Amino acid-dependent increase in hepatic system N activity is linked to cell swelling

Non-Excitatory Amino Acids, Melatonin, and Free Radicals: Examining the Role in Stroke and Aging

The aim of this review is to explore the relationship between melatonin, free radicals, and non-excitatory amino acids, and their role in stroke and aging. Melatonin has garnered significant attention in recent years due to its diverse physiological functions and potential therapeutic benefits by reducing oxidative stress, inflammation, and apoptosis. Melatonin has been found to mitigate ischemic brain damage caused by stroke. By scavenging free radicals and reducing oxidative damage, melatonin may help slow down the aging process and protect against age-related cognitive decline. Additionally, non-excitatory amino acids have been shown to possess neuroprotective properties, including antioxidant and anti-inflammatory in stroke and aging-related conditions. They can attenuate oxidative stress, modulate calcium homeostasis, and inhibit apoptosis, thereby safeguarding neurons against damage induced by stroke and aging processes. The intracellular accumulation of certain non-excitatory amino acids could promote harmful effects during hypoxia-ischemia episodes and thus, the blockade of the amino acid transporters involved in the process could be an alternative therapeutic strategy to reduce ischemic damage. On the other hand, the accumulation of free radicals, specifically mitochondrial reactive oxygen and nitrogen species, accelerates cellular senescence and contributes to age-related decline. Recent research suggests a complex interplay between melatonin, free radicals, and non-excitatory amino acids in stroke and aging. The neuroprotective actions of melatonin and non-excitatory amino acids converge on multiple pathways, including the regulation of calcium homeostasis, modulation of apoptosis, and reduction of inflammation. These mechanisms collectively contribute to the preservation of neuronal integrity and functions, making them promising targets for therapeutic interventions in stroke and age-related disorders.

Hypoxia-induced depression of synaptic transmission becomes irreversible by intracellular accumulation of non-excitatory amino acids

The intracellular accumulation of some amino acids (AAs), mainly glutamine, can contribute to brain edema observed during liver failure. We recently demonstrated that individual applications of high concentrations (10 mM) of some non-excitatory AAs increase the electrical resistance of hippocampal slices, indicating cell swelling. Therefore, we pondered whether an AA mixture's application might cause cell swelling at a physiological concentration range. In rat hippocampal slices, we carried out extra- and intracellular electrophysiological recordings and AAs analysis to address this question. We applied a mixture of 19 AAs at their plasmatic concentrations (Plasma solution: Ala, Gly, Gln, His, Ser, Tau, Thr, Arg, Leu, Met, Pro, Val, Asn, Cys, Phe, Ile, Lys, Tyr, and Trp). This solution was afterward divided into two according to the individual AAs at 10 mM concentration inducing synaptic potentiation (Plasma1, containing the first seven AAs of Plasma) or not (Plasma2, with the remaining AAs). Plasma application increased evoked field potentials requiring extracellular chloride. This effect was mimicked by the Plasma1 but not the Plasma2 solution. Plasma1-induced potentiation was independent of changes in release probability, basic electrophysiological membrane properties, and NMDAR activation. AAs in Plasma1 act cooperatively to accumulate intracellularly and to induce synaptic potentiation. In the presence of Plasma1, the reversible synaptic depression caused by a 40-min hypoxia period turned into an irreversible disappearance of synaptic potentials through an NMDAR-dependent mechanism. The presence of a system A transport inhibitor did not block Plasma1-mediated effects. These results indicate that cell swelling, induced by the accumulation of non-excitotoxic AAs through unidentified transporters, might foster deleterious effects produced by hypoxia-ischemia episodes.

Rapid downregulation of the rat glutamine transporter SNAT3 by a caveolin-dependent trafficking mechanism in Xenopus laevis oocytes

The glutamine transporter SNAT3 is involved in the uptake and release of glutamine in the brain, liver, and kidney. Substrate transport is accompanied by Na(+) cotransport and H(+) antiport. In this study, treatment of Xenopus laevis oocytes expressing rat SNAT3 with the phorbol ester PMA resulted in a rapid downregulation of glutamine uptake in less than 20 min. PMA treatment of oocytes coexpressing SNAT3 and the monocarboxylate transporter MCT1 reduced SNAT3 activity only, demonstrating the specificity of the regulatory mechanism. Single or combined mutations of seven putative phosphorylation sites in the SNAT3 sequence did not affect the regulation of SNAT3 by PMA. Expression of an EGFP-SNAT3 fusion protein in oocytes established that the downregulation was caused by the retrieval of the transporter from the plasma membrane. Coexpression of SNAT3 with dominant-negative mutants of dynamin or caveolin revealed that SNAT3 trafficking occurs in a dynamin-independent manner and is influenced by caveolin. Although system N activity was not affected by PMA in cultured astrocytes, a downregulation was observed in HepG2 cells.

L-Proline transport into renal OK epithelial cells: a second renal proline transport system is induced by amino acid deprivation

Influx of [(3)H]-L-proline into renal OK cells revealed that basal transport was mediated by the transporter SIT1. When cells were submitted for 8 h to amino acid deprivation, uptake of L-proline was now dominated by a low-affinity system with an apparent K (m) of 4.4 +/- 0.6 mM and a V (max) of 10.2 +/- 0.6 nmol/mg of protein/min operating in addition to the high-affinity SIT1 system with a K (m) of 0.12 +/- 0.01 mM and a V (max) of 0.28 +/- 0.04 nmol/mg of protein/min. The low- and high-affinity proline transporting systems were sensitive to inhibitors of JNK and PI-3 kinases, whereas a GSK-3 inhibitor affected only the upregulated transport system. Ion-replacement studies and experiments assessing substrate specificities for both systems provided strong evidence that SNAT2, that showed two- to threefold increased mRNA levels, is the responsible transporter mediating the increased proline influx under conditions of amino acid deprivation.

Chemotransport contributes to the effect of oscillatory fluid flow on human bone marrow stromal cell proliferation

Mechanical loads produce a diverse set of biophysical signals that may regulate bone cell activity, but accumulating evidence suggests that interstitial fluid flow is the primary signal that bone cells perceive. Because we previously demonstrated that oscillatory fluid flow increases human bone marrow stromal cell proliferation, we investigated the contribution of fluid shear stress and chemotransport, two stimuli induced by interstitial fluid flow. Alterations in flow rate at a constant peak shear stress were associated with decreases in oscillatory fluid flow-induced marrow stromal cell proliferation, while variations in peak fluid shear stress had no significant effect. Modulation of marrow stromal cell proliferation by flow rate may be attributed to changes in the release of ATP and intracellular calcium signaling. We found that if the flow rate is decreased while maintaining a constant peak fluid shear stress, marrow stromal cells release less ATP into the extracellular environment. Moreover, as the flow rate decreased fewer cells respond to fluid flow with an increase in intracellular calcium concentration. These data suggest that chemotransport is a prerequisite for marrow stromal cells to respond to interstitial fluid flow.

The glutamine commute: take the N line and transfer to the A

The transfer of glutamine between cells contributes to signaling as well as to metabolism. The recent identification and characterization of the system N and A family of transporters has begun to suggest mechanisms for the directional transfer of glutamine, and should provide ways to test its physiological significance in diverse processes from nitrogen to neurotransmitter release.

Recent molecular advances in mammalian glutamine transport

Much has been learned about plasma membrane glutamine transporter activities in health and disease over the past 30 years, including their potential regulatory role in metabolism. Since the 1960s, discrimination among individual glutamine transporters was based on functional characteristics such as substrate specificity, ion dependence, and kinetic and regulatory properties. Within the past two years, several genes encoding for proteins with these defined activities (termed "systems") have been isolated from human and rodent cDNA libraries and found to be distributed among four distinct gene families. The Na(+)-dependent glutamine transporter genes isolated thus far are System N (SN1), System A (ATA1, ATA2), System ASC/B(0) (ASCT2 or ATB(0)), System B(0,+) (ATB(0,+)) and System y(+)L (y(+)LAT1, y(+)LAT2). Na(+)-independent glutamine transporter genes encoding for System L (LAT1, LAT2) and System b(0,+) (b(0,+)AT) have also been recently isolated, and similar to y(+)L, have been shown to function as disulfide-linked heterodimers with the 4F2 heavy chain (CD98) or rBAT (related to b(0,+) amino acid transporter). In this review, the molecular features, catalytic mechanisms and tissue distributions of each are addressed. Although most of these transporters mediate the transmembrane movement of several other amino acids, their potential roles in regulating interorgan glutamine flux are discussed. Most importantly, these newly isolated transporter genes provide the long awaited tools necessary to study their molecular regulation during the catabolic states in which glutamine is considered to be "conditionally essential."

Hepatic glutamine transporter activation in burn injury: role of amino acids and phosphatidylinositol-3-kinase

Burn injury elicits a marked, sustained hypermetabolic state in patients characterized by accelerated hepatic amino acid metabolism and negative nitrogen balance. The transport of glutamine, a key substrate in gluconeogenesis and ureagenesis, was examined in hepatocytes isolated from the livers of rats after a 20% total burn surface area full-thickness scald injury. A latent and profound two- to threefold increase in glutamine transporter system N activity was first observed after 48 h in hepatocytes from injured rats compared with controls, persisted for 9 days, and waned toward control values after 18 days, corresponding with convalescence. Further studies showed that the profound increase was fully attributable to rapid posttranslational transporter activation by amino acid-induced cell swelling and that this form of regulation may be elicited in part by glucagon. The phosphatidylinositol-3-kinase (PI3K) inhibitors wortmannin and LY-294002 each significantly attenuated transporter stimulation by amino acids. The data suggest that PI3K-dependent system N activation by amino acids may play an important role in fueling accelerated hepatic nitrogen metabolism after burn injury.

Glutamine transport and human hepatocellular transformation

Among other functions, the liver serves to regulate both glucose and nitrogen economy in the body, and in humans, the amino acid glutamine is a major gluconeogenic substrate and the primary extrahepatic ammonia shuttle. Accordingly, the liver acinus possesses a unique heterogeneous metabolic architecture suited to carry out these functions with glutamine-consuming urea cycle and gluconeogenic enzymes in the periportal hepatocytes and a high capacity for glutamine synthesis in the perivenous hepatocytes, resulting in net glutamine balance across the hepatic bed under most conditions. Cytoplasmic levels of glutamine are significantly governed by the activity of the System N transporter in the plasma membrane of parenchymal cells; in this capacity, this glutamine carrier has been shown to represent a rate-limiting step in metabolism via glutaminase. The unique properties of System N allow it to rapidly adapt in support of the dynamic demands of whole body ammonia and glucose homeostasis. In contrast to System N in normal hepatocytes, human hepatoma cells take up glutamine at rates several-fold faster through a broad-specificity higher affinity transporter with characteristics of System ASC or B0. It is currently hypothesized that the expression of this high activity carrier by hepatoma cells combined with accelerated metabolism and tumor-induced derangements in hepatocellular architecture result in net glutamine consumption, and may underlie the diminished plasma glutamine levels observed in patients with hepatocellular carcinoma (HCC). The transport of glutamine through System ASC has been shown to regulate growth in some human hepatoma cells, which suggests this transporter may warrant consideration as a therapeutic target for HCC.

Inhibition of system A amino acid transport and hepatocyte proliferation following partial hepatectomy in the rat

System A, the sodium-dependent neutral amino acid transport activity, has a 3-fold increase in its initial uptake velocity into hepatocytes following partial hepatectomy (PH) in the rat. The purpose of this study was to examine the effect of inhibition of System A-mediated amino acid transport on hepatocyte proliferation and liver regeneration. We describe the in vivo competitive inhibition of System A activity following PH by the nonmetabolizable, System A-specific substrate, alpha-(methylamino)isobutyric acid (MeAIB). Administration of MeAIB 60 minutes before PH decreased the incorporation of [(3)H]thymidine into DNA by 45% +/- 5% and 76% +/- 17% at 24 and 36 hours, respectively. The readministration of MeAIB every 12 hours further decreased DNA synthesis by 92% +/- 18% and 82% +/- 11% at 24 and 36 hours. The recovery of liver mass of rats receiving MeAIB was decreased by 46.4% +/- 5.1% at 24 hours after PH. In vitro, 5 mmol/L MeAIB inhibited proliferation of primary hepatocytes by 56% +/- 4% and 61% +/- 12% 48 hours after incubation with 10% fetal calf serum or epidermal growth factor (5 ng/mL), respectively. Thus, MeAIB inhibition of System A transport activity decreased both in vivo and in vitro inducement of hepatocyte proliferation. Treatment with MeAIB did not significantly change the incorporation of [(3)H]leucine into total liver protein, but changes in serum amino acids and hepatocyte cell volume were observed, suggesting System A transport activity during hepatocyte proliferation functions primarily to provide amino acids to fuel liver-specific biochemical pathways and to increase cell volume.

Integrin and cytoskeletal involvement in signalling cell volume changes to glutamine transport in rat skeletal muscle

1. Muscle glutamine transport is modulated in response to changes in cell volume by a mechanism dependent on active phosphatidylinositol 3-kinase. We investigated the possibility that this mechanism requires interactions between the extracellular matrix (ECM), integrins and the cytoskeleton as components of a mechanochemical transduction system. 2. Using skeletal muscle cells, we studied effects of (a) inactivating integrin-substratum interactions by using integrin-binding peptide GRGDTP with inactive peptide GRGESP as control, and (b) disrupting the cytoskeleton using colchicine or cytochalasin D, on glutamine transport after brief exposure to hypo-osmotic, isosmotic or hyperosmotic medium (170, 300 and 430 mosmol kg-1, respectively). 3. Neither GRGDTP nor GRGESP significantly affected basal glutamine uptake (0.05 mM; 338 +/- 58 pmol min-1 (mg protein)-1) but GRGDTP specifically prevented the increase (71%) and decrease (39%) in glutamine uptake in response to hypo- and hyperosmotic exposure, respectively. 4. Colchicine and cytochalasin D prevented the increase and decrease in glutamine uptake in response to changes in external osmolality. They also increased basal glutamine uptake by 59 +/- 19 and 85 +/- 16%, respectively, in a wortmannin-sensitive manner. 5. These results indicate involvement of ECM-integrin-mediated cell adhesion and the cytoskeleton in mechanochemical transduction of cell volume changes to chemical signals modulating glutamine transport in skeletal muscle. Phosphatidylinositol 3-kinase may function to maintain the mechanotransducer in an active state.

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.

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.

Alanine uptake activates hepatocellular chloride channels

Cells involved in the retrieval and metabolic conversion of amino acids undergo significant increases in size in response to amino acid uptake. The resultant adaptive responses to cell swelling are thought to include increases in membrane K+ and Cl- permeability through activation of volume-sensitive ion channels. This viewpoint is largely based on experimental models of hypotonic swelling, but few mammalian cells experience hypotonic challenge in vivo. Here we have examined volume regulatory responses in a physiological model of cell-swelling alanine uptake in immortalized hepatocytes. Alanine-induced cell swelling was followed by a decrease in cell volume that was temporally associated with an increase in membrane Cl- currents. These currents were dependent both on alanine concentration and Na+, suggesting that currents were stimulated by Na+-coupled alanine uptake. Cl- currents were outwardly rectifying, exhibited an anion permeability sequence of I- > Br- > Cl-, and were inhibited by the Cl- channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid, features similar to those reported for a widely distributed class of volume-sensitive anion channels evoked by experimental hypotonic stress. These findings suggest that volume-sensitive anion channels participate in adaptive responses to amino acid uptake and provide such channels with a new physiological context.

Glucocorticoid-dependent induction of interleukin-6 receptor expression in human hepatocytes facilitates interleukin-6 stimulation of amino acid transport

OBJECTIVE

The authors studied the effects of interleukin-6 (IL-6) and tumor necrosis factor-alpha (TNF-alpha) on glutamine and alanine transport in isolated human hepatocytes. They also evaluated the role of dexamethasone in modulating this response and its effects on the expression of the plasma membrane high-affinity IL-6 receptor.

SUMMARY BACKGROUND DATA

Animal studies indicate that cytokines are important mediators of the increased hepatic amino acid uptake that occurs during cancer and sepsis, but studies in human tissues are lacking. The control of transport by cytokines and cytokine receptor expression in the liver may provide a mechanism by which hepatocytes can modulate amino acid availability during catabolic disease states.

METHODS

Human hepatocytes were isolated from wedge biopsy specimens and plated in 24-well trays. Interleukin-6 and TNF-alpha, in combination with the synthetic glucocorticoid dexamethasone, were added to hepatocytes in culture, and the transport of radiolabeled glutamine and alanine was measured. Fluorescent-activated cell sorter (FACS) analysis was used to study the effects of dexamethasone on IL-6 receptor number in the well-differentiated human hepatoma HepG2.

RESULTS

Both IL-6 and TNF-alpha exerted a small stimulatory effect on alanine and glutamine transport. Dexamethasone alone did not alter transport rates, but pretreatment of cells augmented the effects of both cytokines on carrier-mediated amino acid uptake. Dexamethasone pretreatment and a combination of IL-6 and TNF-alpha resulted in a greater than twofold increase in transport activity. Fluorescent-activated cell sorter analysis demonstrated that dexamethasone induced a threefold increase in the expression of high-affinity IL-6 receptors.

CONCLUSIONS

Interleukin-6 and TNF-alpha work coordinately with glucocorticoids to stimulate amino acid uptake in human hepatocytes. Dexamethasone exerts a permissive effect on cytokine-mediated increases in transport by increasing IL-6 receptor expression on the cell surface. It is likely that this upregulation of IL-6 receptors "primes" human liver cells for subsequent stimulation by cytokines. The resulting increase in hepatic amino acid transport provides the liver with substrate to support key metabolic pathways during catabolic states.

On isolated hepatocytes mitochondrial swelling induced in hypoosmotic medium does not affect the respiration rate

In isolated hepatocytes incubated in hypoosmotic media, a large increase in the mitochondrial volume is not directly involved in the activation of respiration. Moreover, results of the quantification of the various bioenergetic parameters are not in accordance with an activation of the respiratory chain as previously proposed (Halestrap, A.P. (1989) Biochim. Biophys. Acta, 973, 355-382), but point more to an inhibition of respiration. The same respiration rate is obtained in hypoosmolar incubation media in vitro and in situ for a higher overall thermodynamic driving force over the electron transport chain.

Vanadate inhibition of hepatocytic autophagy. Calcium-modulated and osmolality-modulated antagonism by asparagine

The phosphate analogue vanadate, at 10 mM, strongly (approximately 90%) inhibited the autophagic sequestration of endogenous lactate dehydrogenase in isolated rat hepatocytes. The effect of vanadate was markedly (approximately 80%) antagonized by asparagine (20 mM), and to a lesser extent by glutamine, glycine, and alanine. The antagonism was only observed in the presence of Ca2+ when an isotonic standard incubation medium was used, but by increasing the medium osmolality this Ca2+ requirement could be eliminated. Asparagine induced a cell swelling (17% at 20 mM) that might account for at least part of its vanadate antagonism, since hypotonic cell swelling by itself stimulated autophagy (with a maximal effect at approximately 200 mosM). Conversely, hypertonic media inhibited autophagy and were additive to vanadate. In a strongly hypotonic medium (less than 200 mosM), both asparagine and vanadate were inhibitory. However, since vanadate alone had no effect on cell volume, the vanadate-asparagine antagonism could not be exerted exclusively at the level of cell volume regulation. An additional mechanism might be a partial deamination of asparagine, generating ammonia, which was found to oppose the vanadate inhibition of autophagy while having no effect on cell volume. Other metabolizable amino acids, like alanine and glycine, were moderately vanadate-antagonistic while failing to induce cell swelling. These results are compatible with a vanadate-antagonistic effect of asparagine mediated partly through an unknown mechanism (possibly pH change) by its deamination product, ammonia, partly through cell swelling and a secondary Ca2+ influx that could compensate for a vanadate-induced depletion of intracellular calcium stores.

Regulation of cell function by the cellular hydration state

Cellular hydration can change within minutes under the influence of hormones, nutrients, and oxidative stress. Such short-term modulation of cell volume within a narrow range acts per se as a potent signal which modifies cellular metabolism and gene expression. It appears that cell swelling and cell shrinkage lead to certain opposite patterns of cellular metabolic function. Apparently, hormones and amino acids can trigger those patterns simply by altering cell volume. Thus alterations of cellular hydration may represent another important mechanism for metabolic control and act as another second or third messenger linking cell function to hormonal and environmental alterations.

Influence of cloned voltage-gated K+ channel expression on alanine transport, Rb+ uptake, and cell volume

Voltage-gated K+ channels are involved in regulation of action potential duration and in setting the resting membrane potential in nerve and muscle. To determine the effects of voltage-gated K+ channel expression on processes not associated with electrically excitable cells, we studied cell volume, membrane potential, Na(+)-K(+)-ATPase activity, and alanine transport after the stable expression of the Kv1.4 and Kv1.5 human K+ channels in Ltk- mouse fibroblasts (L-cells). The fast-activating noninactivating Kv1.5 channel, but not the rapidly inactivating Kv1.4 channel, prevented dexamethasone-induced increases in intracellular volume and inhibited Na(+)-K(+)-ATPase activity by 25%, as measured by 86Rb+ uptake. Alanine transport, measured separately by systems A and ASC, was lower in Kv1.5-expressing cells, indicating that the expression of this channel modified the Na(+)-dependent amino acid transport of both systems. Expression of the Kv1.4 channel did not alter alanine transport relative to wild-type or sham-transfected cells. The changes specific to Kv1.5 expression may be related to the resting membrane potential induced by this channel (-30 mV) in contrast to that measured in wild-type sham-transfected, or Kv1.4-transfected cells (-2 to 0 mV). Blocking of the Kv1.5 channel by 60 microM quinidine negated the effects of Kv1.5 expression on intracellular volume, Na(+)-K(+)-ATPase, and Na(+)-dependent alanine transport. These results indicate that delayed rectifier channels such as Kv1.5 can play a key role in the control of cell membrane potential, cell volume, Na(+)-K(+)-ATPase activity, and electrogenic alanine transport across the plasma membrane of electrically unexcitable cells.

Regulation of spermidine transport in L1210 cells

1. 1 mM 2-amino isobutyric acid (AIB), glutamine or asparagine when preincubated for 3 hr with L1210 cells promoted a marked increase in the rate of spermidine uptake. 2. Cycloheximide also increased the transport rate and completely prevented the increase due to AIB. 3. Trifluoperazine and iso-H7 inhibited the uptake of spermidine, much less the uptake of AIB. 4. Adenosine promoted an increase in the uptake of AIB, a decrease in that of spermidine. 5. Hypotonic stress also increased the rate of spermidine transport. This modification was only partially prevented by cycloheximide. 6. Okadaic acid had no effect on this increase, whereas it prevented the increase of ODC activity.

Anisoosmostic liver perfusion: redox shifts and modulation of alpha-ketoisocaproate and glycine metabolism

1) In isolated perfused rat liver, 14CO2 production from [1-14C]alpha-ketoisocaproate or [1-14C]glycine as well as ketogenesis from alpha-ketoisocaproate were stimulated upon exposure to hypoosmotic perfusion media, whereas hyperosmotic exposure inhibited. The effects of anisotonicity were preserved when ketogenesis from alpha-ketoisocaproate and 14CO2 production from [1-14C]glycine were already stimulated by glucagon. On the other hand, ketogenesis from tyrosine (2 mM) or octanoate (0.1 mM) were almost unaffected by anisoosmotic exposure. 2) With all ketogenic substrates studied, hypoosmotic (hyperosmotic) cell swelling (shrinkage) decreased (increased) the beta-hydroxybutyrate/acetoacetate ratio in effluent perfusate. A shift of the mitochondrial and cytosolic NADH systems to a more oxidized (reduced) state following hypoosmotic (hyperosmotic) exposure was also found upon infusion of beta-hydroxybutyrate/acetoacetate and lactate/pyruvate as redox indicator metabolite couples. The effects of anisotonicity on the beta-hydroxybutyrate/acetoacetate ratio were reversible upon normoosmotic reexposure and persisted throughout anisoosmotic exposure despite completion of volume regulatory K+ fluxes within 10-15 min. Hepatic oxygen consumption decreased by about 10% during hyperosmotic cell shrinkage and was transiently stimulated during hypoosmotic exposure. 3) There was a close relationship between ketogenesis from alpha-ketoisocaproate (0.5 mM) and the mitochondrial redox state, as assessed by the beta-hydroxybutyrate/acetoacetate ratio in effluent, regardless of whether the pathway was modulated by anisotonicity or glucagon. 4) Isoosmotic cell swelling induced by addition of glutamine (3 mM) was without significant effect on ketogenesis from octanoate and stimulated ketogenesis and 14CO2production from [1-14C]alpha-ketoisocaproate only slightly (i.e. by less than 10%); however, in each case the hydroxybutyrate/acetoacetate ratio in effluent perfusate decreased by about 20% upon addition of glutamine. 5) Stimulation of 14CO2production from [1-14C]glycine by hypoosmotic exposure and glucagon was only slightly affected when the accompanying decrease of the beta-hydroxybutyrate/acetoacetate ratio was reversed by addition of beta-hydroxybutyrate. 6) The data are compatible with a hypotonicity (hypertonicity)-induced shift of the mitochondrial NADH system to a more oxidized (reduced) state, probably due to a alterations of respiration. Mitochondrial swelling probably also occurs under the influence of glutamine. Modulation of ketogenesis from alpha-ketoisocaproate, but not of glycine oxidation by anisoosmotic exposure and glucagon can be related to the accompanying redox shifts. The observations support the concept that cell volume may be an important parameter determining liver cell function.