Regulation of cell volume in the perfused rat liver by hormones

Hepatocyte membrane potential regulates serum insulin and insulin sensitivity by altering hepatic GABA release

Hepatic lipid accumulation in obesity correlates with the severity of hyperinsulinemia and systemic insulin resistance. Obesity-induced hepatocellular lipid accumulation results in hepatocyte depolarization. We have established that hepatocyte depolarization depresses hepatic afferent vagal nerve firing, increases GABA release from liver slices, and causes hyperinsulinemia. Preventing hepatic GABA release or eliminating the ability of the liver to communicate to the hepatic vagal nerve ameliorates the hyperinsulinemia and insulin resistance associated with diet-induced obesity. In people with obesity, hepatic expression of GABA transporters is associated with glucose infusion and disposal rates during a hyperinsulinemic euglycemic clamp. Single-nucleotide polymorphisms in hepatic GABA re-uptake transporters are associated with an increased incidence of type 2 diabetes mellitus. Herein, we identify GABA as a neuro-hepatokine that is dysregulated in obesity and whose release can be manipulated to mute or exacerbate the glucoregulatory dysfunction common to obesity.

Evidence for functional selectivity in TUDC- and norUDCA-induced signal transduction via α5β1 integrin towards choleresis

Functional selectivity is the ligand-specific activation of certain signal transduction pathways at a receptor and has been described for G protein-coupled receptors. However, it has not yet been described for ligands interacting with integrins without αI domain. Here, we show by molecular dynamics simulations that four side chain-modified derivatives of tauroursodeoxycholic acid (TUDC), an agonist of α₅β₁ integrin, differentially shift the conformational equilibrium of α₅β₁ integrin towards the active state, in line with the extent of β₁ integrin activation from immunostaining. Unlike TUDC, 24-nor-ursodeoxycholic acid (norUDCA)-induced β₁ integrin activation triggered only transient activation of extracellular signal-regulated kinases and p38 mitogen-activated protein kinase and, consequently, only transient insertion of the bile acid transporter Bsep into the canalicular membrane, and did not involve activation of epidermal growth factor receptor. These results provide evidence that TUDC and norUDCA exert a functional selectivity at α₅β₁ integrin and may provide a rationale for differential therapeutic use of UDCA and norUDCA.

α5 β1-integrins are sensors for tauroursodeoxycholic acid in hepatocytes

Ursodeoxycholic acid, which in vivo is converted to its taurine conjugate tauroursodeoxycholic acid (TUDC), is a mainstay for the treatment of cholestatic liver disease. Earlier work showed that TUDC exerts its choleretic properties in the perfused rat liver in an α5 β1 integrin-mediated way. However, the molecular basis of TUDC-sensing in the liver is unknown. We herein show that TUDC (20 μmol/L) induces in perfused rat liver and human HepG2 cells the rapid appearance of the active conformation of the β1 subunit of α5 β1 integrins, followed by an activating phosphorylation of extracellular signal-regulated kinases. TUDC-induced kinase activation was no longer observed after β1 integrin knockdown in isolated rat hepatocytes or in the presence of an integrin-antagonistic hexapeptide in perfused rat liver. TUDC-induced β1 integrin activation occurred predominantly inside the hepatocyte and required TUDC uptake by way of the Na(+) /taurocholate cotransporting peptide. Molecular dynamics simulations of a 3D model of α5 β1 integrin with TUDC bound revealed significant conformational changes within the head region that have been linked to integrin activation before.

CONCLUSIONS

TUDC can directly activate intrahepatocytic β1 integrins, which trigger signal transduction pathways toward choleresis. (HEPATOLOGY 2013).

The Src family kinase Fyn mediates hyperosmolarity-induced Mrp2 and Bsep retrieval from canalicular membrane

In perfused rat liver, hyperosmolarity induces Mrp2- (Kubitz, R., D'urso, D., Keppler, D., and Häussinger, D. (1997) Gastroenterology 113, 1438-1442) and Bsep retrieval (Schmitt, M., Kubitz, R., Lizun, S., Wettstein, M., and Häussinger, D. (2001) Hepatology 33, 509-518) from the canalicular membrane leading to cholestasis. The aim of this study was to elucidate the underlying signaling events. Hyperosmolarity-induced retrieval of Mrp2 and Bsep from the canalicular membrane in perfused rat liver was accompanied by an activating phosphorylation of the Src kinases Fyn and Yes but not of c-Src. Both hyperosmotic transporter retrieval and Src kinase activation were sensitive to apocynin (300 μmol/liter), N-acetylcysteine (NAC; 10 mmol/liter), and SU6656 (1 μmol/liter). Also PP-2 (250 nmol/liter), which inhibited hyperosmotic Fyn but not Yes activation, prevented hyperosmotic transporter retrieval from the canalicular membrane, suggesting that Fyn but not Yes mediates hyperosmotic Bsep and Mrp2 retrieval. Neither hyperosmotic Fyn activation nor Bsep/Mrp2 retrieval was observed in livers from p47(phox) knock-out mice. Hyperosmotic activation of JNKs was sensitive to apocynin and NAC but insensitive to SU6656 and PP-2, indicating that JNKs are not involved in transporter retrieval, as also evidenced by experiments using the JNK inhibitors L-JNKI-1 and SP6001255, respectively. Hyperosmotic transporter retrieval was accompanied by a NAC and Fyn knockdown-sensitive inhibition of biliary excretion of the glutathione conjugate of 1-chloro-2,4-dinitrobenzene in perfused rat liver and of cholyl-L-lysyl-fluorescein secretion into the pseudocanaliculi formed by hepatocyte couplets. Hyperosmolarity triggered an association between Fyn and cortactin and increased the amount of phosphorylated cortactin underneath the canalicular membrane. It is concluded that the hyperosmotic cholestasis is triggered by a NADPH oxidase-driven reactive oxygen species formation that mediates Fyn-dependent retrieval of the Mrp2 and Bsep from the canalicular membrane, which may involve an increased cortactin phosphorylation.

Activation of P2Y receptors causes strong and persistent shrinkage of C11-MDCK renal epithelial cells

Purinergic receptors activate diverse signaling cascades and regulate the activity of cell volume-sensitive ion transporters. However, the effects of ATP and other agonists of P2 receptors on cell volume dynamics are only scarcely studied. In the present work, we used the recently developed dual-image surface reconstruction technique to explore the influence of purinergic agonists on cell volume in the C11-Madin-Darby canine kidney cell line resembling intercalated cells from kidney collecting ducts. Unexpectedly, we found that ATP and UTP triggered very robust (55-60%) cell shrinkage that lasted up to 2 h after agonist washout. Purinergic regulation of cell volume required increases in intracellular Ca(2+) and could be partially mimicked by the Ca(2+)-ionophore ionomycin or activation of protein kinase C by 4β-phorbol 12-myristate 13-acetate. Cell shrinkage was accompanied by strong reductions in intracellular K(+) and Cl(-) content measured using steady-state (86)Rb(+) and (36)Cl(-) distribution. Both shrinkage and ion efflux in ATP-treated cells were prevented by the anion channel blocker 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and by the BK(Ca) channel inhibitors charybdotoxin, iberiotoxin, and paxilline. To evaluate the significance of cell-volume changes in purinergic signaling, we measured the impact of ATP on the expression of the immediate-early gene c-Fos. Thirty-minute treatment with ATP increased c-Fos immunoreactivity by approximately fivefold, an effect that was strongly inhibited by charybdotoxin and completely prevented by NPPB. Overall, our findings suggest that ATP-induced cell-volume changes are partially responsible for the physiological actions of purinergic agonists.

Activation of integrins by urea in perfused rat liver

High concentrations of urea were shown to induce a paradoxical regulatory volume decrease response with K(+) channel opening and subsequent hepatocyte shrinkage (Hallbrucker, C., vom Dahl, S., Ritter, M., Lang, F., and Häussinger, D. (1994) Pflügers Arch. 428, 552-560), although the hepatocyte plasma membrane is thought to be freely permeable to urea. The underlying mechanisms remained unclear. As shown in the present study, urea (100 mmol/liter) induced within 1 min an activation of β(1) integrins followed by an activation of focal adhesion kinase, c-Src, p38(MAPK), extracellular signal-regulated kinases, and c-Jun N-terminal kinase. Because α(5)β(1) integrin is known to act as a volume/osmosensor in hepatocytes, which becomes activated in response to hepatocyte swelling, the findings suggest that urea at high concentrations induces a nonosmotic activating perturbation of this osmosensor, thereby triggering a volume regulatory K(+) efflux. In line with this, similar to hypo-osmotic hepatocyte swelling, urea induced an inhibition of hepatic proteolysis, which was sensitive to p38(MAPK) inhibition. Molecular dynamics simulations of a three-dimensional model of the ectodomain of α(5)β(1) integrin in water, urea, or thiourea solutions revealed significant conformational changes of α(5)β(1) integrin in urea and thiourea solutions, in contrast to the simulation of α(5)β(1) in water. These changes lead to an unbending of the integrin structure around the genu, which may suggest activation, whereas the structures of single domains remained essentially unchanged. It is concluded that urea at high concentrations affects hepatic metabolism through direct activation of the α(5)β(1) integrin system.

Volume-sensitive outwardly rectifying chloride channel in white adipocytes from normal and diabetic mice

The volume-sensitive outwardly rectifying (VSOR) chloride channel is ubiquitously expressed and involved in cell volume regulation after osmotic swelling, called regulatory volume decrease (RVD), in various cell types. In adipocytes, the expression of the VSOR channel has not been explored to date. Here, by employing the whole-cell patch-clamp technique, we examined whether or not the VSOR channel is expressed in white adipocytes freshly isolated from epididymal fat pads of normal (C57BL/6 or KK) and diabetic (KKA(y)) mice. Whole cell voltage-clamp recordings revealed that Cl(-) currents were gradually activated upon cell swelling induced by application of a hypotonic solution, both in normal and diabetic adipocytes. Although both the mean cell size (or cell capacitance) and the current magnitude in KKA(y) adipocytes were larger than those in C57BL/6 cells, the current density was significantly lower in KKA(y) adipocytes (23.32 +/- 1.94 pA in C57BL/6 adipocytes vs. 13.04 +/- 2.41 pA in KKA(y) adipocytes at +100 mV). Similarly, the current density in diabetic KKA(y) adipocytes was lower than that in adipocytes from KK mice (a parental strain of KKA(y) mice), which do not present diabetes until an older age. The current was inhibited by Cl(-) channel blockers, 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB) and glibenclamide, or hypertonic solution, and showed outward rectification and inactivation kinetics at large positive potentials. These electrophysiological and pharmacological properties are consistent with those of the VSOR channel in other cell types. Moreover, adipocytes showed RVD, which was inhibited by NPPB. In KKA(y) adipocytes, RVD was significantly slower (tau; 8.42 min in C57BL/6 adipocytes vs. 11.97 min in KKA(y) adipocytes) and incomplete during the recording period (25 min). It is concluded that the VSOR channel is functionally expressed and involved in volume regulation in white adipocytes. RVD is largely impaired in adipocytes from diabetic mice, presumably as a consequence of the lower density of the functional VSOR channel in the plasma membrane.

Effect of creatine supplementation on creatine and glycogen content in rat skeletal muscle

The effects of high dose creatine feeding (5 g kg(-1) BW day(-1), 5 days) on creatine content, glucose transport, and glycogen accumulation in white gastrocnemius, red gastrocnemius and soleus muscles of the rat was investigated. Isolated rat hindquarters of creatine fed and control rats were perfused with a standard medium containing either insulin alone (0, 100 or 20 000 microU mL(-1)) or in combination with creatine (2 or 10 mmol L(-1)). Furthermore, plasma insulin concentration was measured in normal rats during creatine feeding, as well as in anaesthetized rats during intravenous creatine infusion. Five days of creatine feeding increased (P < 0.05) total creatine content in soleus (+ 20%) but not in red gastrocnemius (+15%, n.s.) and white gastrocnemius (+ 10%, n.s.). In parallel, glycogen content was markedly elevated (P < 0.05) in soleus (+ 40%), less (P < 0.05) in red gastrocnemius (+ 15%), and not in white gastrocnemius (+ 10%, n.s.). Glucose transport rate, muscle GLUT-4 content, glycogen synthase activity in perfused muscles and glycogen synthesis rate were not significantly altered by creatine feeding in either muscle type. Furthermore, high dose creatine feeding raised (P < 0.05) plasma creatine concentration fivefold but did not alter circulating insulin level. It is concluded that short-term high dose creatine feeding enhances creatine disposal and glycogen storage in rat skeletal muscle. However, the creatine and glycogen response to creatine supplementation is markedly greater in oxidative than in glycolytic muscles.

Calcium signaling in liver

In hepatocytes, hormones linked to the formation of the second messenger inositol 1,4,5-trisphosphate (InsP3) evoke transient increases or spikes in cytosolic free calcium ([Ca2+]i), that increase in frequency with the agonist concentration. These oscillatory Ca2+ signals are thought to transmit the information encoded in the extracellular stimulus to down-stream Ca2+-sensitive metabolic processes. We have utilized both confocal and wide field fluorescence microscopy techniques to study the InsP3-dependent signaling pathway at the cellular and subcellular levels in the intact perfused liver. Typically InsP3-dependent [Ca2+]i spikes manifest as Ca2+ waves that propagate throughout the entire cytoplasm and nucleus, and in the intact liver these [Ca2+]i increases are conveyed through gap junctions to encompass entire lobular units. The translobular movement of Ca2+ provides a means to coordinate the function of metabolic zones of the lobule and thus, liver function. In this article, we describe the characteristics of agonist-evoked [Ca2+]i signals in the liver and discuss possible mechanisms to explain the propagation of intercellular Ca2+ waves in the intact organ.

The nucleolus exhibits an osmotically regulated gatekeeping activity that controls the spatial dynamics and functions of nucleolin

We demonstrate that physiologically relevant perturbations in the osmotic environment rheostatically regulate a gatekeeping function for the nucleolus that controls the spatial dynamics and functions of nucleolin. HeLa cells and U2-OS osteosarcoma cells were osmotically challenged with 100-200 mm sorbitol, and the intranuclear distribution of nucleolin was monitored by confocal microscopy. Nucleolin that normally resides in the innermost fibrillar core of the nucleolus, where it assists rDNA transcription and replication, was expelled within 30 min of sorbitol addition. The nucleolin was transferred into the nucleoplasm, but it distributed there non-uniformly; locally high levels accumulated in 4',6-diamidino-2-phenylindole-negative zones containing euchromatic (transcriptionally active) DNA. Inositol pyrophosphates also responded within 30 min of hyperosmotic stress: levels of bisdiphosphoinositol tetrakisphosphate increased 6-fold, and this was matched by decreased levels of its precursor, diphosphoinositol pentakisphosphate. Such fluctuations in inositol pyrophosphate levels are of considerable interest, because, according to previously published in vitro data, they regulate the degree of phosphorylation of nucleolin through a novel kinase-independent phosphotransferase reaction ( Saiardi, A., Bhandari, A., Resnick, R., Cain, A., Snowman, A. M., and Snyder, S. H. (2004) Science 306, 2101-2105 ). However, by pharmacologically intervening in inositol pyrophosphate metabolism, we found that it did not supervise the osmotically driven switch in the biological activities of nucleolin in vivo.

Cytosolic glutathione peroxidase from liver of pacu (Piaractus mesopotamicus), a hypoxia-tolerant fish of the Pantanal

Pacu (Piaractus mesopotamicus Holmberg, 1887, Characiformes) dwells in waters of Pantanal, in which it has adapted for alternate concentrations of dissolved oxygen. Intracellular antioxidant protection should be vital for such an adaptation. Accordingly, we found that cytosol from liver of pacu has the highest antioxidant glutathione peroxidase activity so far reported for fish and murine species. To clarify whether this activity was due to a selenium independent glutathione S-transferase or to a glutathione peroxidase, we purified it and studied its kinetics. The substrates cumene hydroperoxide and hydrogen peroxide were promptly reduced by the enzyme, but peroxidized phosphatidylcholine had to undergo previous fatty acid removal with phospholipase A(2). Augmenting concentrations (from 2 to 6 mM) of reduced glutathione activated the pure enzyme. Curves of velocity versus different micromolar concentrations of hydrogen peroxide in the presence of 2, 4 or 8 mM reduced glutathione indicated that at least 2.5 mM reduced glutathione should be available in vivo for an efficient continuous destruction of micromolar concentrations of hydrogen peroxide by this peroxidase. Molecular exclusion HPLC and SDS-polyacrylamide gel electrophoresis indicated that the purified peroxidase is a homotetramer. Data from internal sequences showed selenocysteine in its primary structure and that the enzyme was a homologue of the type-1 glutathione peroxidase found in rat, bull, trout, flounder and zebra fish. Altogether, our data establish that in liver cells of pacu, a hypoxia-tolerant fish from South America, there are high levels of a cytosolic GPX-1 capable of quenching hydrogen peroxide and fatty acid peroxides, providing an effective antioxidant action.

Hyperammonaemia in V1a vasopressin receptor knockout mice caused by the promoted proteolysis and reduced intrahepatic blood volume

An analysis of arginine-vasopressin (AVP) V1a receptor-deficient (V1aR-/-) mice revealed that glucose homeostasis and lipid metabolism were altered in the mutant mice. Here, we used V1aR-/- mice to investigate whether the deficiency of the V1a receptor, which led to altered insulin sensitivity, affected protein metabolism. The serum 3-methylhistidine levels were increased in V1aR-/- mice under feeding conditions, indicating that proteolysis was enhanced in muscle tissue from V1aR-/- mice. Furthermore, serum amino acid profiling revealed that the amino acid levels, including glycogenic and branched-chain amino acids, were reduced in V1aR-/- mice. In addition, an alanine-loading test showed that gluconeogenesis was enhanced in V1aR-/- mice. Blood ammonia, which is a by-product of amino acid catabolism, was two times higher in V1aR-/- mice without hepatopathy under the feeding and fasting conditions than in wild-type mice. Amino acid profiling also revealed that the amino acid pattern was not typical of a urea-cycle enzymatic disorder. An ammonia tolerance test and an indocyanine green elimination test showed that V1aR-/- mice had lower ammonia clearance due to a decreased intrahepatic circulating blood volume. Metabolic acidosis, including lactic- and keto-acidosis, was not observed in V1aR-/- mice. These results provide evidence that proteolysis promotes the production of glucose in the muscles of V1aR-/- mice and that hyperammonaemia is caused by promoted protein catabolism and reduced intrahepatic blood volume. Thus, our study with V1aR-/- mice indicates that AVP plays a physiological role via the V1a receptor in regulating both protein catabolism and glucose homeostasis.

Effect of extracellular osmolality on cell volume and resting metabolism in mammalian skeletal muscle

The purpose of the present investigation was to establish an in vitro mammalian skeletal muscle model to study acute alterations in resting skeletal muscle cell volume. Isolated, whole muscles [soleus and extensor digitorum longus (EDL)] were dissected from Long-Evans rats and incubated for 60 min in Sigma medium 199 (1 g of resting tension, bubbled with 95% O2-5% O2, 30 ± 2°C, and pH 7.4). Medium osmolality was altered to simulate hyposmotic (190 ± 10 mmol/kg) or hyperosmotic conditions (400 ± 10 mmol/kg), whereas an isosmotic condition (290 ± 10 mmol/kg) served as a control. After incubation, relative water content of the muscle decreased with hyperosmotic and increased with hyposmotic condition in both muscle types ( P < 0.05). The cross-sectional area of soleus type I and type II fibers increased ( P < 0.05) in hyposmotic, whereas hyperosmotic exposure led to no detectable changes. The EDL type II fiber area decreased in the hyperosmotic condition and increased after hyposmotic exposure, whereas no change was observed in EDL type I fibers. Furthermore, exposure to the hyperosmotic condition in both muscle types resulted in decreased muscle ATP and phosphocreatine ( P < 0.05) contents and increased creatine and lactate contents ( P < 0.05) compared with control and hyposmotic conditions. This isolated skeletal muscle model proved viable and demonstrated that altering extracellular osmolality could cause acute alterations in muscle water content and resting muscle metabolism.

Osmosensing by integrins in rat liver

Changes in hepatocyte hydration are induced not only by ambient hypo- or hyperosmolarity, but also under isosmotic condition by hormones, substrates, and oxidative stress. The perfused rat liver is a well-established intact organ model with preservation of the three-dimensional hepatocyte anchoring to the extracellular matrix and/or adjacent cells, parenchymal cell polarity, liver cell heterogeneity, acinar construction, and gene expression gradients. Originally, data from the perfused rat liver indicated that changes of cell hydration independent of their origin critically contribute to the control of autophagic proteolysis and canalicular bile acid excretion. Meanwhile, the concept that cell hydration changes trigger signal transduction processes that control metabolism, gene expression, transport, and the susceptibility to stress is well accepted. This chapter summarizes evidence obtained from experiments with the perfused rat liver that integrins are osmosensors in the liver and thereby critically contribute to the Src- and MAP-kinase-dependent inhibition of autophagic proteolysis, stimulation of canalicular taurocholate excretion, and regulatory volume decrease as induced by hypoosmotic swelling. Moreover, integrin-dependent sensing of hepatocyte swelling is essential for signaling and proteolysis inhibition by insulin and glutamine. These findings define a novel role of integrins in insulin and glutamine signaling and set an example for mechanotransduction as an integral part of overall growth factor and nutrient signaling.

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.

Inhibitory effects of berberine on ion channels of rat hepatocytes

AIM: To examine the effects of berberine, an isoquinoline alkaloid with a long history used as a tonic remedy for liver and heart, on ion channels of isolated rat hepatocytes.

METHODS: Tight-seal whole-cell patch-clamp techniques were performed to investigate the effects of berberine on the delayed outward potassium currents (I_K), inward rectifier potassium currents (I_K1) and Ca²⁺ release-activated Ca²⁺ currents (I_CRAC) in enzymatically isolated rat hepatocytes.

RESULTS: Berberine 1-300 μmol/L reduced I_K in a concentration-dependent manner with EC₅₀ of 38.86 ± 5.37 μmol/L and n_H of 0.82 ± 0.05 (n = 8). When the bath solution was changed to tetraethylammonium (TEA) 8 mmol/L, I_K was inhibited. Berberine 30 μmol/L reduced I_K at all examined membrane potentials, especially at potentials positive to +60 mV (n = 8, P < 0.05 or P < 0.01 vs control). Berberine had mild inhibitory effects on I_K1 in rat hepatocytes. Berberine 1-300 μmol/L also inhibited I_CRAC in a concentration-dependent fashion. The fitting parameters were EC₅₀ = 47.20 ± 10.86 μmol/L, n_H = 0.71 ± 0.09 (n = 8). The peak value of I_CRAC in the I-V relationship was decreased by berberine 30 μmol/L at potential negative to -80 mV (n = 8, P < 0.05 vs control). But the reverse potential of I_CRAC occurred at voltage 0 mV in all cells.

CONCLUSION: Berberine has inhibitory effects on potassium and calcium currents in isolated rat hepatocytes, which may be involved in hepatoprotection.

An hypothesis on the consolidation and PGE1-induced deconsolidation of a platelet plug

Consolidation is the final stage in haemostasis in which a platelet plug blocking a bleeding area of a vessel: (a) becomes impermeable to circulating plasma proteins and (b) contracts to resist blood pressure.

HYPOTHESIS

The impermeabilization step of consolidation is accomplished through fluid uptake by the platelets from a hydrated intercellular glue formed during thrombin activation. Dehydration occurs through inhibition of the Na+,K+-ATPase of platelets with sodium and water uptake. However, and uniquely, due to the high cellular density of the platelet plug, access of peripheral plasma fluids to the plug is limited forcing the platelets to take up preferentially the fluid of interplatelet space. The increased adhesion properties of the dehydrated glue simultaneously furthers a decreased hydraulic permeability and an improved coupling of the contractile forces among platelets. In 'Deconsolidation', the fluid uptake process can be reversed and amplified by agents that increase cAMP, reactivating the Na+,K+-ATPase and expressing CFTR or equivalent Cl- secretory channels that force the extrusion of fluid from the platelets, with rehydration of the intercellular polymer and a large increase in the interplatelet space.

Neural control of hepatic osmolytes and parenchymal cell hydration

Liver cell function is strongly influenced by cell hydration and osmolyte content via osmosensing and osmosignaling pathways. Electrical stimulation of hepatic nerves increases the hepatocellular hydration state via an alpha-adrenergic mechanism, promotes taurine release from parenchymal cells and myo-inositol release from hepatic stellate cells. Although changes in liver cell hydration and osmolyte content are known regulators of liver cell function, the functional relevance and integration of nerve-stimulation-dependent alterations of liver cell volume and osmolyte content in the overall hepatic response to towards signals from the nervous system remains to be established.

Involvement of Integrins and Src in Insulin Signaling toward Autophagic Proteolysis in Rat Liver

Cell volume changes critically determine hepatic signal transduction and metabolism. Hepatocyte swelling by insulin contributes to p38(MAPK) activation leading to inhibition of autophagic proteolysis. Recently integrins were shown to sense hypoosmotic hepatocyte swelling. Here the role of integrins, Src, and focal adhesion kinase (FAK) in insulin signaling was investigated using the intact organ model of perfused rat liver. Insulin increases [Tyr(P)(418)]Src, [Tyr(P)(397)]FAK, and dual p38(MAPK) phosphorylation by about 2-fold. Infusion of the integrin-antagonizing hexapeptide GRGDSP or the Src inhibitor PP-2 prevented activation of Src and p38(MAPK) and, consequently, proteolysis inhibition by insulin. However, insulin-induced phosphorylation of IRbeta (Tyr(1158)) and protein kinase B (PKB, Ser(473)), as well as K(+)-uptake and cell swelling, was not reduced by the inhibitors. Both hypoosmotic swelling and insulin increase the plasma membrane levels of activated beta(1) integrin. Inhibition of insulin-induced swelling by furosemide largely abolished activation of beta(1) integrin and phosphorylation of Src, but not of PKB. Rapamycin does not affect either insulin-induced K(+)-retention and cell swelling or proteolysis inhibition, indicating that swelling-dependent proteolysis inhibition occurs independently from the mammalian target of rapamycin. The data suggest that sensing of cell swelling by integrins essentially contributes to insulin signaling, thereby defining a novel way of integrin involvement in growth factor signaling.

Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Effects of palmatine on potassium and calcium currents in isolated rat hepatocytes

AIM: To study the effects of palmatine, a known inhibitor on delayed rectifier potassium current and L-type calcium current (I_Ca,L) in guinea pig ventricular myocytes, on the potassium and calcium currents in isolated rat hepatocytes.

METHODS: Tight-seal whole-cell patch-clamp techniques were performed to investigate the effects of palmatine on the delayed outward potassium currents (I_K), inward rectifier potassium current (I_K1) and Ca²⁺ release-activated Ca²⁺ current (I_CRAC) in enzymatically isolated rat hepatocytes.

RESULTS: Palmatine 0.3-100 μM reduced I_K in a concentration-dependent manner with EC₅₀ of 41.62 ± 10.11 μM and nH, 0.48 ± 0.07 (n = 8). The effect of the drug was poorly reversible after washout. When the bath solution was changed to tetraethylammonium (TEA) 8 mM, I_K was inhibited. Palmatine 10 μM and 100 μM shifted the I-V curves of I_K downward, and the block of I_K was voltage-independent. Palmatine 0.3-100 μM also inhibited I_CRAC in a concentration-dependent manner. The fitting parameters were as follows: EC₅₀ = 51.19 ± 15.18 mM, and n_H = 0.46 ± 0.07 (n = 8). The peak value of I_CRAC in the I-V relationship was decreased by palmatine 10 μM and 100 μM. But the reverse potential of I_CRAC occurred at Voltage = 0 mV in all cells. Palmatine 0.3-100 μM failed to have any significant effect on either inward or outward components of I_K1 at any membrane potential examined.

CONCLUSION: The inhibitory effects on I_K and I_CRAC could be one of the mechanisms that palmatine exerts protective effect on hepatocytes.

Changes in distribution spaces and cell permeability caused by ATP in the rat liver

AIMS/BACKGROUND

Cellular and extracellular volume changes caused by ATP were investigated in the liver as well as the possible formation of diffusion barriers, which could be responsible for some of its metabolic effects.

METHODS

The experimental system was the bivascularly perfused rat liver. [(14)C]Sucrose and [(3)H]water were simultaneously injected into either the portal vein or the hepatic artery. Mean transit times, distribution spaces, variances and linear superimpositions were calculated.

RESULTS

In the portal system, ATP reduced the transit time in the great vessels, had little or no effect on sinusoidal and cellular spaces, but impaired the flow-limited distribution of both [(14)C]sucrose and [(3)H]water. In the arterial bed ATP infused into either the portal vein or the hepatic artery produced vasodilation and increased the aqueous extra-sucrose space. These effects were inhibited by Nomega-nitro-L-arginine methyl ester infused into the hepatic artery.

CONCLUSIONS

Sucrose and extra-sucrose space changes caused in the arterial bed by portally infused ATP are most probably analogous to the transhepatic vasodilation effect already described for the rabbit liver. Impairment of flow-limited distribution of tracers in the sinusoidal bed indicates that ATP induces the formation of permeability barriers, which could be responsible for some of its metabolic effects.

Intracellular calcium in the isolated rat liver: correlation to glucose release, K(+) balance and bile flow

This study correlates whole organ measurements of intracellular calcium concentration ([Ca(2+)](i)) with hormone-induced (epinephrine, vasopressin) changes of liver functions (glucose release, K(+) balance and bile flow). [Ca(2+)](i) was measured in the isolated perfused rat liver using the sensor Fura-2 and applying liver surface fluorescence spectroscopy. The technique was improved by (i) minimizing biliary elimination of the sensor by employing a rat strain deficient in canalicular organic anion transport (TR(-) mutation) and (ii) by correcting for changes of interfering intrinsic organ fluorescence that was shown to depend on the oxidation-reduction state (NAD(P)H content) of the organ. Epinephrine (50 nM) elicits an instantaneous peak rise of [Ca(2+)](i) to approx. 400 nM, followed by a sustained elevation that depends on the presence of extracellular Ca(2+). The rise of [Ca(2+)](i) coincides with initiation of glucose release, transient K(+) uptake, and transient stimulation of bile flow. Vasopressin (2 nM) exerts qualitatively similar effects. The transient rise of bile flow is attributed to Ca(2+)-mediated contraction of the pericanalicular actin-myosin web of hepatocytes.

Insulin resistance induced by loop diuretics and hyperosmolarity in perfused rat liver

Insulin-induced cell swelling was recently suggested to reflect an independent signal for metabolic insulin effects such as inhibition of hepatic proteolysis, which is transmitted at the level of autophagosome formation via p38MAPK activation [Häussinger et al., Gastroenterology 116 (1999), 921-935]. Here, the role of insulin-induced cell swelling in the overall context of insulin signalling towards proteolysis inhibition was studied in perfused rat liver. Loop diuretics and hyperosmolarity, which impair insulin-stimulated cell swelling, strongly blunt Erk-2 and p38MAPK activation as well as proteolysis inhibition by insulin, but are without effect on insulin-induced tyrosine phosphorylation of IR-beta and IRS-1. Inhibitors of phosphatidylinositol-3-kinase (PI3-kinase) also block insulin-induced cell swelling, MAP kinase activation and proteolysis inhibition, but the antiproteolytic response to hypoosmolarity remains unaffected. We suggest that PI3-kinase-mediated cell swelling induced by insulin is required to amplify the insulin signal to MAP kinases and thus proteolysis regulation. The perturbation of insulin-induced cell swelling may be of pathophysiological relevance for the development of insulin resistance in clinical situations associated with hyperosmotic dehydration and loop diuretic treatment.

Cell hydration controls autophagosome formation in rat liver in a microtubule-dependent way downstream from p38MAPK activation

Autophagic proteolysis in rat liver is under the control of the cellular hydration state. Because the morphological site of swelling-dependent proteolysis regulation has not yet been identified, the formation of autophagosomes was investigated with transmission electron microscopy in slices from perfused livers. In livers from fed rats, hypo-osmotic exposure (185 mosmol/l) led within 30 min to a decrease in fractional cytoplasmic autophagosome volume that was sensitive to colchicine and p38(MAPK) inhibition. Similarly, the decrease in autophagosome volume, but not the increase in cell volume caused by insulin or glutamine/glycine, was strongly inhibited by colchicine and SB 203580, an inhibition of p38(MAPK) activation. Immune complex assays from perfused liver showed that hypo-osmotic activation of p38(MAPK) was not inhibited by colchicine. Further, experiments using confocal laser microscopy in cultivated hepatocytes incubated with mouse-derived anti-(alpha-tubulin) showed that microtubular structures were not influenced by the inhibition of p38(MAPK) by SB 203580. It is concluded that the sequestration of autophagic vacuoles is a major site of proteolysis regulation by cell hydration. Swelling-induced activation of p38(MAPK) is required for this process and occurs upstream of the putative microtubule regulation site.

Vacuolar hepatocyte degeneration induced by infusion of 20% glucose solution with insulin after hepatopancreatectomy in rats

To investigate the causes of hepatic dysfunction after extensive resection of the liver together with pancreatectomy, rats were subjected to sham operation, to 68% hepatectomy alone, to 90% pancreatectomy alone, or to 68% hepatectomy combined with 90% pancreatectomy (hepatopancreatectomy). Solutions of 5% or 20% glucose were infused post-operatively for 48 h at a constant rate (250 ml/kg body weight/day) under fasting conditions. To improve the survival rates of pancreatectomized and hepatopancreatectomized rats given 20% glucose, it was necessary to use insulin. In hepatopancreatectomized rats, infusion of 20% glucose with insulin (1 U/5 g glucose) induced prominent hepatocyte vacuolar degeneration and mitochondrial swelling, associated with reduced hepatic protein content. The severity of histological changes was proportional to the insulin dose and the activity of hepatic glucokinase, a key glycolytic enzyme. were observed in These histological changes pancreatectomized rats albeit in a milder form, but not in sham-operated or hepatectomized rats given 20% glucose nor in any rats given 5% glucose. Our results suggest that hepatopancreatectomy followed post-operatively by a high glucose load and exogenously administered insulin enhances the development of hepatocyte swelling.

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.

Prostaglandin E1-induced deconsolidation of thrombin-activated platelet aggregates I: ultrastructure-computer image analysis

We have compared, at an ultrastructural-computer image morphometric level, the relaxation induced by Mg-ethylene-bis-oxyethylenenitrilo-tetracetic acid and prostaglandin E1 on a model of a thrombin-activated platelet aggregate. Mg-ethylene-bisoxyethylenenitrilo-tetracetic acid produced a small increase of 5.0% of the intercellular space over the control levels, and a decrease of 10.0+/-1.3% of the cross-sectional area of the platelets, with no apparent cytoskeletal alterations. In contrast, the prostaglandin El-treated preparation shows a 360% increase in the intercellular space and a decrease of the average platelet cross-sectional area of 30.0+/-2.0% with marked cytoskeletal alterations. We use the term "deconsolidation" to describe this effect. The enlargement of the intercellular space allows the observation of two types of contacts: (1) a type S (segmental) complex, of approximately 200-nm length that maintains a narrow interplatelet gap of 20-30 nm, filled with a dense intercellular material, and (2) a type R (reticular) complex, formed by scant focal regions of the plasma membrane from opposing platelets that are connected through a mesh of fibrillar or granular material contained within a variable-size space. We hypothesize that deconsolidation is caused by fluid loss from the platelets into the intercellular space. As a result, platelet volume decreases and intercellular space increases.

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.

Glucagon increases glutamine uptake without affecting glutamine release in humans

Glucagon causes transient hyperglycemia and persistent hypoaminoacidemia, but the mechanisms of this action are unclear. To address this question, the present study measured the effects of glucagon on glucose, leucine, phenylalanine, and glutamine kinetics. Seven healthy subjects each underwent three pancreatic clamp studies (octreotide 30 ng/kg/min, insulin 0.15 mU/kg/min, and glucagon 1.4 ng/kg/min) lasting 7 hours. During the last 3.5 hours of the studies, glucagon infusion was either unchanged (study 0) or increased to 4 and 7 ng/kg/min (studies 1 and 2). The higher glucagon infusion rates increased the glucagon concentration by 50% and 100%, respectively. [6,6-(2)H2]glucose, [2-(15)N]glutamine, 2H5-phenylalanine, and 2H3-leucine were infused to quantify the respective fluxes. Glucagon transiently increased glucose concentrations by stimulating glucose production, which peaked in 15 minutes to 3.82 +/- 0.36 and 4.21 +/- 0.33 mg/kg/min in studies 1 and 2 and then returned to the postabsorptive levels. Glucagon decreased the glutamine concentration (-10% +/- 2% and -22% +/- 2% in studies 1 and 2 v study 0, P < .05), because glutamine uptake became greater than glutamine release (balance from -1.9 +/- 0.9 in study 0 to -8.1 +/- 1.1 and -13.6 +/- 1.0 micromol/kg/h in studies 1 and 2, P < .01). Glucagon decreased the leucine concentration (-11% +/- 3% in study 2 v study 0, P < .02) and caused a small increment in proteolysis (+6% in study 2 v study 0, P < .01) that was related to the decrement in glutamine concentrations. Phenylalanine kinetics were not significantly affected. These results show that glucagon promotes the uptake of gluconeogenic substrates but does not increase their release, suggesting that glucagon-induced hyperglycemia is short-lived because glucagon fails to provide more fuel for gluconeogenesis. The small increase in proteolysis and the depletion of circulating glutamine prove that physiologic hyperglucagonemia can contribute to protein catabolism.

Roles of tyrosine kinase in insulin action on cell volume of fetal rat type II pneumocyte

The aim of the present study was to investigate the roles of tyrosine kinase (TK) in the insulin action on cell volume in fetal rat (20-day gestational age) type II pneumocyte. Insulin (100 nmol/l) increased cell volume, and this insulin (100 nmol/l) action was completely blocked by 50 micromol/l bumetanide (BMT) and 10 micromol/l amiloride (AML). This observation indicates that 100 nmol/l insulin activates BMT-sensitive Na+/K+/2Cl- cotransporter and AML-sensitive pathways. The stimulatory action of 100 nmol/l insulin on BMT-sensitive Na+/K+/2Cl- cotransporter was completely abolished by 10 micromol/l lavendustin A (LAV-A, an inhibitor of TK), however 100 nmol/l insulin could stimulate AML-sensitive pathways even in LAV-A (10 micromol/l)-treated cells. These observations indicate that the insulin (100 nmol/l) action on the BMT-sensitive Na+/K+/2Cl- cotransporter is mediated through TK-dependent pathways, while 100 nmol/l insulin requires a TK-independent pathway to show the stimulatory action on the AML-sensitive pathways. From these observations we conclude that TK-dependent and -independent pathways are involved in the insulin (100 nmol/l) signaling in fetal rat type II pneumocyte.

Cell volume and the metabolic actions of insulin

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

Effects of hydration state on the synthesis and secretion of triacylglycerol by isolated rat hepatocytes. Implications for the actions of insulin and glucagon on hepatic secretion

The effects of hepatocyte volume on the secretion of triacylglycerol were studied in order to test the suggestion that increases in the portal concentrations of osmolyte amino acids and metal ions during the prandial/early-absorptive phase may be involved in mediating the acute changes in glycerolipid metabolism observed in vivo [Zammit (1995) Biochem Soc. Trans. 23, 506-511]. Incubation of isolated rat hepatocytes with hypo-osmotic medium or in the presence of glutamine (in the presence or absence of leucine), conditions which gave an increase in cell water content of between 8 and 27%, resulted in a decrease in the rate of [14C]triacylglycerol (TAG) secretion when [14C]palmitate was used as substrate. The inhibition was proportional to the increase in cell water content. At low exogenous palmitate concentration (0.05 mM), the inhibition of [14C]TAG secretion was accompanied by a marked shift in the incorporation of label from TAG to phospholipid. In the presence of 0.5 mM palmitate this effect was attenuated, and in the presence of 1 mM palmitate it was abolished. Increased cell volume associated with incubation of hepatocytes with glutamine (in the presence or absence of leucine) also resulted in a decrease in the fraction of newly labelled TAG that was secreted into the medium. Decreased cell volume, achieved by incubation of hepatocytes with hyperosmotic medium (sufficient to decrease cell water content by approx. 9%) decreased overall [14C]TAG secretion, but did not affect the amount of label that was incorporated into phospholipid as a fraction of that incorporated into total glycerolipids. Cell shrinkage, however, diminished the fraction of newly labelled [14C]TAG that was secreted. When intracellular TAG was prelabelled with [3H]glycerol, it was found that cell shrinkage markedly inhibited (preformed) [3H]TAG secretion, whereas cell swelling did not affect this route of TAG secretion. The data are discussed in terms of the possible action of changes in cell hydration at the different loci at which hepatocyte TAG secretion is controlled, with reference to previous observations that both insulin and glucagon are able to inhibit TAG secretion in cultured rat hepatocytes and HepG2 cells.

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)

The synergistic action (cross-talk) of glucagon and vasopressin induces early bile flow and plasma-membrane calcium fluxes in the perfused rat liver

A study was made of the initial responses of perfusate Ca2+ fluxes and bile flow to Ca(2+)-mobilizing agonists, following refinements to the methods for analysing these parameters in the perfused rat liver. Net Ca2+ efflux induced by vasopressin commences at 15 s, reaches a maximal rate at 35 s and declines to zero by 55 s, when Ca2+ influx commences. Vasopressin-induced increases in bile flow commence by 20 s, attain a maximal rate by 35 s and begin to decline at 50 s, to reach basal values by 90 s. Concomitant administration of glucagon modifies each of these actions of vasopressin in the following ways: it decreases by 5 s the time of onset of net Ca2+ efflux, and the time and magnitude of such efflux, and the time of onset of bile flow is decreased to 15 s, and the flow reaches maximal rates by 30 s. When the alpha 1-adrenergic agonist phenylephrine is used in place of vasopressin, Ca2+ efflux commences at 17-18 s and is greater in magnitude; little bile flow is induced by this agonist. Glucagon modifies the action of phenylephrine in the following ways: the onset of Ca2+ efflux is brought forward by 2-3 s, it is of lower magnitude and Ca2+ influx begins by 45 s; bile flow commences by 15-20 s, and reaches a maximum at 30 s, where the rate is much greater than in the absence of glucagon; this rate gradually declines to be near basal by 80 s. The onset of agonist-induced oxygen uptake was also brought forward by the co-administration of glucagon. Comparison of agonist-induced plasma-membrane Ca2+ fluxes and bile flow (with or without glucagon administration) suggests that correlations can be made between net Ca2+ fluxes and the transient increases seen in bile flow.

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.

Alkalinization of acidic cellular compartments following cell swelling

Osmotic swelling of rat hepatocytes increases fluorescence of Acridine orange and of fluorescein isothiocyanate (FITC)-dextran, both indicative of alkalinization of acidic intracellular vesicles. Similar to osmotic cell swelling, insulin and glutamine lead to an increase in Acridine orange fluorescence, an effect virtually abolished upon osmotic reversal of glutamine-induced cell swelling. Barium, which blocks K+ channels in the plasma membrane, similarly leads to cell swelling and increase of Acridine orange fluorescence. Since proteolysis is governed by lysosomal pH, these observations indicate that the anti-proteolytic action of osmotic cell swelling is mediated by lysosomal alkalinization. Thereby, insulin, glutamine and barium probably exert their anti-proteolytic action by cell swelling and subsequent lysosomal alkalinization.

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.

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.

Effects of catecholamines on plasma potassium: the role of alpha- and beta-adrenoceptors

The sympathetic nervous system plays an important role in the control of plasma potassium levels. Administration of adrenaline or noradrenaline evokes, in the majority of mammal species a dual response: first a short transient hyperkalaemia, followed by a maintained hypokalaemia. Alpha 1- and alpha 2-adrenoceptors mediate the initial hyperkalaemia through the activation of hepatic Ca(2+)-dependent-K(+)-channels. Stimulation of beta 1- and beta 2-adrenoceptors induces the late hypokalaemia by stimulation of skeletal muscle Na(+)-K(+)-ATPase. Beta 3-adrenoceptor stimulation may also have an effect on plasma potassium control since administration of selective beta 3-adrenoceptor agonists induces a decrease in plasma potassium. The simultaneous infusion of phenyleprine (alpha-adrenoceptor agonist) and isoprenaline (beta-adrenoceptor agonist) increases plasma potassium levels: this effect is several times larger than the algebric summation of the changes in plasma potassium when each agent is infused separately, thus suggesting potentiation. The physiological (changes in cell volume and function secondary to changes in ion fluxes) and clinical implications (pathophysiological conditions with hypo or hyperkalaemia, hyperkalaemic periodic paralysis, ventricular arrythmias) of these findings are discussed.

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.

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)