Characterization of H+ efflux pathways in rat hepatocytes

Control of hepatocyte DNA synthesis by intracellular pH and its role in the action of tumor promoters

The mechanisms of tumor promotion in liver by various xenobiotics of diverse structure are not well understood. However, these tumor promoters share the ability to exert growth-stimulatory effects on hepatocytes. Our laboratory has been utilizing normal rat hepatocytes under defined conditions of primary cultures, to investigate growth-stimulatory actions of liver tumor promoters. We have shown that most, if not all, of the liver tumor promoters tested stimulate hepatocyte DNA synthesis when added in combination with epidermal growth factor (EGF), insulin, and glucocorticoids. In the present study, we sought evidence for the role of the Na(+)/H(+) antiporter and cytoplasmic alkalinization in the direct growth-stimulatory actions of tumor promoters on hepatocytes. Hepatocytes cultured under conditions (bicarbonate-buffered medium) where intracellular pH (pH(i)) was independent of extracellular pH (pH(e)), EGF- and insulin-stimulated rates of DNA synthesis were unaffected by modest changes in pH(e). However, under conditions (HEPES-buffered medium) where pH(i) varied in a linear fashion with pH(e), rates of EGF- and insulin-stimulated DNA synthesis were highly dependent on pH(e). Similarly, 12-O-tetradecanoylphorbol-13-acetate (TPA) and alpha-hexachlorocyclohexane (HCH)-stimulated DNA synthesis were pH(e)-dependent but were stimulatory over different pH(e) ranges, suggesting that these promoters may act by distinct mechanisms. Chemicals that are capable of inducing rapid cytoplasmic alkalinization, ammonium chloride (1 and 15 mM) and monensin (0.5 microM), were found to stimulate hepatocyte DNA synthesis. The role of the Na(+)/H(+) antiport in controlling pH(i) of hepatocytes was demonstrated by artificially acidifying 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein acetoxymethyl (BCECF)-loaded isolated hepatocytes with 20 mM sodium acetate and the use of specific inhibitors. Amiloride and its analogues inhibited pH(i) recovery from the acid load in a dose dependent manner and the relative potency of these inhibitors paralleled their K(i) values for the Na(+)/H(+) antiport. At concentrations that stimulate hepatocyte DNA synthesis, some liver tumor promoters phenobarbital (PB) and HCH, were found to cause a rapid rise pH(i) in isolated hepatocytes which was sensitive to amiloride and its analogues. Taken together, our data suggest that activation of Na(+)/H(+) antiport activity may be one mechanism whereby some liver tumor promoters stimulate hepatocytes DNA synthesis. This study has implications for the mechanisms of tumor promotion in liver carcinogenesis.

Alterations of cell volume regulation in the development of hepatocyte necrosis

Intracellular Na+ accumulation has been shown to contribute to hepatocyte death caused by anoxia or oxidative stress. In this study we have investigated the mechanism by which Na+ overload can contribute to the development of cytotoxicity. ATP depletion in isolated hepatocytes exposed to menadione-induced oxidative stress or to KCN was followed by Na+ accumulation, loss of intracellular K+, and cell swelling. Hepatocyte swelling occurred in two phases: a small amplitude swelling (about 15% of the initial size) with preservation of plasma membrane integrity and a terminal large amplitude swelling associated with cell death. Inhibition of Na+ accumulation by the use of a Na+-free medium prevented K+ loss, cell swelling, and cytotoxicity. Conversely, blocking K+ efflux by the addition of BaCl2 did not influence Na+ increase and small amplitude swelling, but greatly stimulated large amplitude swelling and cytotoxicity. Menadione or KCN killing of hepatocytes was also enhanced by inducing cell swelling in an hypotonic medium. However, increasing the osmolarity of the incubation medium did not protect against large amplitude swelling and cytotoxicity, since stimulated Na+ accumulation and K+ efflux. Altogether these results indicate that the impairment of volume regulation in response to the osmotic load caused by Na+ accumulation is critical for the development of cell necrosis induced by mitochondrial inhibition or oxidative stress.

N,N-Dimethylformamide modulates acid extrusion from murine hepatoma cells

N,N-Dimethylformamide (DMF) affects cellular differentiation, causes hepatotoxicity and gastric irritation, and may be carcinogenic. Since these processes involve changes in cellular pH homeostasis, we investigated the effects of DMF on H+ extrusion and cytosolic pH (pHi) of mouse hepatoma cells (Hepa 1C1C7). Extracellular pH was monitored using a silicon-based sensor system (Cytosensor microphysiometer) and pHi was monitored by fluorescence spectrophotometry. Superfusion of cells with DMF (0.25 to 0.5 M) suppressed the extracellular acidification rate (ECAR) below baseline. Following washout of DMF there was a rapid, concentration-dependent, prolonged overshoot of ECAR above baseline rates. Removal of extracellular Na+ or superfusion with amiloride abolished the overshoot in acidification rate, indicating involvement of Na+/H+ exchange. The overshoot was dependent on extracellular glucose, suggesting that it arises from an increase in metabolic acid production. Fluorescence measurements showed that DMF did not change pHi. Furthermore, DMF did not alter the rate of pHi recovery of cells acid loaded using nigericin, indicating that DMF does not directly alter Na+/H+ exchange activity in these cells. In summary, these data suggest that suppression of acidification rate by DMF is likely due to decreased metabolic acid production. Washout of DMF is then accompanied by increased glucose metabolism and H+ efflux via Na+/H+ exchange. It is possible that alterations in H+ production and transport contribute to the hepatotoxicity of DMF and its effects on cellular differentiation.

Cytosolic pH regulation in perfused rat liver: role of intracellular bicarbonate production

The contribution of metabolic bicarbonate to cytosolic pH (pHcyto) regulation was studied on isolated perfused rat liver using phosphorus-31 NMR spectroscopy. Removal of external HCO3- decreased proton efflux from 18.6+/-5.0 to 1.64+/-0.29 micromol/min per g liver wet weight (w.w.) and pHcyto from 7.17+/-0.06 to 6.87+/-0.06. In the nominal absence of bicarbonate, inhibition of carbonic anhydrase by acetazolamide induced a further decrease of proton efflux of 0.69+/-0.26 micromol/min per g liver w.w. reflecting a reduction in metabolic CO2 hydration, and hence a decrease of H+ and HCO3- supplies. Even though 27% of the proton efflux was amiloride-sensitive under bicarbonate-free conditions, amiloride did not change pHcyto, revealing the contribution of additional regulatory processes. Indeed, pH regulation was affected by the combined use of 4-acetamido-4'-isothiocyanostilbene-2,2'-disulfonic acid (SITS) and amiloride since pHcyto decreased by 0.16+/-0.05 and proton efflux by 0.60+/-0.14 micromol/min per g liver w.w. The data suggest that amiloride-sensitive or SITS-sensitive transport activities could achieve, by themselves, pHcyto regulation. The involvement of two mechanisms, most likely Na+/H+ antiport and Na+:HCO3 symport, was confirmed in the whole organ under intracellular and extracellular acidosis. The evidence of Na-dependent transport of HCO3- in the absence of exogenous bicarbonate implies that the amount of metabolic bicarbonate is sufficient to effectively participate to pHcyto regulation.

Cytosolic pH variations in perfused rat liver at 4 degrees C: role of intracellular buffering power

The effect of low temperature on cytosolic pH regulation and buffering capacity was evaluated in the isolated rat liver. The pH changes were followed by phosphorus-31 nuclear magnetic resonance. Cooling from 37 to 4 degrees C, with Krebs-Heinseleit perfusion at an external pH of 7.35, induced an alkaline shift in cytosolic pH (pHcyt) of 0.13 or 0.75 pH units in the presence of bicarbonate, respectively (dpH cys/dT values were 0.004 and 0.022 unit/degrees C. With 4 degrees C perfusion, in the presence or absence of bicarbonate, acute changes of external pH (from 7.40 to 5.90) did not affect pHcyt. In contrast, intracellular loading with isobutyric acid or NH4Cl induced rapid pHcyt variations. The intrinsic buffering power value (10 to 50 slykes) measured in the absence of bicarbonate depended on pHcyt. The larger value was observed for pHcyt 7.30, a value near the pK value of the imidazole group of intracellular proteins at 4 degrees C. The presence of bicarbonate modified the amplitude of the pHcyt change by increasing the total buffering power. It was demonstrated that during hypothermia, ionic carriers are inactivated and the charged forms of molecules are unable to cross the cell membrane; thus, the pHcyt homeostasis depends essentially on intracellular buffering power.

Effect of perfusate pH on the influx of 5-5'-dimethyl-oxazolidine-2,4-dione and dissociation of epidermal growth factor from the cell-surface receptor: the existence of the proton diffusion barrier in the Disse space

The influx clearance (PSinf.MID) of the weak acid 5,5'-dimethyl-oxazolidine-2,4-dione (DMO) was determined by the multiple indicator dilution method with the isolated perfused rat liver under various perfusate pH conditions, ranging from 6.4 to 7.6. Although the pH partition theory predicted an increase in influx clearance of ten times in proportion to the change in the unionized fraction of DMO, there was no measurable change in this value. The effect of medium pH on the steady-state cell/medium concentration ratio (C/M) ratio of DMO was also investigated using isolated hepatocytes. The C/M ratio increased while medium pH decreased, but this change was less marked than predicted by the pH partition theory. Finally the pH dependency of the dissociation rate constant (koff) of epidermal growth factor from its receptor was also investigated using both isolated rat hepatocytes and the perfused rat liver. When the extracellular pH was changed from 6.4 to 5.6, the koff value of isolated hepatocytes increased 44 times, while that of the perfused rat liver increased only 9 times. Therefore, the effect of changing the extracellular pH on pH-dependent dissociation of epidermal growth factor from its cell-surface receptor was less in the perfused liver than in isolated hepatocytes. These findings, in addition to the well-known existence of the Na(+)-H+ exchanger on the sinusoidal membrane and the possible existence of the unstirred water layer in the Disse space, seem to suggest the existence of the proton diffusion barrier in the rat liver, which remains stronger in the perfused liver than in isolated hepatocytes.

Intracellular calcium-mediated activation of hepatic Na+/H+ exchange by arginine vasopressin and phenylephrine

The effect of Ca++ mobilizing agonists arginine vasopressin and phenylephrine on Na+/H+ exchange was studied in freshly isolated hepatocytes and isolated perfused rat livers. The activity of Na+/H+ exchange was determined from the rate of H+ efflux, 22Na uptake and pHi recovery. Arginine vasopressin and phenylephrine stimulated H+ efflux and 22Na uptake in isolated rat hepatocytes and increased the rate of pHi recovery from acid-loaded hepatocytes. These effects were inhibited by amiloride. Arginine vasopressin- and phenylephrine-induced increases in H+ efflux were also dependent on extracellular Na+. Arginine vasopressin- and phenylephrine-induced increases in intracellular Ca++ concentration, H+ efflux, 22Na uptake and intracellular pH recovery were decreased in hepatocytes preloaded with the Ca(++)-buffering agent [bis-(2-amino-5-methylphenoxy)-ethane-N,N,N',N'-tetraacetic acid] (MAPTA). Na+/H+ exchange-dependent intracellular pH recovery from cytosolic acidification was stimulated by thapsigargin, which increases intracellular calcium concentration by inhibiting endoplasmic reticulum Ca++ ATPase. Arginine vasopressin- and phenylephrine-induced increases in intracellular pH recovery were not dependent on extracellular Ca++ and were inhibited by calmidazolium, a calmodulin inhibitor. Arginine vasopressin and phenylephrine also increased H+ efflux in the absence but not in the presence of amiloride in perfused rat livers without affecting biliary HCO3- excretion. These results indicate that arginine vasopressin and phenylephrine activate Na+/H+ exchange in rat hepatocytes, an effect mediated in part by intracellular Ca++ and calmodulin kinase. Furthermore, sinusoidal Na+/H+ exchange does not appear to be involved in biliary HCO3- excretion.

Mechanism of inhibition of hepatic bile acid uptake by amiloride and 4,4'-diisothiocyano-2,2'-disulfonic stilbene (DIDS)

The mechanisms by which amiloride and 4,4'-diisothiocyano-2,2'-disulfonic stilbene (DIDS) inhibit hepatic uptake of cholate and taurocholate (TC) were investigated in isolated rat hepatocytes. Amiloride inhibited Na(+)-dependent uptake of cholate and TC only when hepatocytes were preincubated with amiloride, indicating an indirect effect of amiloride. Time-dependent studies showed that the inhibition of bile acid uptake was associated with a parallel increase in intracellular Na+ concentration ([Na+]i). Although amiloride decreased intracellular pH, this decrease preceded amiloride-induced inhibition of bile acid uptake and increase in [Na+]i. Amiloride inhibited bile acid uptake, decreased membrane potential, and increased [Na+]i with comparable concentration dependency. DIDS inhibited Na(+)-dependent uptake of cholate and TC non-competitively. Neither DIDS nor amiloride inhibited Na(+)-independent uptake of cholate and TC. These results indicate that amiloride inhibits Na(+)-dependent cholate and TC uptake by decreasing the transmembrane Na(+)-gradient, and further support the hypothesis that two different transporters may be involved in hepatic bile acid uptake by Na(+)-dependent and Na(+)-independent mechanisms.

Nafenopin, a hypolipidemic and non-genotoxic hepatocarcinogen increases intracellular calcium and transiently decreases intracellular pH in hepatocytes without generation of inositol phosphates

Addition of nafenopin (30-300 microM to 45Ca2+ preloaded cultured hepatocytes caused a rapid and concentration-dependent increase in 45Ca2+ efflux in a manner similar to vasopressin, as evidenced by the loss of radioactivity from the cells. In contrast to vasopressin, addition of nafenopin to [3H]inositol prelabelled hepatocytes in culture did not increase [3H]inositol phosphate production. When added simultaneously with vasopressin, nafenopin inhibited the vasopressin-stimulated [3H]inositol phosphate production. In hepatocyte suspensions isolated from rats treated for 1 week with a carcinogenic dose of nafenopin (1000 ppm in their daily food) the incorporation of [3H]inositol into the phosphoinositide fraction, particularly phosphatidylinositol 4-phosphate and phosphatidylinositol 4,5-bisphosphate, was much less than that in hepatocytes isolated from untreated rats. The vasopressin-stimulated [3H]inositol phosphate production was also decreased. Experiments with hepatocyte suspensions preloaded with Ca2+ or pH sensitive fluorescent indicators demonstrated that addition of nafenopin caused an increase in intracellular free Ca2+ and transient acidification of the cells. The increase in [Ca2+]i was decreased by only about 25% when extracellular calcium was removed indicating that nafenopin mainly mobilizes Ca2+ from intracellular stores. The recovery to basal pH was amiloride-sensitive indicating the importance of Na+/H+ exchange in pH recovery after intracellular acidification. Amiloride also inhibited DNA synthesis induced by nafenopin and by epidermal growth factor in cultured hepatocytes; but this effect occurred concomitantly with inhibition of basal DNA synthesis. We suggest that hepatic Ca2+ mobilization induced by nafenopin may play an important role in the mechanism by which nafenopin exerts its physiological as well as its tumour promotive activity upon chronic treatment with carcinogenic doses.

Vasopressin, insulin and peroxide(s) of vanadate (pervanadate) influence Na+ transport mediated by (Na+, K+)ATPase or Na+/H+ exchanger of rat liver plasma membrane vesicles

Uptake of 22Na+ by liver plasma membrane vesicles, reflecting Na+ transport by (Na+, K+)ATPase or Na+/H+ exchange was studied. Membrane vesicles were isolated from rat liver homogenates or from freshly prepared rat hepatocytes incubated in the presence of [Arg8]vasopressin or pervanadate and insulin. The ATP dependence of (Na+, K+)ATPase-mediated transport was determined from initial velocities of vanadate-sensitive uptake of 22Na+, the Na(+)-dependence of Na+/H+ exchange from initial velocities of amiloride-sensitive uptake. By studying vanadate-sensitive Na+ transport, high-affinity binding sites for ATP with an apparent Km(ATP) of 15 +/- 1 microM were observed at low concentrations of Na+ (1 mM) and K+ (1mM). At 90 mM Na+ and 60 mM K+ the apparent Km(ATP) was 103 +/- 25 microM. Vesiculation of membranes and loading of the vesicles prepared from liver homogenates in the presence of vasopressin increased the maximal velocities of vanadate-sensitive transport by 3.8-fold and 1.9-fold in the presence of low and high concentrations of Na+ and K+, respectively. The apparent Km(ATP) was shifted to 62 +/- 7 microM and 76 +/- 10 microM by vasopressin at low and high ion concentrations, respectively, indicating that the hormone reduced the influence of Na+ and K+ on ATP binding. In vesicles isolated from hepatocytes preincubated with 10 nM vasopression the hormone effect was conserved. Initial velocities of Na+ uptake (at high ion concentrations and 1 mM ATP) were increased 1.6-1.7-fold above control, after incubation of the cells with vasopressin or by affinity labelling of the cells with a photoreactive analogue of the hormone. The velocity of amiloride-sensitive Na+ transport was enhanced by incubating hepatocytes in the presence of 10 nM insulin (1.6-fold) or 0.3 mM pervanadate generated by mixing vanadate plus H2O2 (13-fold). The apparent Km(Na+) of Na+/H+ exchange was increased by pervanadate from 5.9 mM to 17.2 mM. Vesiculation and incubation of isolated membranes in the presence of pervanadate had no effect on the velocity of amiloride-sensitive Na+ transport. The results show that hormone receptor-mediated effects on (Na+, K+)ATPase and Na+/H+ exchange are conserved during the isolation of liver plasma membrane vesicles. Stable modifications of the transport systems or their membrane environment rather than ionic or metabolic responses requiring cell integrity appear to be involved in this regulation.

Amiloride inhibits constitutive internalization and increases the surface number of epidermal growth factor receptors in intact rat hepatocytes

In previous experiments the surface expression of epidermal growth factor (EGF) receptors in freshly isolated rat hepatocytes varied temperature- and time-dependently and was depleted by monensin and cycloheximide in a way suggesting that a subpopulation of these receptors are subject to constitutive cycling (Gladhaug and Christoffersen; 1988). We here report the finding that pretreatment of the hepatocytes with amiloride exerts marked effects on cellular EGF receptor movements. After 2 h incubation with 1 mM amiloride, the receptor level was approximately 270,000 sites/cell surface vs. 140,000 in the untreated cell, with no change in receptor affinity. Amiloride thus stabilized the surface EGF receptor pool at an elevated level. In cells pretreated with amiloride for 60 min, the relative endocytosis decreased from about 2.6 EGF molecules internalized per receptor during 15 min endocytosis in untreated cells to about 1.5 molecules/receptor in amiloride-treated cells. These results suggest that amiloride causes an accumulation of EGF receptors at the hepatocyte surface due to inhibition of constitutive receptor internalization. In addition, it was found that in amiloride-treated hepatocytes the phorbol ester TPA strongly inhibited high-affinity EGF binding without affecting the total surface receptor number. In control cells, TPA did not consistently affect binding. Pretreatment with amiloride prevented surface EGF receptor depletion induced by cycloheximide and puromycin, but it did not significantly inhibit surface receptor depletion caused by monensin. Although the underlying mechanism of the amiloride effect on intracellular receptor trafficking is not clear, the results provide further evidence for a continuous, ligand-independent EGF receptor cycling pathway in hepatocytes.

Epidermal growth factor and cyclic AMP stimulate Na+/H+ exchange in isolated rat hepatocytes

Na+/H+ exchange in acid-loaded isolated hepatocytes was measured using the intracellular pH indicator biscarboxyethyl-carboxyfluorescein (BCECF) to follow intracellular pH (pHi). The rate of amiloride-sensitive Na(+)-dependent recovery from cytoplasmic-acid-loading was found to be increased in cells treated with epidermal growth factor (EGF), 8-(4-chlorophenylthio)adenosine 3',5'-monophosphate (ClPhScAMP) or phorbol 12-myristate 13-acetate (PMA). These three agents increased the rate of Na+/H+ exchange to similar extents and their effects were not additive. The stimulation was shown in all three cases to be due an alkaline shift of 0.1 in the set point pH of the Na+/H+ exchanger. Experiments measuring the uptake of 22Na+ into acid-loaded primary hepatocyte monolayer cultures confirmed these results. EGF, ClPhScAMP and PMA significantly increased the amiloride-inhibitable accumulation of 22Na+, thus providing further evidence that Na+/H+ exchange is stimulated by these effectors.