The effects of the stress caused by experimental procedures on alanine, aspartate, glutamate and glutamine in rat liver

The effects of stress and injury on the activity of phosphoenolpyruvate carboxykinase in the liver of the rat

The effects of stress (diethyl ether anaesthesia for 4-8 min, or intravenous injection of 0.05 ml of a dimethyl sulphoxide/water mixture) and of a scald injury given under ether anaesthesia on hepatic PEPCK (phosphoenolpyruvate carboxykinase, EC 4.1.1.32) were studied in the post-absorptive rat. Injury raised PEPCK activity by about 70% in 2 h and by over 100% in 4 h, over three times as fast as in animals that had only been handled (controls). The two stresses, both of types commonly imposed in animal experiments, had almost as much effect as injury for the first 2 h, although much less thereafter. The roles of sympathetic stimulation and corticosterone in mediating these rises were studied by using alpha beta-blockers and trilostane respectively as inhibitors. (Trilostane only decreased corticosterone concentrations to a little above control values.) The ether-induced increase was somewhat decreased by alpha beta-blockade, but was only eliminated by combined alpha beta-blockade and trilostane. After injury, however, PEPCK synthesis was unaffected by either alpha beta-blockade or trilostane, although it was decreased by their combined action; and it seems that either corticosterone or sympathetic stimulation was sufficient to stimulate PEPCK synthesis maximally. Stimulation by corticosterone was much greater than reported previously by others, for reasons that are discussed. Sympathetic stimulation may have been mediated by glucagon and cyclic AMP, since injury raised portal glucagon concentrations, and stress and injury raised those of hepatic cyclic AMP. PEPCK synthesis was, however, stimulated despite increases in portal insulin concentration, and was not related to the [insulin]/[glucagon] ratio. Thus stress and injury over-rode normal control mechanisms.

[14C]bicarbonate fixation into glucose and other metabolites in the liver of the starved rat under halothane anaesthesia. Metabolic channelling of mitochondrial oxaloacetate

Previous attempts to account for the labelling in vivo of liver metabolites associated with the citrate cycle and gluconeogenesis have foundered because proper allowance was not made for the heterogeneity of the liver. In the basal state (anaesthetized after 24h starvation) this heterogeneity is minimal, and we show that labelling by [14C]bicarbonate can be interpreted unambiguously. [14C]Bicarbonate was infused to an isotopic steady state, and measurements were made of specific radioactivities of blood bicarbonate, alanine, glycerol and lactate, of liver alanine and lactate, and of individual carbon atoms in blood glucose and liver aspartate, citrate and malate. (Existing methods for several of these measurements were extensively modified.) The results were combined with published rates of gluconeogenesis, uptake of gluconeogenic precursors by the liver, and citrate-cycle flux, all measured under similar conditions, and with estimates of other rates made from published data. To interpret the results, three ancillary measurements were made: the rate of CO2 exchange by phosphoenolpyruvate carboxykinase (PEPCK; EC 4.1.1.32) under conditions that simulated those in vivo; the 14C isotope effect in the pyruvate carboxylase (EC 6.4.1.1) reaction (14C/12C = 0.992 +/- 0.008; S.E.M., n = 8); the ratio of labelling by [2-14C]- to that by [1-14C]-pyruvate of liver glutamate 1.5 min after injection. This ratio, 3.38, is a measure of the disequilibrium in the mitochondria between malate and oxaloacetate. The data were analysed with due regard to experimental variance, uncertainties in values of fluxes measured in vitro, hepatic heterogeneity and renal glucose output. The following conclusions were reached. The results could not be explained if CO2 fixation was confined to pyruvate carboxylase and there was only one, well-mixed, pool of oxaloacetate in the mitochondria. Addition of the other carboxylation reactions, those of PEPCK, isocitrate dehydrogenase (EC 1.1.1.42) and malic enzyme (EC 1.1.1.40), was not enough. Incomplete mixing of mitochondrial oxaloacetate had to be assumed, i.e. that there was metabolic channelling of oxaloacetate formed from pyruvate towards gluconeogenesis. There was some evidence that malate exchange across the mitochondrial membrane might also be channelled, with incomplete mixing with that in the citrate cycle. Calculated rates of exchange of CO2 by PEPCK were in agreement with those measured in vitro, with little or no activation by Fe2+ ions.(ABSTRACT TRUNCATED AT 400 WORDS)

The design of experiments using isotopes for the determination of the rates of disposal of blood-borne substrates in vivo with special reference to glucose, ketone bodies, free fatty acids and proteins

1. The two well-known methods of estimating rates of irreversible disposal (R) of blood-borne substrates in vivo by isotope experiments involve estimating the specific radioactivity (S) of the substrate in blood either after single intravenous injection of labelled substrate or during its infusion at a constant rate. The value of R is calculated from the S-time curve, usually by assuming: (i) a metabolic steady state with respect to substrate, (ii) the passage of all substrate through the blood, and (iii) the absence of certain types of recycling via blood. 2. In a theoretical investigation we show how experiments can be performed and R calculated from analyses of blood when one or more of the above assumptions is unjustified, by using glucose, ketone bodies, plasma free fatty acids and proteins as examples. In general the methods require single injection procedures, with estimation of the total quantity of label in the substrate in blood and the substrate concentration instead of only S. Such values give estimates of R with standard errors even when only one blood specimen is taken from each of a group of animals, as is convenient when working with small animals or substrates in low concentration, and when the animals are in a non-steady state in which constant infusion procedures are invalid. 3. Similar methods give the fraction of label injected as one compound which passes through another (the isotopic yield). 4. The methods are not always applicable, and cannot be applied to plasma proteins in some pathological conditions. A questionnaire for assessing their applicability is given.

Disequilibrium in the malate dehydrogenase reaction in rat liver mitochondria in vivo

1. When [2-(14)C]pyruvate is injected into rats the C3-position of liver glutamate becomes more heavily labelled than the C2-position, thus establishing that oxaloacetate and fumarate are not in equilibrium in rat liver mitochondria in vivo. The amount of disequilibrium was shown to be simply related to the value that the C3-label/C2-label ratio would have were no label recycled. This ratio, z, was calculated for post-absorptive rats in environmental temperatures of 20 degrees and 30 degrees C from determinations of the distribution of label within glutamate 1, 3 and 10min after intravenous injection of [2-(14)C]pyruvate. The values of z (best estimate and range) were 1.65 (1.60-1.69) in rats at 20 degrees C and 2.43 (2.23-2.63) in rats at 30 degrees C. These values of z imply the following rates of interconversion in mitochondria of fumarate and oxaloacetate (in terms of the oxaloacetate-->citrate flux, R) in rats at 20 degrees C: [Formula: see text] and in rats at 30 degrees C: [Formula: see text] 2. The kinetic parameters of malate dehydrogenase and fumarate hydratase and the intramitochondrial concentrations of NAD(+) and NADH under (as far as could be judged) conditions in vivo were collated. From them and the best estimates of R now available were calculated the rates of interconversion of fumarate, malate and oxaloacetate required to give the found values of z. These rates showed that the fumarate hydratase reaction was nearly in equilibrium, but that the malate dehydrogenase reaction was considerably out of equilibrium. The calculations also led to the following conclusions. 3. In livers of rats at 20 degrees and 30 degrees C mitochondrial malate concentrations were respectively about 5 and 1.5 times mean cellular concentrations. 4. Mitochondrial oxaloacetate concentrations were less than 0.2 of the mean cellular concentrations. They were also only 0.65 and 0.55 of the equilibrium concentrations for the malate dehydrogenase reaction in rats at 20 degrees and 30 degrees C respectively. 5. Malate dehydrogenase activity was low because of the very low oxaloacetate concentrations in the mitochondria and the very small fraction of the enzyme complexed with NAD(+), i.e. in each direction one substrate concentration was very sub-optimal.