Role of eicosanoids, inositol phosphates and extracellular Ca2+ in cell-volume regulation of rat liver

Investigating cell-ECM contact changes in response to hypoosmotic stimulation of hepatocytes in vivo with DW-RICM

Hepatocyte volume regulation has been shown to play an important role in cellular metabolism, proliferation, viability and especially in hepatic functions such as bile formation and proteolysis. Recent studies on liver explants led to the assumption that cell volume changes present a trigger for outside-in signaling via integrins, a protein family involved in mediating cellular response to binding to the extracellular matrix (ECM). However, it remains elusive how these volume change related signaling events are transducted on a single cell level and how these events are influenced and controlled by ECM interactions. One could speculate that an increase in cell volume leads to an increase in integrin/ECM contacts which causes activation of integrins, which act as mechano-sensors. In order to test this idea, it was an important issue to quantify the cell volume-dependence of the contact areas between the cell and the surrounding ECM. In this study we used two wavelength reflection interference contrast microscopy (DW-RICM) to directly observe the dynamics of cell-substrate contacts, mimicking cell-ECM interactions, in response to a controlled and well-defined volume change induced by hypoosmotic stimulation. This is the first time a non-invasive, label-free method is used to uncover a volume change related response of in vitro hepatocytes in real time. The cell cluster analysis we present here agrees well with previous studies on ex vivo whole liver explants. Moreover, we show that the increase in contact area after cell swelling is a reversible process, while the reorganisation of contacts depends on the type of ECM molecules presented to the cells. As our method complements common whole liver studies providing additional insight on a cell cluster level, we expect this technique to be particular suitable for further detailed studies of osmotic stimulation not only in hepatocytes, but also other cell types.

Calcium and cytoskeleton signaling during cell volume regulation in isolated nematocytes of Aiptasia mutabilis (Cnidaria: Anthozoa)

Cell volume regulation has not been completely clarified in Coelenterates. The present investigation focuses on cell volume regulation under anisosmotic conditions, both hyposmotic and hypertonic, and on the underlying signals in nematocytes isolated from the Coelenterate Aiptasia mutabilis living in sea water. Nematocytes, once isolated from acontia, that were submitted to either hyposmotic (35%) and hypertonic shock (45%) show RVD and RVI capabilities, respectively. In order to ascertain the role of Ca2+ in triggering such regulatory mechanisms and the possible involvement of cytoskeleton components, tests were performed by employing either Ca2+ free conditions, Gd3+ as Ca2+ channel blockers, TFP as calmodulin inhibitor, colchicine as microtubule inhibitor and cytochalasin B as microfilament polymerization inhibitor. Results show that isolated nematocytes of A. mutabilis can regulate their volume upon both hyposmotic and hypertonic challenge. Ca2+ both from external medium and from internal stores is needed to perform RVD mechanisms, whereas, intracellular Ca2+ seems to be mainly involved in RVI. Moreover cytoskeletal components may play an important role since a significant RVD and RVI inhibition was observed in treated cells. On the basis of our observations further studies are warranted to further verify the role of signals, including phosphatases and phosphorylases, in cell volume regulation of primitive eukaryotic cells.

Effects of high-K+, Na+-deficient solution on contractility of the smooth muscles of the bovine trachea

A high-K+, Na+-deficient (I-154 K+) solution induced contraction followed by gradual relaxation of the smooth muscles of the bovine trachea, while hyperosmotic addition of 65 mM KCl induced a large sustained contraction. Exposure of the muscle to the I-154 K+ solution induced an increase in the ratio of cellular water content and a sustained increase in oxidized flavoprotein fluorescence or reduced pyridine nucleotide fluorescence. The I-154 K+ solution also induced a sustained increase in [Ca2+]i level. Decreases in developed tension and increases in cellular water content were both prevented by the addition of sucrose or NaCl but not pyruvate. Substitution of KI for KCl in the I-154 K+ solution produced a greater inhibition of contraction, while substitution with K-propionate produced no inhibition of contraction. Moreover, decreases in developed tension and increases in cellular water content were both prevented by addition of 100 microM 4,4'-diisothiocyanatostilbene-2,2'-disulphonic acid (DIDS), but not 10 microM bumetanide or 1 mM acetazolamide. In conclusion, I-154 K+ solution induced-relaxation in the bovine trachea may be due to swelling of smooth muscle cells and the mechanism of swelling is probably involved in DIDS-sensitive anion movement.

Modulation of hepatocellular swelling-activated K+currents by phosphoinositide pathway-dependent protein kinase C

K+channels participate in the regulatory volume decrease (RVD) accompanying hepatocellular nutrient uptake and bile formation. We recently identified KCNQ1 as a molecular candidate for a significant fraction of the hepatocellular swelling-activated K+current ( IKVol). We have shown that the KCNQ1 inhibitor chromanol 293B significantly inhibited RVD-associated K+flux in isolated perfused rat liver and used patch-clamp techniques to define the signaling pathway linking swelling to IKVolactivation. Patch-electrode dialysis of hepatocytes with solutions that maintain or increase phosphatidylinositol 4,5-bisphosphate (PIP2) increased IKVol, whereas conditions that decrease cellular PIP2decreased IKVol. GTP and AlF4−stimulated IKVoldevelopment, suggesting a role for G proteins and phospholipase C (PLC). Supporting this, the PLC blocker U-73122 decreased IKVoland inhibited the stimulatory response to PIP2or GTP. Protein kinase C (PKC) is involved, because K+current was enhanced by 1-oleoyl-2-acetyl- sn-glycerol and inhibited after chronic PKC stimulation with phorbol 12-myristate 13-acetate (PMA) or the PKC inhibitor GF 109203X. Both IKVoland the accompanying membrane capacitance increase were blocked by cytochalasin D or GF 109203X. Acute PMA did not eliminate the cytochalasin D inhibition, suggesting that PKC-mediated IKVolactivation involves the cytoskeleton. Under isotonic conditions, a slowly developing K+current similar to IKVolwas activated by PIP2, lipid phosphatase inhibitors to counter PIP2depletion, a PLC-coupled α1-adrenoceptor agonist, or PKC activators and was depressed by PKC inhibition, suggesting that hypotonicity is one of a set of stimuli that can activate IKVolthrough a PIP2/PKC-dependent pathway. The results indicate that PIP2indirectly activates hepatocellular KCNQ1-like channels via cytoskeletal rearrangement involving PKC activation.

Leukotriene and purinergic receptors are involved in the hyperpolarizing effect of glucagon in liver cells

The pancreatic hormone glucagon hyperpolarizes the liver cell membrane. In the present study, we investigated the cellular signalling pathway of glucagon-induced hyperpolarization of liver cells by using the conventional microelectrode method. The membrane potential was recorded in superficial liver cells of superfused mouse liver slices. In the presence of the K+ channel blockers tetraethylammonium (TEA, 1 mmol/l) and Ba2+ (BaCl2, 5 mmol/l) and the blocker of the Na+/K+ ATPase, ouabain (1 mmol/l), no glucagon-induced hyperpolarization was observed confirming previous findings. The hyperpolarizing effect of glucagon was abolished by the leukotriene B4 receptor antagonist CP 195543 (0.1 mmol/l) and the purinergic receptor antagonist PPADS (5 micromol/l). ATPgammaS (10 micromol/l), a non-hydrolyzable ATP analogue, induced a hyperpolarization of the liver cell membrane similar to glucagon. U 73122 (1 micromol/l), a blocker of phospholipase C, prevented both the glucagon- and ATPgammaS-induced hyperpolarization. These findings suggest that glucagon affects the hepatic membrane potential partly by inducing the formation and release of leukotrienes and release of ATP acting on purinergic receptors of the liver cell membrane.

Cell volume regulation: osmolytes, osmolyte transport, and signal transduction

In recent years, it has become evident that the volume of a given cell is an important factor not only in defining its intracellular osmolality and its shape, but also in defining other cellular functions, such as transepithelial transport, cell migration, cell growth, cell death, and the regulation of intracellular metabolism. In addition, besides inorganic osmolytes, the existence of organic osmolytes in cells has been discovered. Osmolyte transport systems-channels and carriers alike-have been identified and characterized at a molecular level and also, to a certain extent, the intracellular signals regulating osmolyte movements across the plasma membrane. The current review reflects these developments and focuses on the contributions of inorganic and organic osmolytes and their transport systems in regulatory volume increase (RVI) and regulatory volume decrease (RVD) in a variety of cells. Furthermore, the current knowledge on signal transduction in volume regulation is compiled, revealing an astonishing diversity in transport systems, as well as of regulatory signals. The information available indicates the existence of intricate spatial and temporal networks that control cell volume and that we are just beginning to be able to investigate and to understand.

Expression and regulation of the Na(+)/K(+)/2Cl(-) cotransporter NKCC1 in rat liver and human HuH-7 hepatoma cells

The expression of sodium potassium chloride cotransporter 1 (NKCC1) was studied in different liver cell types. NKCC1 was found in rat liver parenchymal and sinusoidal endothelial cells and in human HuH-7 hepatoma cells. NKCC1 expression in rat hepatic stellate cells increased during culture-induced transformation in the myofibroblast-like phenotype. NKCC1 inhibition by bumetanide increased alpha(1)-smooth muscle actin expression in 2-day-cultured hepatic stellate cells but was without effect on basal and platelet-derived-growth-factor-induced proliferation of the 14-day-old cells. In perfused rat liver the NKCC1 made a major contribution to volume-regulatory K(+) uptake induced by hyperosmolarity. Long-term hyperosmotic treatment of HuH-7 cells by elevation of extracellular NaCl or raffinose concentration but not hyperosmotic urea or mannitol profoundly induced NKCC1 mRNA and protein expression. This was antagonized by the compatible organic osmolytes betaine or taurine. The data suggest a role of NKCC1 in stellate cell transformation, hepatic volume regulation, and long-term adaption to dehydrating conditions.

The involvement of calcium mobilization in the calcium-activated potassium currents activated by hyposmotic swelling in gastric antral circular myocytes of the guinea-pig

In our study of the effects of hyposmotic swelling on the Ca(2+)-activated potassium currents [I(K(Ca))] and its mechanism, we employed the whole-cell patch clamp technique using the gastric antral circular myocytes of the guinea-pig. Hyposmotic swelling efficiently increased I(K(Ca)), and the extent of changes in I(K(Ca)) was sharply dependent on the osmolarity of the perfusion solutions. When the calcium-free solution (EGTA 10 microM added in calcium-free solution) was superfused, I(K(Ca)) was not increased by the hyposmotic swelling. Gadolinium (Gd(3+)) 100 nM, a blocker of the stretch-activated nonselective cation channel, blocked the activation of I(K(Ca)) induced by hyposmotic swelling, but nicardipine 5 microM (the L-type calcium channel blocker) did not. Heparin 3 mg/ml, a potent inhibitor of inositol triphosphate receptor (InsP(3)R), did not inhibit the response, and caffeine 1 mM (the agonist for calcium-induced calcium release [CICR]) imitated the effect of hyposmotic swelling. Ryanodine (15 microM), markedly inhibited the effect. These results suggest that hyposmotic swelling activates I(K(Ca)), and the activation is associated with CICR, which is triggered by extracellular calcium influx through the stretch-activated channel (SA channel).

Hyposmotically induced amino acid release from the rat cerebral cortex: role of phospholipases and protein kinases

In an evaluation of the contribution of swelling-induced amino acid release, through the regulatory volume decrease (RVD) process, to cerebral ischemic injury, studies of the role of phospholipases and protein kinases in the response to hyposmotic stress were undertaken using an in vivo rat cortical cup model. Hyposmotic stress induced significant releases of aspartate, glutamate, glycine, phosphoethanolamine, taurine and GABA from the rat cerebral cortex. Taurine release was most affected, exhibiting a greater than 9-fold increase during the hyposmotic stimulus. The phospholipase A2 (PLA2) inhibitors 4-bromophenacyl bromide (1 microM) and 7,7-dimethyleicosadienoic acid (5 microM) had no significant effects on hyposmotically induced amino acid release. AACOCF3 (50 microM), an inhibitor of cytosolic PLA2 decreased taurine release to 84% of DMSO controls. The release of the other amino acids was not affected. The phospholipase C inhibitor U73122 (5 microM) had no significant effects on amino acid release. The protein kinase C (PKC) inhibitor chelerythrine (5 microM) significantly reduced hyposmotically induced taurine release to 72% of saline controls but had no significant effects on the other amino acids. Stimulation of PKC with phorbol 12-myristate, 13-acetate (10 microM) did not significantly change taurine, glutamate, glycine or phosphethanolamine release. The releases of aspartate and GABA were enhanced 2 to 3 fold. Phorbol 12,13-didecanoate (10 microM), another potent stimulator of PKC, significantly increased taurine release to 122% of DMSO controls. The releases of aspartate, glutamate and glycine were enhanced 2.5 to 3.5 fold. Similarly, stimulation of protein kinase A with forskolin (100 microM) significantly increased taurine, aspartate, and glycine release 1.5- to 2-fold compared to DMSO controls. In summary, phospholipases may play a minor role in volume regulation. These studies also support the hypothesis that protein kinases play a modulatory role in the RVD response. The results show that although RVD may play a role, additional mechanisms, including phospholipase activation, must be involved in the ischemia-evoked release of excitotoxic amino acids.

Release of osmolytes from perfused rat liver on perivascular nerve stimulation: alpha-adrenergic control of osmolyte efflux from parenchymal and nonparenchymal liver cells

The effects of perivascular nerve stimulation and phenylephrine on osmolyte release were studied in the intact perfused rat liver and isolated liver parenchymal cells (PC) and nonparenchymal cells. In the perfused liver, electrical stimulation of perivascular nerves (20 Hz/2 ms/20 V) led to a phentolamine-sensitive increase of cell hydration by 6.5% +/- 1.2% (n = 3) and a transient phentolamine-sensitive stimulation of taurine and inositol, but not betaine, release. These nerve effects were mimicked by phenylephrine, but not prostaglandin F2alpha, and were not affected by sodium nitroprusside (SNP) or ibuprofen. Nerve stimulation-induced taurine, but not inositol, release was inhibited by 4, 4'-di-isothiocyanatostilbene-2,2'-disulphonic acid (DIDS) (50 micromol/L). Single-cell fluorescence studies with isolated liver PC, Kupffer cells (KC), sinusoidal endothelial cells (SEC), and hepatic stellate cells (HSC) revealed that phenylephrine induced an increase in cytosolic free Ca2+ only in PC and HSC, but not in KC and SEC, whereas extracellular uridine triphosphate (UTP) produced Ca2+ transients/oscillations in all liver cell types studied. Phenylephrine had no effect on osmolyte release from isolated KC and SEC, but increased taurine (but not inositol) release from PC and inositol (but not taurine) efflux from HSC. The data suggest that: 1) liver cell hydration and-consecutively-osmolyte content are modulated by hepatic nerves via an alpha-adrenergic mechanism, which does not involve eicosanoids or hemodynamic changes; 2) that PC and HSC are the primary targets for nerve-dependent alpha-adrenergic activation, whereas 3) KC and SEC probably do not express alpha-adrenoceptors coupled to Ca2+ mobilization or osmolyte efflux.

Hypotonic Ca2+ signaling and volume regulation in proliferating and quiescent cells from multicellular spheroids

Hypotonicity-induced Ca2+ signals and volume regulation were studied in proliferating and quiescent subpopulations of multicellular prostate cancer spheroids. Enzymatic dissociation of multicellular spheroids 100+/-19 microm in diameter, which are entirely proliferative, yielded a population of cells with a mean cell diameter of 17.5+/-1.4 microm. After dissociation of spheroids in a size class of 200+/-30, 300+/-60, and 400+/-65 microm in diameter, two subpopulations of cells with mean cell diameters corresponding to 12.9+/-1.9 microm and 16.7+/-2 microm were discriminated. The subpopulation of large cells was shown to be proliferative by positive Ki-67 antibody staining; the subpopulation of small cells was Ki-67 negative, indicating cell quiescence. In a spheroid size class of 100+/-19 microm, a distinct subpopulation of quiescent cells was absent. Superfusion by hypotonic solutions revealed that only the proliferating cell fraction showed a regulatory volume decrease (RVD) and a [Ca2+]i transient. Both effects were absent in the quiescent cell population. The [Ca2+]i transient persisted in low (10 nM) Ca2+ solution and in the presence of 4 mM extracellular Ni2+ but was abolished in the presence of the endoplasmic reticulum Ca2+-ATPase blocker 2,5-di-tert-butyl-hydrochinone (t-BHQ). The t-BHQ likewise inhibited RVD, indicating that Ca2+ release from intracellular stores was necessary for RVD. Moreover, [Ca2+]i and RVD were dependent on an intact microfilament cytoskeleton because after 30 min of preincubation with cytochalasin B the [Ca2+]i transient was significantly reduced and RVD was abolished. The absence of RVD and [Ca2+]i transient in quiescent cells may be due to differences in the amount and the cytosolic arrangement of F-actin observed in quiescent cells.

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

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

Existence, properties, and functional expression of "Maxi-K"-type, Ca2+-activated K+ channels in short-term cultured hepatocytes

A large-conductance, Ca2+-activated K+ channel was identified and characterized in embryonic chick hepatocytes using the patch-electrode voltage-clamp technique. The channel conductance was 213 pS in excised patches bathed in symmetrical 145 mM KCI and 1 mM Ca2+. Current-voltage relationships were linear with high K+ on both sides of the membrane but showed constant field rectification as the K+ gradient was increased. The reversal potential shifted 58 mV per 10-fold change in the ratio of external to internal K+. Channel openings occurred at potentials higher than +50 mV in cell-attached patches. The open probability X voltage relationship shifted to more negative potentials in excised, inside-out patches exposed to a solution containing high Ca2+. The voltage sensitivity of the channel was not significantly affected by changes in internal Ca2+ concentration. Conversely, channel gating, reflected in the half-activation potential, shifted 118 mV per 10-fold change in internal Ca2+ at concentrations less than approximately 2 microM, although at higher Ca2+, this parameter was Ca2+ insensitive. Channel open probability in cell-attached patches increased significantly following exposure of the cells to either the Ca2+ ionophore A-23187 or L-alanine, a cell-volume modulator. Channel density increased with time spent in culture from no observations in 10-hr cells, through 13 and 80% of patches in 24-and 48-hr cultured cells, respectively. The implications of delayed functional expression for ion channel studies in acutely dissociated cells is discussed.

Current concepts of brain edema. Review of laboratory investigations

Klatzo's classification of brain edema into two types, vasogenic and cytotoxic, has been in general use since 1967. The former involves overall brain swelling due to fluid entry from the vasculature because of openings in the blood-brain barrier (BBB), whereas the latter refers to cell swelling without any loss of the normal impermeability of the BBB. This review principally covers new work that identifies the intracellular swelling of astrocytes as a major form of cytotoxic edema seen in many different kinds of brain injury. The term edema should be retained because of its familiarity; however, because such intracellular swelling is usually not a response to toxins, it is suggested that the term cellular edema is preferable to cytotoxic edema. The difficulties involved in measuring cellular edema clinically are discussed, and the belief that a "pure" form of either edema is unlikely to exist. It is emphasized that the mechanisms and direct consequences of vasogenic and cellular edema are so different that the connection is mainly semantic. Studies conducted in vitro have identified several potentially damaging secondary consequences of astrocytic swelling. One of the most important of these seems likely to be the increased release of excitatory amino acids from swollen astrocytes. Potential mechanisms for inhibition of the increased release of amino acids have been identified in vitro and could prove therapeutically useful.

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.

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.

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.

Regulation of liver cell volume and proteolysis by glucagon and insulin

The effects of insulin and glucagon on liver cell volume and proteolysis were studied in isolated perfused rat liver. The rate of proteolysis was assessed as [3H]leucine release from single-pass-perfused livers from rats which had been prelabelled in vivo by intraperitoneal injection of [3H]leucine. The intracellular water space was determined from the wash-out profiles of simultaneously added [3H]inulin and [14C]urea. In normo-osmotic (305 mosM) control perfusions the intracellular water space was 548 +/- 10 microliters/g wet mass (n = 44) and was increased by 16.5 +/- 2.6% (n = 6), i.e. by 85 +/- 14 microliters/g, after hypoosmotic exposure (225 mosM). Glucagon (0.1 microM) decreased the intracellular water space by 17 +/- 4% (n = 4), whereas insulin (35 nM) increased the intracellular water space by 9.3 +/- 1.4% (n = 15). Also, in isolated rat hepatocyte suspensions insulin (100 nM) caused cell swelling by 10.7 +/- 1.8% (n = 16), which was fully reversed by glucagon. In perfused liver, insulin-induced cell swelling was accompanied by a hepatic net K+ uptake (4.5 +/- 0.2 mumol/g) and an inhibition of proteolysis by 21 +/- 2% (n = 12); further addition of glucagon led to a net K+ release of 3.8 +/- 0.2 mumol/g (n = 7) and fully reversed the insulin effects on both cell volume and proteolysis. Similarly, insulin-induced cell swelling and inhibition of proteolysis were completely antagonized by hyperosmotic (385 mosM) cell shrinkage. Furthermore, cell swelling and inhibition of proteolysis after hypo-osmotic exposure or amino acid addition were reversed by glucagon-induced cell shrinkage. There was a close relationship between the extent of cell swelling and the inhibition of proteolysis, regardless of whether cell volume was modified by insulin, glucagon or aniso-osmotic exposure. The data show that glucagon and insulin are potent modulators of liver cell volume, at least in part by alterations of cellular K+ balance, and that their opposing effects on hepatic proteolysis can largely be explained by opposing effects on cell volume. It is hypothesized that hormone-induced alterations of cell volume may represent an important, not yet recognized, mechanism mediating hormonal effects on metabolism.