Metabolic inter-relationships of intracellular pH measured by double-barrelled micro-electrodes in perfused rat liver

Hepatic intralobular mapping of fructose metabolism in the rat liver

Detailed mapping of glucose and lactate metabolism along the radius of the hepatic lobule was performed in situ in rat livers perfused with 1.5 mM lactate before and during the addition of 5 mM fructose. The majority of fructose uptake occurred in the periportal region; 45% of fructose taken up in the periportal half of the lobular volume being converted into glucose. Periportal lactate uptake was markedly decreased by addition of fructose. Basal perivenous lactate output, which was derived from glucose synthesized periportally, was increased in the presence of fructose. During fructose infusion there was a small decrease in cell pH periportally, but acidification of up to 0.5 pH units perivenously. The evidence suggests that in situ the apparent direct conversion of fructose into lactate represents, to a substantial extent, the result of periportal conversion of fructose into glucose and the subsequent uptake and glycolysis to lactate in the perivenous zone of some of that glucose. (31)P NMR spectroscopy showed that the cellular concentration of phosphomonoesters changes very little periportally during fructose infusion, but there was an approximate twofold increase perivenously, presumably due to the accumulation of fructose 1-phosphate. It may be inferred that fructokinase activity is expressed throughout the hepatic lobule.

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.

The effect of hypoxia and acidosis on propranolol clearance in the isolated perfused rat liver preparation

The effect of hypoxia and acidosis on the elimination of an oxidatively metabolized drug, S-propranolol, was examined in the single-pass isolated perfused rat liver (IPRL). The experiments (N = 6) consisted of four consecutive 30 min phases: normal pH (pH 7.4)/normal oxygen delivery, normal pH/hypoxia, hypercapnic acidosis (pH 7.1)/normal oxygenation and hypercapnic acidosis/hypoxia. Hypoxia and acidosis were produced by equilibrating the perfusate with appropriate mixtures of O2, N2 and CO2. With normal oxygen delivery there was no difference in hepatic clearance of propranolol between normal pH and acidosis (9.65 +/- 0.34 and 9.78 +/- 0.11 mL/min, respectively. P < 0.05). During hypoxia, propranolol clearance was impaired to a similar extent under both pH conditions (7.41 +/- 0.97 and 8.06 +/- 0.81 mL/min, respectively, P > 0.05). Therefore, respiratory acidosis does not affect the clearance of propranolol by the IPRL, nor does it influence the sensitivity of propranolol clearance to hypoxia. Neither acidosis nor hypoxia resulted in a significant reduction in bile flow compared with the normal pH/normal oxygen phase and there was no correlation between bile flow and perfusate bicarbonate concentration (P > 0.05).

Intracellular pH regulation and metabolic interactions in hepatic tissues

Intracellular pH (pHi) regulation in the vertebrate liver relies heavily on ionic transport mechanisms. Liver, in common with many tissues, has plasma membrane Na(+)-H+ and Cl(-)-HCO3- electroneutral exchangers which work in opposition to tightly control pHi. Mammalian livers also possess electrogenic Na(+)-HCO3- exchangers, capable of base uptake, which, when coupled to pHi-mediated changes in membrane potential, probably confer an additional measure of pHi control, compared to fish livers, where the transporter appears to be functionally absent. It is suggested that this may be a fundamental difference between aquatic and aerial breathing. pHi regulation has barely been examined in invertebrate hepatic tissues, but already some interesting differences are apparent. Notably, an electrogenic 2Na(+)-1H+ acid-extrusion system is present in apical membranes of crustacean hepatopancreas. Despite these ionic control systems, complex acid-base disturbances (e.g., "metabolic" acidosis) have been known for some time to influence hepatic metabolism in vertebrates, but few studies have carefully examined the independent effects of the acid-base variables involved. Thus mechanistic explanations for the effects of acid-base disturbances are scarce. Ureogenesis in mammals has been well studied, and several pH-related mechanisms are evident. In contrast, the pH-insensitivity of ureogenesis in fish liver may represent a second difference between aquatic and terrestrial species. In summary, by virtue of its metabolic diversity, liver represents a potentially important organ in acid-base balance, and an interesting study tissue for interrelationships between metabolism and acid-base balance.

pH-sensitive control of arginase by Mn(II) ions at submicromolar concentrations

The manganese dependence of arginase was reinvestigated with extracts of mouse liver to see whether more physiological properties were displayed than have been reported for the purified enzyme. In a preincubation with Mn(II) ions at 37 degrees C the enzyme underwent a slow and reversible activation. At least 90-95% of the activation achieved was dependent on Mn2+. However, no Mn2+ was required for catalytic activity in the assay. The activation showed little dependence upon pH over the range 6.5-9.5, whereas the catalytic activity increased 12-fold in apparent accord with the titration curve of an ionizable group of pKa 7.9. The Mn2+ dependence of arginase activation obeyed Michaelis-Menten kinetics, with Kd varying from 0.3 microM at pH 6.8 to 0.08 microns at pH 7.7. Free Mn2+ concentrations were established in these assays with a trimethylenediaminetetraacetate-Mn buffer. Vmax increased about three-fold over this range. The calculated arginase activity at 0.05 microM Mn2+ increases about nine-fold over this physiological pH range. An enzyme model is proposed to explain these findings. The activity of arginase at "physiological" [Mn2+] and the pronounced pH dependence conferred upon it are consistent with a recently revised role for the urea cycle in the control of bicarbonate and pH in the body. It appears possible that arginase loses Mn2+ sensitivity during the usual purification.

Degradation of insulin in isolated liver endosomes is functionally linked to ATP-dependent endosomal acidification

The degradation of insulin in isolated liver endosomes and the relationships of this process with ATP-dependent endosomal acidification have been studied. Incubation of endosomal fractions containing 125I-insulin in isotonic KCl at 30 degrees C resulted in a rapid loss of insulin integrity as judged from trichloroacetic acid precipitability, Sephadex G-50 chromatography, immunoreactivity and receptor binding ability, with a maximum at pH 5-6 (t1/2: 10, 10, 6 and 6 min, respectively). On a log/log plot, the amount of acid-soluble products generated was linearly related to the amount of insulin associated with endosomes (slope, 0.80). Upon incubation, virtually all acid-soluble products diffused out of endosomes as judged from their solubility in aqueous poly(ethyleneglycol). In permeabilized endosomes, intact insulin was also released in part extraluminally, but only when degradation was inhibited did this release increase with lowering pH. ATP shifted the pH for maximal insulin degradation to about 7.5-8.5 and caused endosomal acidification as judged from the uptake of acridine orange and the fluorescence of internalized fluorescein-labeled dextran and galactosylated bovine serum albumin (delta pH about 0.8-0.9). GTP, ITP and UTP exerted comparable effects but with lower potencies. The ability of ATP to alter the pH dependence of insulin degradation was maximal in the presence of Cl-, other anions being less effective (Br- greater than gluconate = SO4(2-) greater than NO3- = sucrose = mannitol) and/or inhibitory (NO3-). Na+, K+ and Li+ supported more effectively ATP-dependent insulin degradation than did choline. Divalent cations were required for the ATP effect (Mg2+ = Mn2+ greater than Co2+ greater than Ni2+ = Zn2 greater than Ca2+). Little or no effects of ATP occurred in the presence of proton ionophores such as monensin and carbonyl cyanide chlorophenylhydrazone, and inhibitors of the proton ATPase such as N-ethylmaleimide. The abilities of nucleotides, ions and inhibitors to support or inhibit ATP-dependent insulin degradation were well correlated with their abilities to affect ATP-dependent acidification. The acidotropic agents chloroquine and quinacrine caused a leftward shift in the pH dependence of insulin degradation and a decrease in maximal degradation; in the presence of ATP, chloroquine almost completely inhibited degradation at pH 5-9. It is concluded that ATP-dependent acidification, in part by enhancing the dissociation of the insulin-receptor complex, is required for optimum degradation of insulin within liver endosomes.

Handling of tracer bicarbonate by the liver. The relative impermeability of hepatocyte cell membranes to the ionic species

The multiple indicator dilution technique was used to study transfer of labeled HCO3- across the hepatocyte membrane in the anesthetized mongrel dog. A bolus of H[14C]O3-, 51Cr-labeled erythrocytes, [36Cl-] and/or [3H]sucrose, and [3H]OH was injected through a catheter in the portal vein, and timed anaerobic blood samples were obtained from a catheter in the hepatic vein. Experiments were carried out in untreated controls and after intravenous infusion of acetazolamide (100 mg/kg). In the controls, the H[14C]O3- curve was very similar to the [3H]OH curve. The dilution curves were all linear transformations of each other, indicating that HCO3-, as had previously been shown for the other diffusible tracers, undergoes delayed-wave flow-limited distribution. The distribution space for H[14C]O3- in the control situation includes the blood plasma and interstitial spaces, the erythrocyte interior modified by a Donnan equilibrium, and the available liver cellular space. The calculated HCO3- concentration in the liver cells was somewhat lower than that in the plasma space; the difference implied a cellular pH lower than that of plasma by approximately 0.08 pH units. When the carbonic anhydrases were inhibited with acetazolamide, the dilution curve for H[14C]O3- changed radically, approaching that for [36Cl-], which does not enter the liver cells. The change indicates that although HCO3-, like Cl-, is rapidly exchanged between plasma and erythrocytes, it also does not readily penetrate hepatocytes unless previously transformed to carbon dioxide by the carbonic anhydrases.

Influence of cytosolic pH on receptor-mediated endocytosis of asialoorosomucoid

Inhibitors of specific steps in the endocytosis of galactose-terminating glycoproteins (asialoglycoproteins) by cultured rat hepatocytes have been described (J. Cell Biol. 98: 375-381, 1984). In particular, substitution of K+ for Na+ in the culture medium results in reduced delivery of ligand to lysosomes; ligand-receptor internalization, dissociation, and segregation remain normal. We have now demonstrated by direct microelectrode measurement that incubation of hepatocytes in K+-substituted medium results in a reduction of intracellular pH by greater than or equal to 0.5 U. In addition, we have shown that reduced intracellular pH in these cells produced by either direct (CO2 diffusion) or indirect (K+ substitution) acidification inhibits ligand delivery to lysosomes. Return of internalized ligand-receptor complex to the cell surface (diacytosis) is also inhibited by these manipulations. These studies suggest that intracellular pH may modulate specific steps involving vesicle translocation and fusion in the receptor-mediated endocytosis of asialoglycoproteins. Similar effects of direct acidification of hepatocytes by CO2 diffusion and indirect acidification by K+ substitution for Na+, on diacytosis and ligand delivery to lysosomes, suggest that K+ substitution may influence these events by altering intracellular pH.

Thermogenesis and the effect of injected catecholamines on the oxygen consumption of cafeteria-fed rats

1. The oxygen consumption (VO2) of unrestrained rats given a 'cafeteria' (high energy, high fat) or control diet was studied. The resting values of VO2 were the same in each dietary group, whether maintained at 26 degrees C or 6 degrees C. This negative finding suggests that cafeteria feeding is not an important cause of diet-induced thermogenesis (DIT). 2. The response of each group of rats to injected noradrenaline or dopamine was also studied. Each catecholamine could increase VO2 values but the response was much less in cold-adapted rats measured at 6 degrees C. In all experimental circumstances the dopamine response exceeded that of noradrenaline. There was no evidence that the cafeteria diet consistently increased the response to either catecholamine. 3. These results suggest that DIT cannot be equated with non-shivering thermogenesis (NST). Furthermore, it is suggested that dopamine would be a better agent for measuring the oxygen equivalent of NST, since it would stimulate the dopamine receptors as well as the alpha- and beta-adrenoreceptors of brown fat.

Intracellular pH as measured by 19F NMR

19F NMR pH measurements with the fluorinated pH indicators we have described are rapid and sensitive, work with readily available commercial instruments, and extend the applicability of nondestructive NMR measurements to systems for which 31P NMR measurements are presently impractical. The family of 19F pH indicators is useful for independent confirmation of pHi values obtained by 31P NMR, distribution of radioactive weak acids, or other methods. The necessity for using exogenous indicators, which at first appears as a liability, can be turned to advantage also. Our future goal is to direct our measurements unambiguously to compartments (in cells or in tissues) of particular interest, by matching the indicator precursor molecule to the hydrolytic enzyme activities inherent in the target cell or compartment, so that the pH indicator is generated in situ.

NMR as a metabolic tool

We have briefly reviewed the broad range of applications of NMR spectroscopy to metabolism in tissues and biological fluids. Most of these studies are in the exploratory stage, though the potential of NMR for non-invasive and non-destructive monitoring of certain important substrates and reaction pathways is considerable. The limitations of the technique lie in its relative insensitivity and the rather restricted range of substances that it can detect, as well as the current expense. So far, the main clinically useful applications have been in the diagnosis and monitoring of treatment of certain inborn errors of metabolism, namely those that result in altered energy of pH states or the abnormal accumulation of significant amounts of metabolites in body fluids. It might be expected that as localization techniques improve, clinically useful information will be obtained in a wide range of ischaemic or hypoxic states, e.g. stroke and myocardial infarction. The possibility of producing a detailed spatial image of metabolite concentrations (e.g. ATP), in the way that NMR imaging techniques currently do using features of the water proton resonance, is attractive and the initial results are very encouraging (Bogusky et al, 1986; Bailes et al, 1987; Blackledge et al, 1987).

Cell membrane and transepithelial voltages and resistances in isolated rat hepatocyte couplets

The basic electrical properties of an isolated rat hepatocyte couplet (IRHC) system have been analyzed using classical techniques of epithelial electrophysiology, including measurement of electric potentials, resistances and intracellular ion activities. Applications of these techniques are discussed with respect to their limitations in small isolated cells. Mean intracellular and intracanalicular membrane potentials ranged from -23.7 to -46.7 and -4.3 to -5.9 mV, respectively. Membrane resistances were determined using an equivalent circuit analysis modified according to the geometry of the IRHC system. Resistances of the sinusoidal (basolateral) and canalicular (luminal) cell membranes and tight junctions averaged 0.15 and 0.78 G omega and 25 m omega, respectively. The cells are electrically coupled via low resistance intercellular communications (approximately 58 M omega). Intracellular ion activities for Na+, K+ and Cl- averaged 12.2, 88.1 and 17.7 mmol/liter, respectively. The basolateral membrane potential reveals a permeability sequence of PK greater than PCl greater than PNa. The luminal potential showed minimal dependence on changes in transjunctional ion gradients, indicating a poor ion selectivity of the paracellular pathway. The electrogenic (Na+-K+)-ATPase contributes little to the luminal and cellular negative electric potential. Therefore, the luminal potential probably results from the secretion of impermeant ions and a Donnan distribution of permeant ions, a mechanism which provides the osmotic driving force for bile formation. By providing the unique opportunity to measure luminal potentials, this isolated hepatocyte system permits study of secretory mechanisms for the first time in a mammalian gland using electrophysiologic techniques.

Diet-induced thermogenesis in cafeteria-fed rats: a myth?

Oxygen consumption (VO2), carbon dioxide production (VCO2), and respiratory quotient were measured in rats given a high-fat cafeteria diet of the type that is said to promote diet-induced thermogenesis. No significant difference in the measurements as compared with controls was found at room temperature, at 5 degrees C, or in animals exposed to cold for several weeks. The result was the same whether open- or closed-circuit methods were used. The stimulatory effect of norepinephrine on the VO2 was identical in each dietary group. These results cast doubt on the alleged identity of diet-induced and nonshivering thermogenesis and may reflect the change in body composition of the animals rather than a primary response to dietary variation.

Electrophysiological properties of gap junctions between dissociated pairs of rat hepatocytes

Physiological properties of isolated pairs of rat hepatocytes were examined within 5 h after dissociation. These cells become round when separated, but cell pairs still display membrane specializations. Most notably, canaliculi are often present at appositional membranes which are flanked by abundant gap and tight junctions. These cell pairs are strongly dye-coupled; Lucifer Yellow CH injected into one cell rapidly diffuses to the other. Pairs of hepatocytes are closely coupled electrically. Conductance of the junctional membrane is not voltage sensitive: voltage clamp studies demonstrate that gj is constant in response to long (5 s) transjunctional voltage steps of either polarity (to greater than +/- 40 mV from rest). Junctional conductance (gj) between hepatocyte pairs is reduced by exposure to octanol (0.1 mM) and by intracellular acidification. Normal intracellular pH (pHi), measured with a liquid ion exchange microelectrode, was generally 7.1-7.4, and superfusion with saline equilibrated with 100% CO2 reduced pHi to 6.0-6.5. In the pHi range 7.5-6.6, gj was constant. Below pH 6.6, gj steeply decreased and at 6.1 coupling was undetectable. pHi recovered when cells were rinsed with normal saline; in most cases gj recovered in parallel so that gj values were similar for pHs obtained during acidification or recovery. The low apparent pK and very steep pHi-gj relation of the liver gap junction contrast with higher pKs and more gradually rising curves in other tissues. If H+ ions act directly on the junctional molecules, the channels that are presumably homologous in different tissues must differ with respect to reactive sites or their environment.

The sodium and bicarbonate dependence of bile secretion in the guinea-pig

The effects of the total substitution of sodium by lithium, and of bicarbonate by a mixture of chloride and phosphate, on the flow and composition of bile was studied in the isolated perfused guinea-pig liver using a single-pass perfusion system. 60% of secretion was bicarbonate dependent and 80% specifically required the presence of sodium. The secretion of bicarbonate in bile against a concentration gradient was inhibited by the presence of lithium. In contrast to the results of similar studies in the rat, lithium is not an effective substitute for sodium in maintaining bile secretion.

The mechanism of inhibition of ureogenesis by acidosis

In perfused rat liver a decrease of cytosol pH, determined with pH-sensitive microelectrodes, from 7.2 to 6.85 is associated with a 50% fall in ureogenesis from ammonium chloride. In isolated rat hepatocytes the fall in ureogenesis due to acidosis is associated with decrease in the mitochondrial and cytosolic concentration of citrulline. Limitation of carbamoyl phosphate synthesis and thus citrulline supply could be responsible for the inhibition of ureogenesis observed.

The effects of supplementation of the diet with highly palatable foods upon energy balance in the rat

Full energy balance studies have been performed for 9 weeks on four groups of four adult female rats housed in a continuously running indirect calorimeter; for four weeks two of the groups received highly palatable foods in addition to a standard pelleted diet. A further sixteen groups, of which eight received the palatable foods, provided additional carcass composition data. All practicable precautions were taken to measure energy exchange accurately. Comparison of 'start-to-finish' apparent energy balance with carcass composition changes showed a systematic error of approx. +3% of energy throughput. This was most probably caused by losses of energy in food and excreta, which led to over-estimation of energy intake. Variations among individual balance periods added a standard error of approx. +/- 1%: the source of error here was probably imperfect matching of animals analysed at intermediate stages. The rats offered the palatable foods increased their metabolizable energy (m.e.) intake by 106 kJ/day, 51% of the control groups' intake, in the first week of supplementation. Over the whole 4 weeks of supplementation the increase was 64 kJ/day, or 31%. Withdrawal of the palatable foods led to an immediate fall in intake to about two-thirds of control level, and a return to control level over the next 2-3 weeks. Energy expenditure rose more slowly than intake, reaching a fairly steady level ca. 5 days after introduction of the palatable foods. Expenditure was then ca. 22 kJ/day above control level; an increase of 12% above control expenditure or, allowing for systematic and random errors, 33-37% of the additional m.e. intake. Expenditure returned to control level over the 2 weeks after ceasing supplementation. The experimental groups gained weight at a declining rate throughout the period of supplementation. The gain in live body weight at the end was ca. 32 g, but this comprised a carcass weight gain of 37 g and a loss of 5 g gastrointestinal tract contents. The carcass weight gain comprised 27 g fat (i.e. ca. 70% of the weight gained), 9 g lean tissue and 1 g additional water. The gain of carcass energy was 1100 kJ. There was a small increase in body length, an increase in liver weight, and an increase in the weight of the interscapular brown adipose tissue pad. Regression analysis showed that the increase in the weight of the interscapular brown adipose tissue pad reflected the increase in total body fat.(ABSTRACT TRUNCATED AT 400 WORDS)