The application of microcalorimetry to the assessment of metabolic efficiency in isolated rat hepatocytes

Growth yield homeostasis in respiring yeast is due to a strict mitochondrial content adjustment

In living cells, growth is the result of coupling between substrate catabolism and multiple metabolic processes taking place during net biomass formation and cell property maintenance. A crucial parameter for growth description is its yield, i.e. the efficiency of the transformation from substrate consumption to biomass formation. Using numerous yeast strains growing on different respiratory media, we have shown that the growth yield is identical regardless of the strain, growth phase, and respiratory substrate used. This homeostasis is the consequence of a strict linear relationship between growth and respiratory rates. Moreover, in all conditions tested, the oxygen consumption rate was strictly controlled by the cellular content of respiratory chain compounds in such a way that, in vivo, the steady state of oxidative phosphorylation was kept constant. Thus, the growth yield homeostasis depends on the tight adjustment of the cellular content of respiratory chain compounds to the growth rate. Any process leading to a defect in this adjustment allows an energy waste and consequently an energy yield decrease.

The calorimetric-respirometric ratio is an on-line marker of enthalpy efficiency of yeast cells growing on a non-fermentable carbon source

Although on-line calorimetry has been widely used to detect transitions in global metabolic activity during the growth of microorganisms, the relationships between oxygen consumption flux and heat production are poorly documented. In this work, we developed a respirometric and calorimetric approach to determine the enthalpy efficiency of respiration-linked energy transformation of isolated yeast mitochondria and yeast cells under growing and resting conditions. On isolated mitochondria, the analysis of different phosphorylating and non-phosphorylating steady states clearly showed that the simultaneous measurements of heat production and oxygen consumption rates can lead to the determination of both the enthalpy efficiency and the ATP/O yield of oxidative phosphorylation. However, these determinations were made possible only when the net enthalpy change associated with the phosphorylating system was different from zero. On whole yeast cells, it is shown that the simultaneous steady state measurements of the heat production and oxygen consumption rates allow the enthalpy growth efficiency (i.e. the amount of energy conserved as biomass compared to the energy utilised for complete catabolism plus anabolism) to be assessed. This method is based on the comparison between the calorimetric-respirometric ratio (CR ratio) determined under growth versus resting conditions during a purely aerobic metabolism. Therefore, in contrast to the enthalpy balance approach, this method does not rely on the exhaustive and tedious determinations of the metabolites and elemental composition of biomass. Thus, experiments can be performed in the presence of non-limiting amounts of carbon substrate, an approach which has been successfully applied to slow growing cells such as yeast cells expressing wild-type or a mutant rat uncoupling protein-1.

Hepatic energy and substrate metabolism: a possible metabolic basis for early nutritional support in cirrhotic patients

In the liver, the in vivo assessment of metabolic functions is limited by methodologic problems. The present evidence suggests that the liver contributes to 20-30% of whole body energy expenditure. Hepatic fuel selection can change considerably under different circumstances. During tissue catabolism (i.e., depletion of glycogen stores, increased lipid oxidation), the "hepatic respiratory quotient (RQ)" is lower than whole body RQ, suggesting that hepatic catabolism exceeds whole body catabolism. By contrast, the hepatic RQ may exceed whole body RQ during tissue anabolism (i.e., after full repletion of hepatic glycogen stores and significant lipogenesis). In cirrhosis, both the hepatic RQ and the whole body RQ are markedly reduced. When compared with the whole body level, the cirrhosis-induced decrease in the hepatic RQ is more pronounced. Given that liver catabolism exceeds (or possibly precedes) whole body catabolism, early nutritional support is mandatory in cirrhotic patients. The assessment of hepatic, in addition to whole body, energy metabolism may provide a basis for future recommendations of more specific nutritional support in patients with liver diseases.

Fructose 1,6-bisphosphate protects against D-galactosamine toxicity in isolated rat hepatocytes

Incubation of hepatocytes with D-galactosamine (GalN) produced a dose-dependent alteration in cell viability and a fall in ATP and fructose 2,6-bisphosphate (Fru-2,6-P2) levels. The reduction in Fru-2,6-P2 can be explained by changes in the substrates or modulators of 6-phosphofructo-2-kinase/fructose 2,6-bisphosphatase, because neither the adenosine 3',5'-cyclic monophosphate level nor the activity ratio of the enzyme was modified. Microcalorimetric measurements showed that GalN produced an exothermic peak followed by a progressive decrease in heat dissipation. Simultaneous administration of GalN and fructose 1,6-bisphosphate (Fru-1,6-P2) significantly increased cell viability, and concentrations of ATP and Fru-2,6-P2 and led to stable heat production. In the presence of Fru-1,6-P2 alone, hepatocytes kept ATP and Fru-2,6-P2 levels constant, whereas they increased the oxygen uptake-to-heat output ratio. Our results suggest that GalN initiates the hepatotoxic effect by means of an energy-dissipating interaction, produced before its metabolism and presumably at the membrane level, whereas Fru-1,6-P2 protects the cells against this injury in a way that prevents the initial interaction and increases the metabolic efficiency of the cell.

Some effects of different extracellular proteins on oxygen consumption and heat production in isolated rat hepatocytes

When hepatocytes prepared from 24-h-fasted rats were washed, suspended and incubated in Krebs-Henseleit bicarbonate-buffered saline, the endogenous rates of O2 consumption and heat production were 2.13 +/- 0.13 mumol/min per g wet wt. and 1.00 +/- 0.05 J/min per g wet wt. respectively. The inclusion of 2.5% (w/v) defatted and dialysed bovine serum albumin in either the cell suspension (washing) buffer or the cell incubation buffer produced a 20-25% increase in O2 consumption and heat production: these rates were increased by an additional 20-25% when the albumin (2.5%) was present in both the cell suspension and the cell incubation buffers. There was an inverse relationship between the increases in O2 consumption and heat production and the leakage of lactate dehydrogenase from the isolated hepatocytes: the inclusion of purified bovine serum albumin decreased lactate dehydrogenase leakage from 40% to 15% of total enzyme content. The calorimetric-respirometric ratios for hepatocytes incubated both in the absence (-461 +/- 19 kJ/mol O2) and presence (-477 +/- 8 kJ/mol O2) of the purified protein are very similar to the theoretical, thermochemically derived oxycaloric equivalents.

Effects of pure sugar vs. mixed starch fructose loads on food intake

Using a within-subject design, we gave subjects three different 520-530 kcal preloads in the form of puddings in a randomized fashion at weekly intervals. The puddings contained either 50 g of fructose or glucose as the sole carbohydrate source in a protein and fat mixture, or 50 g fructose plus 15 g of starch. Food intake was assessed 2.25 h after the preload was completed. Blood was drawn throughout and assayed for concentrations of glucose and insulin. When the preload contained fructose alone as the sole source of carbohydrate, subjects ate significantly fewer calories and less fat than when the preload contained glucose alone. When starch was added to the fructose preload, there was no significant reduction in calorie and fat intake. Effects on food intake paralleled the rise in plasma insulin levels produced by the different preloads. Implications for use of fructose as an adjunct to weight control efforts are discussed.

Microcalorimetric studies on metabolism of hepatic tissue. I. A methodological study of normal tissue

In the present study, the heat production of liver biopsies (5-8 mg) was measured by a microcalorimetric technique. Tissue incubated in Leibowitz L-15 medium (L-15) showed a higher metabolic rate compared to tissue incubated in a medium without substrate 2.8 microW/mg and 1.75 microW/mg, respectively. Heat production was found to be related to weight density. No difference in the metabolic rate was found after organ perfusion in comparison to nonperfused liver. Storage in medium L-15 at 4 degrees C caused a lower rate of heat production, but if the tissue was stored in an electrolyte balance solution without substrate, no difference was seen compared to fresh tissue. Recording heat production with the present calorimetric technique is relatively simple and rapid and allows measurement of small samples.

Anaerobic metabolism in aerobic mammalian cells: information from the ratio of calorimetric heat flux and respirometric oxygen flux

Calorimetric and respirometric studies of cultured cells show that both neoplastic and non-neoplastic cell types maintain an anaerobic contribution to their total heat flux. In many mammalian cells this can be explained quantitatively by lactate production observed under fully aerobic conditions. Uncoupling and enhanced futile substrate cycling increase the ratio of heat flux to oxygen flux, the calorimetric-respirometric (CR) ratio. The interpretation of calorimetric and respirometric measurements requires an energy balance approach in which experimentally measured CR ratios are compared with thermochemically derived oxycaloric equivalents. The oxycaloric equivalent is the enthalpy change per mole of oxygen consumed, and equals -470 kJ/mol O2 in the aerobic catabolism of glucose, assuming that catabolism is 100% dissipative (the net efficiency of metabolic heat transformation is zero). CR ratios more negative than -470 kJ/mol O2 have been reported in well-oxygenated cell cultures and are discussed in terms of integrated aerobic and anaerobic metabolism.

Microcalorimetric measurements of tissue cells in vitro

A culture flask was designed for the microcalorimetric measurements of tissue cells by an MS 80 standard calvet microcalorimeter. Tissue cells cultured in this flask behaved in the same manner as in the common culture flask used in cytobiological studies. The thermograms of human adenocarcinoma gastric cells (SGc 7901) and HeLa cells were obtained. The heat output power of SGc 7901 cells continuously increased for 70 h with an initial cell number of 3.0 X 10(5). The thermogram was reproducible under strictly controlled conditions. The relationship between the heat output power and the number of SGc 7901 cells within 48 h was obtained. The heat output power was 40 pW/cell to 49 pW/cell when the cell number was in the range 4.5 X 10(5) to 10.4 X 10(5). It was 62.3 +/- 2.9 pW/cell for HeLa cells when the cell number was 6 X 10(5).

Microcalorimetric investigation of metabolism in rat hepatocytes cultured on microplates and in cell suspensions

In the present work, heat production rate in rat hepatocytes has been measured by use of thermopile heat conduction calorimeters. Both hepatocytes cultured in monolayers on microplates and hepatocytes in suspensions were used for microcalorimetric measurements. The highest heat production rate was found in newly cultured cells; thereafter, a gradual decrease was noted. After 1 day of culture, metabolic activity had reached a steady state that lasted about 4 days. A cell-density dependence of heat production was found, both in cell suspensions and in cultured hepatocytes on microplates. Higher cell concentration in the calorimeter ampoule was accompanied by decreasing heat production per cell. The heat output recorded for hepatocytes cultured on microplates (25 X 10(3) cells) was found to be 0.327 +/- 0.13 nW per cell after 24-48 h. Addition of sodium azide and sodium fluoride to tissue culture medium reduced heat production rate in cultured hepatocytes by 60 and 20%, respectively. Recording of heat production with the present calorimetric technique is relatively simple and fast, and offers the possibility to perform measurements in small samples of cultured hepatocytes on microplates, thus allowing long-term as well as repeated measurements on the same cell population.

Microcalorimetric measurements of heat production in brown adipocytes from control and cafeteria-fed rats

The effects of the sequential addition of glucose, noradrenaline, propranolol and oleic acid on the rates of O2 consumption and heat production by isolated interscapular brown adipocytes from control and cafeteria-fed rats were compared. Although the chemical agents produced very similar changes in oxidative metabolism, the actual rates of O2 uptake and heat output in adipocytes from the cafeteria-fed rats, when expressed per g dry wt. of cells, were approx. 65% less than those obtained with cells from the control rats. However, when the same results were expressed per 10(8) multiloccular brown adipocytes, rather than gravimetrically, rates of O2 consumption and heat production were equivalent. Further interpretation of these data is complicated, because the average volume of multiloccular brown adipocytes from cafeteria-fed rats was 2.5 times that for multiloccular cells from control animals.

Direct and indirect calorimetry measurements during the annual growth cycle of juvenile frogs Rana ridibunda P

A thermodynamic study of the energetic metabolism in juvenile amphibians was conducted over an annual cycle of growth. The total energy flux (delta H = enthalpy variation) was measured with direct microcalorimetry and accounted for the sum of all metabolic pathways. Indirect calorimetry, oxygen consumption measurements (VO2), provides the values for the oxidative metabolism-dependent energy. A comparison between the results of both techniques simultaneously performed, provides for the calculation of the energetic efficiency, which indicates the respective importance of aerobic and anaerobic metabolisms during the annual biological cycle. It is clear that oxidative metabolism does not represent all, nor even a constant percentage of the total energy production of the frog, Rana ridibunda. The ration (Formula: see text) varied between 0.72 and 1.00, according to the season and the feeding or fasting periods. These results are discussed with special reference to the ecophysiology and behaviour of this species.

Role of fructose 2,6-bisphosphate in the control by glucagon of gluconeogenesis from various precursors in isolated rat hepatocytes

Hepatocytes from overnight-starved rats were incubated with 1-20 mM-fructose, -dihydroxyacetone, -glycerol, -alanine or -lactate and -pyruvate with or without 0.1 microM-glucagon. The production of glucose and lactate was measured, as was the content of fructose 2,6-bisphosphate. The concentrations of fructose (below 5 mM) and dihydroxyacetone (above 1 mM) that gave rise to an increase in fructose 2,6-bisphosphate were those at which a glucagon effect on the production of glucose and lactate could be observed. Glycerol had no effect on fructose 2,6-bisphosphate content or on production of lactate, and glucagon did not stimulate the production of glucose from this precursor. With alanine or lactate/pyruvate as substrates, glucagon stimulated glucose production whether the concentration of fructose 2,6-bisphosphate was increased or not. The extent of inactivation of pyruvate kinase by glucagon was not affected by the presence of the various gluconeogenic precursors. The role of fructose 2,6-bisphosphate in the effect of glucagon on gluconeogenesis from precursors entering the pathway at the level of triose phosphates or pyruvate is discussed.

No major thermogenic role for (Na+ + K+)-dependent adenosine triphosphatase apparent in hepatocytes from hyperthyroid rats

(Na+ + K+)-dependent ATPase activity, heat production and oxygen consumption were increased by 59%, 62% and 75% respectively in hepatocytes from tri-iodothyronine-treated rats. Ouabain at concentrations of 1 and 10 mM decreased oxygen uptake by 2--8% in hepatocytes from euthyroid rats and by 5--15% in hepatocytes from hyperthyroid animals. Heat output was decreased by 4--9% with the glycoside in isolated liver parenchymal cells from the control animals and by 11% in the cells from the tri-iodothyronine-treated animals. These results do not support the hypothesis that hepatic (Na+ + K+)-ATPase plays a major role in increased heat production in hepatocytes from hyperthyroid rats.

Effects of fructose concentration on carbohydrate metabolism, heat production and substrate cycling in isolated rat hepatocytes

1. Hepatocytes from starved rats were incubated with 5mm-glucose, labelled uniformly with (14)C and specifically with (3)H at positions 1, 2, 3 or 6, and with fructose at concentrations of 2.5, 7.5 or 25mm. 2. In the absence of other substrates only 1% of the radioactivity initially present in [U-(14)C]glucose appeared in the metabolic products, CO(2), lactate, pyruvate, amino acids and glycogen. 3. Fructose at 2.5mm caused a 30% increase in the glucose concentration and a 4-fold increase in the apparent oxidation of [U-(14)C]-glucose. 4. The formation of (3)H(2)O from [1-(3)H]-, [2-(3)H]-, [3-(3)H]- or [6-(3)H]-glucose was 2.4, 4.3, 2.15 or 1.6% respectively in the control incubations and 4.1, 10.4, 7.7 or 5.1% with 2.5mm-fructose. 5. Fructose at 7.5 and 25mm decreased the (3)H(2)O yields to less than the control values, but had no apparent effect on the amount of [U-(14)C]glucose metabolized. 6. In the incubations with 5mm-glucose and 25mm-fructose there were significant decreases in heat production, O(2) consumption and in the ratio of O(2) uptake to heat output. 7. Fructose at 2.5mm caused a 64% increase in heat output, but only a 43% increase in O(2) uptake. 8. The radioisotopic and calorimetric data demonstrate that physiological concentrations of fructose greatly increase metabolism in hepatocytes from starved rats. These data also indicate increased cycling at glucose/glucose 6-phosphate and at fructose 6-phosphate/fructose 1,6-bisphosphate in the presence of 2.5mm-fructose, although the rates of cycling were actually decreased relative to the amount of glucose catabolized. 9. At concentrations of 2.5, 7.5 and 25mm, fructose depressed hepatocyte ATP concentrations by 20, 65 and 80% respectively. Although fructose at 7.5 and 25mm increased glucose and lactate release, O(2) consumption, production of heat and formation of(3)H(2)O from [1-(3)H]-, [2-(3)H]-, [3-(3)H]- or [6-(3)H]-glucose were lowered to values equal to, or less than, controls. These effects probably reflect a severe derangement of hepatic metabolism due to excess phosphorylation of fructose when present at high concentrations.