) significantly increased the benzylamine-induced net K+ release as well as the benzylamine-induced release of GSSG into bile, but had no effect on benzylamine oxidation at monoamine oxidase. In the presence of Ba2+ (1 mM) or in Ca(2+)-free perfusions, the benzylamine-induced net K+ efflux was diminished by 60-70% or about 30%, respectively. This was not explained by the 20-30% decrease in flux through monoamine oxidase observed under these conditions. The results suggest that metabolic generation of H2O2 inside the liver leads to a net K+ efflux and subsequent hepatocellular shrinkage. Net K+ efflux under these conditions is enhanced when catalase is inhibited, suggesting that the rate of both intracellular H2O2 generation and degradation can modulate cellular K+ balance and cellular volume. The data support the idea that oxidative stress may affect hepatocellular functions also by lowering the hepatocellular hydration state.

Mechanism of activation of liver acetyl-CoA carboxylase by cell swelling

The activation of hepatic glycogen synthase by the amino-acid-induced cell swelling has been attributed to the stimulation of [glycogen-synthase]-phosphatase resulting from an increase in the intracellular content in glutamate and aspartate, and a decrease in intracellular Cl-, which is a compensatory response to cell swelling [Meijer, A. J., Baquet, A., Gustafson, L., van Woerkom, G. M. & Hue, L. (1992) J. Biol. Chem. 267, 5823-5828]. Here we studied whether the activation of acetyl-CoA carboxylase by cell swelling could be explained by the same mechanism. The activation of endogenous or purified acetyl-CoA carboxylase was measured in gel-filtered liver extracts or cytosols. No activation could be observed under basal conditions but a fivefold stimulation was obtained with concentrations of glutamate (20-25 mM) found in hepatocytes incubated with glutamine. A similar stimulation was also observed with other dicarboxylic acids such as malonate and succinate, or with metal ions like Mg2+, Ca2+ and Mn2+ (10 mM). The addition of 50-100 mM Cl- was found to inhibit the activation of acetyl-CoA carboxylase by some 20-30%. Mg2+ was also found to stimulate the activation of the endogenous glycogen synthase. The glutamate-stimulated and Mg(2+)-stimulated activation of glycogen synthase and acetyl-CoA carboxylase was unaffected by 10 microM inhibitor-2, a specific inhibitory protein of protein phosphatase-1, but could be nearly completely blocked by the phosphatase inhibitor microcystin-LR. Our data suggest that the amino-acid-induced activation of acetyl-CoA carboxylase and glycogen synthase in the liver occurs by a common ionic mechanism.

A quantitative analysis of the control of glutamine catabolism in rat liver cells. Use of selective inhibitors

1. At a physiological concentration of glutamine (0.5 mM), 87% of the total transport across the plasma membrane of liver cells isolated from fed rats involved the Na(+)-dependent system N; this was substantially inhibited by L-histidine. The residual Na(+)-independent component was attributed to system L on the basis of inhibition by 2-amino-2-norbornanecarboxylate and L-tryptophan. 2. Catabolism of glutamine by intact liver cells or by isolated mitochondria was inhibited by glutamate gamma-hydrazide with IC50 values of 13.7 +/- 3.5 microM and 22.6 +/- 3.8 microM respectively and a maximal inhibition of approx. 75%. The site of inhibition was identified as glutaminase; glutamate gamma-hydrazide inhibited this enzyme in cell-free extracts (IC50 37.8 +/- 7.7 microM) but had no activity against glutamate dehydrogenase or transport of glutamine, whether across mitochondrial or plasma membranes. 3. The major control site in cells from fed animals incubated with 0.5 mM L-glutamine was glutaminase (flux control coefficient 0.96). Appreciable control also resided in both plasma membrane transport systems, with coefficients of 0.51 for system N and -0.46 for system L, such that both interacted to provide a fine control of the intracellular concentration of the amino acid. Similar values were obtained by computer simulation based on theoretical determination of elasticities. 4. Previous controversy about the locus of regulation of hepatic glutamine metabolism is resolved by this distribution of control.

An automatic monitoring system for epithelial cell height

This paper describes an automatic method to measure cell height (h) of epithelia grown as monolayers on transparent filter supports. Tissues are mounted in an Ussing-type chamber enabling solution exchange on both sides. The apical and basal side of the epithelial cells are marked with fluorescent beads. The image of the fluospheres is captured with a video camera and processed by a computer-based video imaging system. One basal reference bead in a gelatin layer on the filter support and up to three beads attached at the apical surface are used to monitor changes in cell height of three cells simultaneously. The focusing of the microbeads is done automatically by moving the objective with a piezoelectric device mounted on the nosepiece of the microscope. The algorithm for locating the bead is based on the changes in fluorescent light intensity emitted by the fluospheres. The method has an accuracy higher than 0.1 micron and a time resolution as low as 6 s if measurements are restricted to one bead at the apical side. The method was tested on artificial model systems and used to measure volume changes in renal cultured epithelia (A6) after exposing the serosal surface to hypotonic solutions and replacing cell-impermeable sucrose by an organic compound (glycerol) with a smaller reflection coefficient. Serosal hypotonicity elicited a rapid volume increase followed by regulatory volume decrease, whereas the organic compound replacement caused a steady increase in cell volume.

Effect of cell volume on Acridine Orange fluorescence in hepatocytes

Hepatic proteolysis is inhibited by cell swelling following a variety of experimental manoeuvres, such as reduction of extracellular osmolarity, concentrative uptake of amino acids, or blockade of K+ channels by barium. On the other hand, proteolysis is known to be accomplished by pH-sensitive lysosomal proteases. Accordingly, NH3/NH4+ inhibits proteolysis by intralysosomal alkalinization. The present study has been performed to test for an effect of cell volume on the pH of acidic intracellular compartments, as assessed by Acridine Orange fluorescence at > 520 nm (F > 520). F > 520 is enhanced by NH3/NH4+ (2 and 20 mmol/l respectively), by glutamine (2 mmol/l), by the K(+)-channel blocker barium (10 mmol/l) and by reduction of extracellular osmolarity (by 20 and 80 mosmol/l respectively). The observations point to release of Acridine Orange from acidic cellular compartments, which is indicative of alkalinization of these compartments during cell swelling. This effect may contribute to the regulation of proteolysis.

Cell swelling and the sensitivity of autophagic proteolysis to inhibition by amino acids in isolated rat hepatocytes

In the isolated perfused rat liver, autophagic proteolysis is inhibited by hypo-osmotic perfusion media [Häussinger, D., Hallbrucker, C., vom Dahl, S., Lang, F. & Gerok, W. (1990) Biochem. J. 272, 239-242]. Here we report that in isolated hepatocytes, incubated in the absence of amino acids to ensure maximal proteolytic flux, proteolysis was not inhibited by hypo-osmolarity while the synthesis of glycogen from glucose, a process known to be very sensitive to changes in cell volume [Baquet, A., Hue, L., Meijer, A. J., van Woerkom, G. M. & Plomp, P. J. A. M. (1990) J. Biol. Chem. 265, 955-959], was stimulated under identical conditions. However, in isolated hepatocytes, hypo-osmolarity increased the sensitivity of autophagic proteolysis to inhibition by low concentrations of amino acids. The anti-proteolytic effect of hypo-osmolarity in our experiments was not due to stimulation of amino-acid transport into the hepatocytes: neither the consumption of most amino acids, nor the rate of urea synthesis was appreciably affected by hypo-osmotic incubation conditions. In the course of these studies we also found that hypo-osmolarity increased the affinity of protein synthesis for amino acids. In the presence of amino acids the intracellular level of ATP was not much affected. However, because of cell swelling under these conditions the intracellular concentration of ATP decreased. It is proposed that a small part of the inhibition of proteolysis by amino acids may be due to this fall in ATP concentration.

Hepatocyte heterogeneity in response to extracellular adenosine

Metabolic and haemodynamic effects of adenosine were studied in antegrade and retrograde rat liver perfusions with influent nucleoside concentrations either below (i.e. 20 microM) or exceeding (i.e. 200-300 microM) the single-pass clearance capacity of the liver. Adenosine (20 microM) increased in antegrade perfusions the perfusion pressure and markedly stimulated prostaglandin D2, thromboxane B2 and glucose output, whereas in retrograde perfusions no pressure and eicosanoid response occurred and glucose output was stimulated only slightly. The perfusion-direction-dependent differences in the glucose and pressure response to adenosine (20 microM) were fully abolished in presence of ibuprofen (50 microM). When the adenosine concentration in influent was raised to 200-300 microM, i.e. to a concentration exceeding single-pass clearance of the nucleoside, the adenosine-induced prostaglandin D2 release was about 10-fold higher in retrograde perfusions than in antegrade perfusions. On the other hand, both adenosine (20-300 microM)-induced cyclic AMP (cAMP) and K+ release from the liver were not affected by the direction of perfusion, and maximal effects on cAMP release were observed at influent adenosine concentrations of 100 microM. The basal rate (adenosine absent) of prostaglandin D2 and thromboxane B2 release was about 10-fold higher in retrograde than in antegrade perfusion experiments, whereas the basal cAMP release from the liver was not affected by the direction of perfusion. Maximal adenosine-stimulated glucose output was significantly higher in antegrade than in retrograde perfusions at all adenosine concentrations tested (range 10-300 microM). Ibuprofen abolished this difference, indicating that eicosanoids liberated under the influence of adenosine contribute to the glycogenolytic response in antegrade, but not in retrograde, perfusion. Desensitization occurred following repetitive adenosine infusion; this was more pronounced for adenosine-induced prostaglandin release than for cAMP or K+ efflux. The data suggest the following. (i) Both cAMP and eicosanoids are involved in the stimulation of glycogenolysis by adenosine. (ii) Eicosanoids are probably liberated under the influence of extracellular adenosine from a portal pre-sinusoidal compartment and accordingly stimulate glycogenolysis only in antegrade perfusions. Thus signals derived from portal vein structures can modulate hepatocellular function. (iii) Contractile elements are probably located also inside the liver acinus. (iv) Eicosanoids released into the hepatic vein reflect less than 10% of hepatic eicosanoid formation, because of marked clearance by perivenous hepatocytes.

Uptake of phosphate by rat hepatocytes in primary culture: a sodium-dependent system that is stimulated by insulin

Primary cultures of rat hepatocytes take up phosphate by a saturable Na(+)-dependent process. Thus the plasma membrane possesses an N(+)-Pi cotransporter of the type described for many cell types, e.g., kidney proximal tubular cells and enterocytes. Coupling to Na+ overcomes the barrier to anion entry represented by the membrane potential. At 0.12 mM Pi, the effect of Na+ is cooperative with a Hill coefficient of 1.7 suggesting two sodium sites per molecule of carrier. At 37 degrees C, the Km (for Pi) and Vmax for the sodium-dependent fraction of Pi uptake are approx. 1 mM and 0.35 nmol Pi/min per mg cell protein, respectively. Insulin stimulates Vmax four-fold with no significant effect on Km. Pi uptake in the absence of sodium is not affected by insulin. The stimulation by insulin could be of metabolic significance. Glucose phosphorylation at the expense of ATP is raised in liver following insulin stimulation, and thus, initially there may be an increased demand for Pi for oxidative phosphorylation until new steady-state conditions of hexose phosphate concentrations and of ATP turnover become established.

Agonist-stimulated Cl- efflux from human neutrophils. A common phenomenon during neutrophil activation

When human peripheral blood neutrophils were stimulated with various agonists which activate and/or prime neutrophils, we found that Cl- efflux was enhanced with a dramatic (50%) loss of intracellular Cl-. Interestingly, the Cl- efflux was enhanced by both agonists which induce a rapid transient increase in intracellular Ca2+ concentration ([Ca2+]i) [class I, e.g. N-formyl-methionyl-leucyl-phenylalanine (fMLP), interleukin-8 (IL8), platelet-activating factor, leukotriene B4 and C5a] and those which do not induce such an [Ca2+]i elevation [class II, e.g. tumor necrosis factor alpha (TNF) and granulocyte-macrophage colony-stimulating factor (GM-CSF)]. The time course of agonist-stimulated Cl- efflux differed depending on the agonist. Class I agonists such as IL8 and fMLP exhibited a 1 min lag phase before the onset of Cl- efflux; class II agonists such as GM-CSF and TNF displayed a 2 and 5 min lag phase, respectively. Both IL8 (class I)- and TNF (class II)-stimulated Cl- efflux exhibited similar sensitivity to inhibition by different types of ion transport inhibitors [ethacrynic acid (EA), amiloride, 4-acetamido-4'-isothiocyanatostilbene-2,2'-disulfonic acid, anthracene-9-carboxylic acid, and 4-4'-diisothiocyanatostilbene-2,2'-disulfonic acid]. On the other hand, natural Cl- efflux, which is thought to be mainly mediated by Cl-/Cl- self exchange, was not inhibited by EA (0.5 mM) or amiloride (0.3 mM). These results imply that both class I and class II agonist-stimulated Cl- efflux occurs via a common Cl- transporter which is different from that reported previously in resting human neutrophils. Although all agonists which induced a Cl- efflux also induced shape change of neutrophils, there did not appear to be a causal relationship between shape change and agonist-stimulated Cl- efflux. However, a temporal correlation was found to exist between agonist-stimulated Cl- efflux and intracellular alkalinization following agonist stimulation. Agonist-stimulated Cl- efflux therefore seems to be a common phenomenon activated by several agonists which act through different signal transduction pathways.

Okadaic acid inhibits insulin stimulation of both ornithine decarboxylase and spermidine transport in hepatocyte cultures

1. Okadaic acid inhibited basal ODC activity in rat hepatocytes in culture and prevented any increase in ODC activity and in the rate of spermidine uptake promoted by both insulin and hypotonicity. 2. The increase promoted by AIB was not counteracted by okadaic acid.

Involvement of microtubules in the swelling-induced stimulation of transcellular taurocholate transport in perfused rat liver

An increase of the hepatocellular hydratation state, induced by hypotonic exposure, amino acids or tauroursodeoxycholate, was shown to increase within minutes the Vmax of transcellular taurocholate transport and excretion into bile [Häussinger, Hallbrucker, Saha, Lang and Gerok (1992) Biochem. J. 288, 681-689]. This stimulatory effect of cell swelling on taurocholate excretion into bile is abolished in the presence of colchicine (5 microM). On the other hand, colchicine did not affect the stimulatory action of hypotonic cell swelling on 14CO2 production from [1-14C]glycine or [1-14C]glucose. Likewise, volume regulatory K+ fluxes following anisotonic exposure were not influenced in the presence of colchicine. Lumicolchicine (5 microM), a stereoisomer of colchicine without an inhibitory effect on microtubules, did not abolish the stimulation of taurocholate excretion into bile following hypo-osmotic exposure. Hypertonic cell shrinkage decreased taurocholate excretion into bile by about 35%; this effect was fully reversible upon normotonic re-exposure. With colchicine pretreatment, however, the hypertonicity-induced inhibition of taurocholate excretion was blunted and was no longer reversible upon normotonic re-exposure. The results suggest that stimulation of taurocholate excretion into bile in response to cell swelling involves a colchicine-sensitive, probably microtubule-dependent, mechanism, but not the stimulation of other cell-volume-sensitive pathways such as glycine oxidation or the pentose-phosphate shunt. It is hypothesized that the swelling-induced stimulation of taurocholate excretion into bile is due to a microtubule-dependent insertion of bile acid transporter molecules into the canalicular membrane.

Hyperosmolarity leads to an increase in derepressed system A activity in the renal epithelial cell line NBL-1

Hyperosmolarity induced an increase in Na(+)-dependent L-alanine uptake in confluent monolayers of the established renal epithelial cell line NBL-1. This induction was attributable to system A and was only seen when the cells had been previously deprived of amino acids in the culture medium to derepress system A activity. It was additive to the adaptive regulation induction, and both were inhibited by cycloheximide. However, the hyperosmolarity effect was inhibited by colcemid (an inhibitor of microtubular function), but adaptive regulation was not. Otherwise, when cell monolayers were incubated in a control medium, basal Na(+)-dependent L-alanine uptake mediated by system B0 decreased. The results of this study show that: (i) system A activity was not induced by cell shrinkage and subsequent swelling due to extracellular hyperosmolarity when cells were incubated in control medium; (ii) previous expression of system A activity induced by amino acid starvation seems to be a prerequisite for further induction due to hyperosmolarity; and (iii) the effects of adaptive regulation and hyperosmotic stress are mediated by different mechanisms.

Structural reaction pattern of hepatocytes following exposure to hypotonicity

Isolated rat hepatocytes were exposed to hypotonic media (225 mosmol/l) for 5 and 15 min and processed for a quantitative electron microscopic stereologic analysis. Within 5 min of hypotonicity, the hepatocyte volume increased by 25% and thereafter displayed a volume regulatory decrease leading to mean cellular volume, which was 16% above that of controls. Stereologic analysis of the major subcellular compartment, the cytosol, showed an identical change as the whole cell. In contrast to that, the mitochondrial compartment increased in volume by 30% within the first 5 min of exposure and returned by regulatory volume decrease back to values of the isotonic controls after 15 min of hypotonicity. In contrast, hypotonicity (220 mosmol/l)-induced stimulation of flux through mitochondrial glutaminase and the glycine cleavage enzyme complex, as assessed by 14CO2 production from [1-14C]glutamine or [1-14C]glycine in isolated perfused rat liver persisted throughout a 15-min period of hypotonic exposure. Thus hypotonicity-induced alterations of mitochondrial metabolism apparently do not parallel the time course of mitochondrial volume changes. This suggests that persistent mitochondrial swelling is not required for functional alterations, but that the latter may be triggered by the initial swelling of mitochondria. Hypotonic exposure did not alter the nuclear volume of isolated hepatocytes. Cell membrane surface nearly doubled after 5 min of hypotonic exposure, but returned within 15 min of exposure to values observed in normotonic media. This may reflect the participation of exocytosis in hepatocyte volume regulation.

Hydroperoxide metabolism in rat liver. K+ channel activation, cell volume changes and eicosanoid formation

Addition of t-butylhydroperoxide (0.2 mM) to isolated perfused rat liver led to a net K+ release of 7.2 +/- 0.2 mumol/g within 8 min and a net K+ reuptake of 6.6 +/- 0.4 mumol/g following withdrawal of the hydroperoxide, in line with earlier findings by Sies et al. [Sies, H., Gerstenecker, C., Summer, K. H., Menzel, H. & Flohé, R. (1974) in Glutathione (Flohé, L., Benöhr, C., Sies, H., Waller, H. D., eds) pp. 261-276, G. Thieme Publ. Stuttgart]. Net K+ release roughly paralleled the amount of GSSG released from the liver under the influence of the hydroperoxide. The t-butylhydroperoxide-induced K+ efflux was inhibited by approximately 70% in the presence of Ba2+ (1 mM), by 30% in Ca(2+)-free perfusions and was decreased by 50-60% when the intracellular Ca2+ stores were simultaneously depleted by repeated additions of phenylephrine. t-Butylhydroperoxide-induced K+ efflux was accompanied by a decrease of the intracellular water space by 58 +/- 14 microliter/g (n = 4), corresponding to a 10% cell shrinkage. The effect of t-butylhydroperoxide on cell volume was inhibited by 70-80% in the presence of Ba2+. In isolated rat hepatocytes treatment with t-butylhydroperoxide led to a slight hyperpolarization of the membrane at concentrations of 100 nM, but marked hyperpolarization occurred at t-butylhydroperoxide concentrations above 10 microM. t-Butylhydroperoxide (0.2 mM) transiently increased the portal-perfusion pressure by 3.3 +/- 0.6 cm H2O (n = 18), due to a slight stimulation of prostaglandin-D2 release under the influence of the hydroperoxide. In the presence of Ba2+ (1 mM), t-butylhydroperoxide increased the perfusion pressure by 12.7 +/- 1.2 cm H2O (n = 9) and produced an approximately tenfold increase of prostaglandin-D2 and thromboxane-B2 release. Under these conditions, glucose output from the liver rose from 0.9 +/- 0.03 to 2.9 +/- 0.7 mumol.g-1.min-1 (n = 4) with a time course roughly resembling that of portal-pressure increase and prostaglandin-D2 overflow. These effects were largely abolished in the presence of ibuprofen or the thromboxane-receptor-antagonist BM 13.177. The t-butylhydroperoxide effects on perfusion pressure, glucose and eicosanoid output were also enhanced in the presence of insulin or during hypotonic exposure; i.e. conditions known to swell hepatocytes, but not during hyperosmotic exposure. The data suggest that t-butylhydroperoxide induces liver-cell shrinkage and hyperpolarization of the plasma membrane due to activation of Ba(2+)-sensitive K+ channels.(ABSTRACT TRUNCATED AT 400 WORDS)

Plasma-depleted platelet concentrates prepared with a new washing solution

In certain clinical situations, complete removal of the plasma proteins from the platelet concentrates (PCs) is necessary by washing prior to transfusion. A simple electrolyte solution with a pH of 6.5 was developed for washing PCs. The platelet-rich plasma collected with acid-citrate-dextrose solution by apheresis in a 0.6-liter polyolefin bag was centrifuged. After removal of the supernatant plasma from pelleted platelet buttons, 200 ml of a washing solution consisting of 90 mM NaCl, 5 mM KCl, 3 mM MgCl2, 17 mM NaH2PO4, 8 mM Na2HPO4, 23 mM Na acetate, 17 mM Na3 citrate, 23.5 mM glucose, 2 mM adenine, 0.1% dextran, and 28.8 mM maltose (pH 6.5) was added to the pelleted platelet button. Steam sterilization of the solution was carried out under nitrogen to avoid caramelization of glucose. After resuspension of the pelleted platelet button with a washing solution and a second centrifugation, Seto additive solution (Seto sol, pH 7.4) was introduced into the bag to resuspend the platelet buttons for storage for 3 days at 22 degrees C. All of these procedures were completed within 3 h using a sterile docking device. In washed PCs, 99.1% of the plasma was removed and platelet recovery was 96%. The washed PCs were compared for 3 days with plasma-poor PCs consisting of 11% plasma and 89% Seto solution. There were no significant differences in percent hypotonic shock response, aggregation, energy metabolism, and morphology of platelets between the two groups during 3 days, except for significant swelling of 3-day-old platelets in washed PCs.(ABSTRACT TRUNCATED AT 250 WORDS)

Cell volume and bile acid excretion

The interaction between cell volume and taurocholate excretion into bile was studied in isolated perfused rat liver. Cell swelling due to hypo-osmotic exposure, addition of amino acids or insulin stimulated taurocholate excretion into bile and bile flow, whereas hyperosmotic cell shrinkage inhibited these. These effects were explained by changes in Vmax of taurocholate excretion into bile: Vmax. increased from about 300 to 700 nmol/min per g after cell swelling by 12-15% caused by either hypo-osmotic exposure or addition of amino acids under normo-osmotic conditions. Steady-state taurocholate excretion into bile was not affected when the influent K+ concentration was increased from 6 to 46 mM or decreased to 1 mM with iso-osmoticity being maintained by corresponding changes in the influent Na+ concentration. Replacement of 40 mM-NaCl by 80 mM-sucrose decreased taurocholate excretion into bile by about 70%; subsequent hypo-osmotic exposure by omission of sucrose increased taurocholate excretion to 160%. Only minor, statistically insignificant, effects of aniso-osmotic cell volume changes on the appearance of bolus-injected horseradish peroxidase in bile were observed. Taurocholate (400 microM) exhibited a cholestatic effect during hyperosmotic cell shrinkage, but not during hypo-osmotic cell swelling. Both taurocholate and tauroursodeoxycholate increased liver cell volume. Tauroursodeoxycholate stimulated taurocholate (100 microM) excretion into bile. This stimulatory effect was strongly dependent on the extent of tauroursodeoxycholate-induced cell swelling. During continuous infusion of taurocholate (100 microM) further addition of tauroursodeoxycholate at concentrations of 20, 50 and 100 microM increased cell volume by 10, 8 and 2% respectively, in parallel with a stimulation of taurocholate excretion into bile by 29, 27 and 9% respectively. There was a close relationship between the extent of cell volume changes and taurocholate excretion into bile, regardless of whether cell volume was modified by tauroursodeoxycholate, amino acids or aniso-osmotic exposure. The data suggest that: (i) liver cell volume is one important factor determining bile flow and biliary taurocholate excretion; (ii) swelling-induced stimulation of taurocholate excretion into bile is probably not explained by alterations of the membrane potential; (iii) bile acids modulate liver cell volume; (iv) taurocholate-induced cholestasis may depend on cell volume; (v) stimulation of taurocholate excretion into bile by tauroursodeoxycholate can largely be explained by tauroursodeoxycholate-induced cell swelling.

Hepatocyte swelling leads to rapid decrease of the G-/total actin ratio and increases actin mRNA levels

Exposure of isolated rat hepatocytes to hypotonic (190 mosmol/l) incubation media lowered the cellular G-actin level without affecting the total actin content: here the G-/total actin ratio decreased by 15.5 +/- 1.4% (n = 7). Similar effects were observed following isotonic cell swelling by either addition of glutamine (10 mM) or insulin (100 nM), resulting in a decrease of the G-/total actin ratios by 13.5 +/- 2.1% (n = 5) and 14.1 +/- 1.1% (n = 11), respectively. The effects of hypotonic exposure, glutamine and insulin on the G-/total actin ratio largely occurred within 1 min and persisted for at least 2 h in presence of the respective effectors. After a 120 min exposure to hypotonic media, glutamine or insulin the actin mRNA levels were increased 2.4-, 2.0- and 3.6-fold, respectively. Hypertonic exposure lowered the G-/total actin ratio by only 4.9 +/- 2.5% (n = 4) and increased actin mRNA levels only 1.2-fold. There was a close relationship between glutamine- and hypotonicity-induced cell swelling and the decrease of G-/total actin ratios. The data suggest that cell swelling exerts rapid and marked effects on the state of actin polymerization and increases actin mRNA levels. Thus, cytoskeletal alterations in response to cell swelling may be involved in the regulation of hepatic metabolism by cell volume.

Effect of anisotonic cell-volume modulation on glutathione-S-conjugate release, t-butylhydroperoxide metabolism and the pentose-phosphate shunt in perfused rat liver

1. Addition of 1-chloro-2,4-dinitrobenzene to isolated perfused rat liver results in the rapid formation of its glutathione-S-conjugate [S-(2,4-dinitrophenyl)glutathione], which is released into both, bile and effluent perfusate. Anisotonic perfusion did not affect total S-conjugate formation, but release of the S-conjugate into the perfusate was increased (decreased) following hypertonic (hypotonic) exposure at the expense of excretion into bile. Stimulation of S-conjugate release into the perfusate following hypertonic exposure paralleled the time course of volume-regulatory net K+ uptake. 2. Basal steady-state release of oxidized glutathione (GSSG) into bile was 1.30 +/- 0.12 nmol.g-1.min-1 (n = 18) during normotonic (305 mOsmol/l) perfusion and was 3.8 +/- 0.3 nmol.g-1.min-1 in the presence of t-butylhydroperoxide (50 mumol/l). Hypotonic exposure (225 mOsmol/1) lowered both, basal and t-butylhydroperoxide (50 mumol/l)-stimulated GSSG release into bile by 35% and 20%, respectively, whereas hypertonic exposure (385 mOsmol/l) increased. Anisotonic exposure was without effect on t-butylhydroperoxide removal by the liver. GSSG release into bile also decreased by 33% upon liver-cell swelling due to addition of glutamine plus glycine (2 mmol/l, each). 3. Hypotonic exposure led to a persistent stimulation 14CO2 production from [1-14C]glucose by about 80%, whereas 14CO2 production from [6-14C]glucose increased by only 10%. Conversely, hypertonic exposure inhibited 14CO2 production from [1-14C]glucose by about 40%, whereas 14CO2 production from [6-14C]glucose was unaffected. The effect of anisotonicity on 14CO2 production from [1-14C]glucose was also observed in presence of t-butylhydroperoxide (50 mumol/l), which increased 14CO2 production from [1-14C]glucose by about 40%. 4. t-Butylhydroperoxide (50 mumol/l) was without significant effect on volume-regulatory K+ fluxes following exposure to hypotonic (225 mOsmol/l) or hypertonic (385 mOsmol/l) perfusate. Lactate dehydrogenase release from perfused rat liver under the influence of t-butylhydroperoxide was increased by hypertonic exposure compared to hypotonic perfusions. 5. The data suggest that hypotonic cell swelling stimulates flux through the pentose-phosphate pathway and diminishes loss of GSSG under conditions of mild oxidative stress. Hypotonically swollen cells are less prone to hydroperoxide-induced lactate dehydrogenase release than hypertonically shrunken cells. Hypertonic cell shrinkage stimulates the excretion of glutathione-S-conjugates into the sinusoidal circulation at the expense of biliary secretion.

Liver cell volume and protein synthesis

Protein synthesis in isolated rat hepatocytes was determined from the incorporation of [3H]leucine (4 mM) into acid-precipitable material in the presence of amino acids at twice their physiological concentration. Protein synthesis increased linearly with time and incubated cell protein, and was inhibited by cycloheximide by more than 95%. In normo-osmotic incubations containing amino acids at twice the physiological concentration the rate of [3H]leucine incorporation was 5.8 +/- 0.2 nmol/h per mg of cell protein (n = 26). Hyperosmotic cell shrinkage due to addition of 60 mM-NaCl or 120 mM-raffinose inhibited [3H]leucine incorporation into acid-precipitable material by 60 and 74% respectively, whereas hypo-osmotic cell swelling was ineffective. Inhibition of protein synthesis by adding 120 mM-raffinose was largely counteracted by simultaneous lowering of the NaCl concentration by 60 mM. Glutamine (10 mM) had no effect on protein synthesis in normo-osmotic incubations (320 mosM), but stimulated protein synthesis in hyperosmotically (440 mosM) pre-shrunken cells almost to rates found in normo-osmotic (320 mosM) control incubations. Cyclic AMP and vasopressin inhibited protein synthesis by 23% and 8% respectively, whereas insulin and phenylephrine were ineffective. However, inhibition of protein synthesis by cyclic AMP was about twice as strong in the presence of vasopressin or phenylephrine. When protein synthesis was preinhibited by cyclic AMP, [3H]leucine incorporation was stimulated by glutamine (10 mM), insulin or hypo-osmotic exposure. There was a close relationship between the inhibition of protein synthesis and the extent of hepatocyte shrinkage induced by the above-mentioned effectors, suggesting a role of cell volume in the regulation of hepatic protein synthesis.

Cell shrinkage follows, rather than mediates, the short-term effects of glucagon on carbohydrate metabolism

The initial effects of glucagon on glycogen breakdown in isolated hepatocytes were found to be independent of cell volume and, when it occurred, cell shrinkage followed rather than mediated the glycogenolytic effect of glucagon. Similar conclusions could be drawn for the effect of glucagon on glycolysis/gluconeogenesis and for the antagonistic effect of insulin on glucagon action.

Arginine catabolism by Leishmania donovani promastigotes

Leishmania donovani promastigotes were grown to late log phase, washed and resuspended in iso-osmotic buffer containing L-arginine, and the rate of urea formation was then measured under various conditions. Addition of glucose or mannose activated urea formation, whereas 2-deoxyglucose inhibited and 6-deoxyglucose had no effect. Addition of alanine or of alpha-aminoisobutyrate inhibited urea formation, alanine causing a greater inhibition than alpha-aminoisobutyrate. Addition of leucine, proline, glycine, or lysine had no effect on urea formation. The presence of glutamate also increased the rate of urea formation from arginine, but to a lesser extent than did glucose. The presence of both glucose and alanine caused no net change in urea formation, whereas the inhibitory effect of alanine exceeded the activating effect of glutamate, so that a small inhibition in the rate of urea formation occurred in the presence of both alanine and glutamate. Cells grown to 3-day stationary phase had a markedly reduced rate of arginine catabolism to urea, but the activating effect of glucose and the inhibitory effect of alanine were qualitatively similar to their effects on late log phase cells. Addition of water to cells suspended in buffer also inhibited urea formation, but this appeared to be due primarily to the release of alanine caused by the hypo-osmotic stress. Addition of mannitol to cells suspended in buffer caused a small inhibition of arginine catabolism. Addition of dibutyrylcyclic AMP, 3',5'-cyclic GMP, phorbol myristic acid, or A23187 had no effect on the rate of urea formation from arginine.(ABSTRACT TRUNCATED AT 250 WORDS)

Eicosanoid-mediated contractility of hepatic stellate cells

To approach experimentally the problem of contractility, stellate cells from rats were isolated and grown on a flexible silicone rubber substrate. Increases or decreases in the number of wrinkles of the silicone membrane beneath the cells that were easily observable by microscopy was employed as semi-quantitative measure of stellate cell motility. Contraction of stellate cells accompanied by diminution of cell body size was induced by U46619 (a thromboxane A2 analogue) and prostaglandin (PG) F2 alpha. Wrinkle formation became detectable 1.5 min after addition of 2 microM-U46619 and reached its maximum 10-15 min later. The effect of PGF2 alpha was not so striking, but lasted for a longer period of time. On the other hand, dibutyryl cyclic AMP, Iloprost (a PGI2 analogue) and PGE2 led to the disappearance or decrease in the number of wrinkles, indicating relaxation of contracted stellate cells. For instance, after addition of 2 microM-Iloprost, 47, 75 and 82% of contracted stellate cells had relaxed within 5, 10 and 20 min respectively. Moreover, dibutyryl cyclic AMP induced disappearance of alpha-smooth muscle actin stress fibres. This response became recognizable 10 min after addition of dibutyryl cyclic AMP; 40 min later, 97% of stellate cells were devoid of stress fibres. Thus stellate cells are able to undergo reversible contraction in primary culture, and the contraction of these cells may be mediated by eicosanoids that can be produced within the liver.