Interrelations between ureogenesis and gluconeogenesis in isolated hepatocytes. The role of antion transport and the competition for energy

Hypothalamic sensing of circulating fatty acids is required for glucose homeostasis

Increased glucose production is a hallmark of type 2 diabetes and alterations in lipid metabolism have a causative role in its pathophysiology. Here we postulate that physiological increments in plasma fatty acids can be sensed within the hypothalamus and that this sensing is required to balance their direct stimulatory action on hepatic gluconeogenesis. In the presence of physiologically-relevant increases in the levels of plasma fatty acids, negating their central action on hepatic glucose fluxes through (i) inhibition of the hypothalamic esterification of fatty acids, (ii) genetic deletion (Sur1-deficient mice) of hypothalamic K(ATP) channels or pharmacological blockade (K(ATP) blocker) of their activation by fatty acids, or (iii) surgical resection of the hepatic branch of the vagus nerve led to a marked increase in liver glucose production. These findings indicate that a physiological elevation in circulating lipids can be sensed within the hypothalamus and that a defect in hypothalamic lipid sensing disrupts glucose homeostasis.

Abolition of the inhibitory effect of ethanol oxidation on gluconeogenesis from lactate by asparagine or low concentrations of ammonia

When isolated hepatocytes from fasted rats were incubated with 10 mM lactate, the [lactate]/[pyruvate] ratio measured at the beginning of the incubation was raised above 70:1 but declined to a steady level of about 8:1 within 40 min. The rate of gluconeogenesis from lactate was initially slow but gradually increased over the incubation period becoming maximal by 30 min. The simultaneous addition of lactate and ethanol resulted in an initial [lactate]/[pyruvate] ratio above 250:1 which by 60 min had declined to a new steady-state level of approx. 60:1. The lactate, ethanol combination also brought about a prolongation of the lag phase before glucose synthesis became maximal; however, by 40 min the rate of gluconeogenesis was independent of the presence of ethanol. Thus the inhibitory effect of ethanol on glucose synthesis was manifest only over the early portion of the incubation period. When asparagine, a precursor of malate/aspartate components, was added to the incubation mixture, the lag before maximal rates of glucose formation from lactate in the absence or presence of ethanol was almost abolished. The presence of asparagine also rapidly lowered the [lactate]/[pyruvate] ratio of hepatocytes incubated with lactate plus ethanol establishing a steady-state level of 15:1 within 10-15 min. Asparagine enhanced the rate of lactate-stimulated ethanol oxidation, particularly during the early part of the incubation. In endeavouring to elucidate which of the products of asparagine catabolism (i.e. ammonia and aspartate) were responsible for these effects, we found that a small and constant level of ammonia, formed by the degradation of urea by urease, almost reproduced the effects of asparagine on the [lactate]/[pyruvate] ratio, glucose synthesis and ethanol oxidation. A bolus addition of 10 mM aspartate or 4 mM ammonia to cells metabolising lactate and ethanol were less effective than a steady-state low ammonia concentration, generated from urea/urease. Our studies suggest that asparagine or a low concentration of ammonia, by providing components of the malate/aspartate shuttle, can ameliorate some of the metabolic effects of ethanol on the liver.

Reciprocal changes in gluconeogenesis and ureagenesis induced by fatty acid oxidation

Fatty acids produced a stimulation of gluconeogenesis and either inhibition or no effect on ureagenesis in livers perfused with gluconeogenic substrates and having NH4Cl plus ornithine as the nitrogen source. This finding indicates that stimulation of flux through pyruvate carboxylase is not sufficient to enhance urea production from ammonia. The metabolic action of fatty acids showed the following characteristics: (1) it was concentration-dependent, showing saturation-type kinetics similar to those described for fatty acid oxidation; (2) the stimulatory action on gluconeogenesis was constant and independent of NH4Cl concentration, whereas the inhibition of ureagenesis was variable and dependent on NH4Cl concentration and the degree of reduction of the gluconeogenic substrate; and (3) fatty acids produced apparent reciprocal changes in the state of reduction of the cytosolic and mitochondrial NAD systems. Fatty acid oxidation exerted its effect mainly, if not exclusively, by preventing the gluconeogenic substrate-induced stimulation of ureagenesis. Fatty acids also inhibited ureagenesis without stimulating gluconeogenesis (lactate < 1 mmol/L), ruling out a limiting energy availability as the cause of the inhibition. One or both of the following two mechanisms seem to account for the fatty acid-induced inhibition of ureagenesis from NH4Cl. First, a decreased uptake of ornithine, and second, decreased flux through pyruvate dehydrogenase and probably other NAD(P)-linked mitochondrial dehydrogenases. The correlation found between the ability of fatty acids to inhibit ureagenesis and the state of activation of pyruvate dehydrogenase supports the latter point.

Interrelationships between ureogenesis and gluconeogenesis in perfused rat liver

Stimulation of ureogenesis by ornithine and/or NH4Cl inhibited gluconeogenesis from lactate but not from equimolar concentrations of pyruvate in perfused rat liver. Neither a shortage of energy nor a decrease in alpha-ketoglutarate availability seems to be responsible for this inhibition. With lactate as substrate the extracellular concentration of pyruvate attained was approximately equal to 0.15 mM that assuming reflects its cytosolic concentration it would be limiting for its mitochondrial transport. Stimulation of ureogenesis from NH4Cl enhances flux through pyruvate dehydrogenase. Furthermore, activation of pyruvate dehydrogenase by dichloroacetate led to stimulation of ureogenesis and inhibition of glucose production. Conversely, inhibition of pyruvate dehydrogenase flux by fatty acid enhanced glucose production and inhibited ureogenesis. Thus, ornithine and/or NH4Cl seem to inhibit lactate to glucose flux by shifting the mitochondrial partitioning of pyruvate from carboxylation towards decarboxylation with the result of a decreased oxaloacetate formation. Gluconeogenic substrates enhanced the hepatic uptake of ornithine. However, no correlation seems to exist between the uptake of ornithine, ornithine-induced stimulation of ureogenesis and total rates of urea production. Ornithine produced a concentration-dependent acidification of the hepatic outflow perfusate, suggesting that it may be transported in exchange for H+.

On the mechanism of stimulation of ureagenesis by gluconeogenic substrates: role of pyruvate carboxylase

Gluconeogenic substrates, lactate or pyruvate, or ornithine produced 100% increase of urea synthesis from NH4Cl. The combined administration of ornithine and lactate (or pyruvate) produced more than additive effects, indicating that they acted at different steps in a potentiating manner. The uptake of ornithine was enhanced by gluconeogenic substrates. This finding may explain, at least in part, the stimulating effect of these substrates on ureagenesis from NH4Cl and ornithine. The gluconeogenic substrate-induced stimulation of ureagenesis from NH4Cl was still observed under conditions of reduced flux through pyruvate carboxylase, ruling out that their action was exclusively mediated by the anaplerotic effect of this enzyme. Pyruvate was a more potent stimulator of ureagenesis than lactate and its effect less sensitive to pyruvate carboxylase inhibition. These observations indicate that a correlation exists between stimulation of ureagenesis by gluconeogenic substrates and flux through pyruvate dehydrogenase. It is concluded that gluconeogenic substrates may stimulate ureagenesis from NH4Cl by 1) increasing intracellular ornithine availability and/or 2) enhancing flux through pyruvate dehydrogenase and consequently the tricarboxylic acid cycle activity.

Inhibition of gluconeogenesis, ureogenesis and drug oxidation by redox cycler quinone in isolated mouse hepatocytes

1. The effect of a redox cycler and arylator (menadione) and a pure arylator quinone (benzoquinone) was studied on different NADPH generating and consuming processes in isolated mouse hepatocytes. 2. Menadione inhibited gluconeogenesis from alanine but not from fructose or glycerol. 3. Drug oxidation measured as aniline hydroxylation and aminopyrine N-demethylation could be inhibited by menadione in microsomal membrane and in isolated hepatocytes both from fed or fasted animals. 4. Ureogenesis in isolated hepatocytes from fed mice could not be inhibited even by high concentration of menadione, while in cells from fasted animals menadione was inhibitory at high concentration in the presence of gluconeogenic precursor and at lower concentration in the absence of it. 5. Benzoquinone did not inhibit the above mentioned processes.

Stimulation of alanine metabolism by ammonia in the perfused rat liver. Quantitative analysis by means of a mathematical model

The effect of ammonia on the catabolism of alanine was studied in the perfused rat liver. Addition of 0.5 mM NH4Cl to the perfusion medium containing 5 mM alanine plus 0.1 mM octanoate produced drastic changes in the metabolite concentrations in the efflux medium. Not only the rate of ureogenesis was activated, but also the formation of glucose, lactate and pyruvate. Additionally, respiration was stimulated, the output of ketone bodies decreased, and the redox ratios lactate/pyruvate as well as 3-hydroxybutyrate/acetoacetate became more oxidized. To interpret the causes of these metabolic changes, a mathematical model was developed. It contains kinetic equations by which fluxes through essential pathways of alanine catabolism, gluconeogenesis and energy metabolism were related to the intracellular concentrations of pyruvate, oxaloacetate and ammonia, as well as to the redox ratios lactate/pyruvate and 3-hydroxybutyrate/acetoacetate. Using a nonlinear regression procedure, the model was suitable to be fitted to the data found in the experiments. The consistency of the model and experiment allowed the changes caused by ammonia to be explained. Primarily, ammonia stimulated ureogenesis hence accelerating the deamination of alanine which led to the increased formation of pyruvate, lactate and glucose. The enhanced energetic load resulting from ureogenesis and gluconeogenesis shifted the mitochondrial and cytosolic NAD systems towards more oxidized states which additionally modified the flux rates. The results demonstrate that there is a high degree of cooperativity between the metabolic pathways.

Effect of propionate on the utilization of nitrogen from 15NH4Cl for urea synthesis in hepatocytes isolated from sheep liver

1. The effect of ornithine (2.0 mM) and propionate (5.0 mM) on the utilization of N from 15NH4Cl (5.0 mM) for urea synthesis in hepatocytes isolated from sheep liver was investigated. 2. The capacity of sheep hepatocytes to utilize [15N]ammonia in the absence of the other exogenous substrates was very low and amounted 132 +/- 37.3 mumol/hr per 1 g dry wt. 3. Ornithine failed to affect the total [15N]ammonia uptake and total urea synthesis, but at the same time it markedly increased the utilization of [15N]ammonia for ureagenesis and diminished the rate of urea synthesis from endogenous sources. 4. Propionate markedly increased total [15N]ammonia utilization and total urea formation; this increase resulted from the rise of ammonia utilization for urea synthesis and it was similar in the presence or absence of ornithine. 5. The capacity of sheep liver cells to utilize ammonia in the presence of propionate (in the presence or absence of ornithine) amounted to 256 mumol/hr per 1 g dry wt, thus being similar to the values in vivo. 6. It is concluded that in sheep hepatocytes both ornithine and propionate stimulate the utilization of ammonia for urea synthesis and these effects take place independently and occur by different mechanisms.

Significance of the alanine aminotransferase reaction in the formation of alpha-ketoglutarate in rat liver mitochondria

The total production of alpha-ketoglutarate from glutamate and isocitrate was estimated in isolated rat liver mitochondria. Mitochondrial alanine aminotransferase converts glutamate to alpha-ketoglutarate [A.K. Groen et al. (1982) Eur. J. Biochem. 122, 87-93], thus participating in the net formation of the tricarboxylic acid cycle intermediates from glutamate. The present investigation indicates a significant contribution of the alanine aminotransferase reaction to glutamate oxidation by isolated rat liver mitochondria in the presence of bicarbonate. It amounted to 41-74 and 7-31% of the total utilization of glutamate in States 4 and 3, respectively, in various conditions in vitro, at pyruvate concentrations in the range of 0.1-10 mM. The participation of glutamate in the total production of alpha-ketoglutarate at physiological concentrations of glutamate, citrate, and isocitrate varied in the range of 72-82%. It was calculated that alpha-ketoglutarate formation by the reaction of alanine aminotransferase amounted to 30 and 5% of the total mitochondrial alpha-ketoglutarate production in States 4 and 3, respectively, at physiological concentrations of its precursors and in the presence of 0.5 mM malate and 0.1 mM pyruvate. It constituted 77-97% of the net production of the tricarboxylic acid cycle intermediates from glutamate in rat liver mitochondria. The importance of alpha-ketoglutarate production via the alanine aminotransferase reaction under various physiological conditions is discussed.

Interrelationship between drug oxidation ureogenesis and gluconeogenesis in isolated hepatocytes

1. The effect of increased ureogenesis--provoked by NH4Cl and ornithine--on gluconeogenesis and aminopyrine oxidation was studied in isolated hepatocytes prepared from 24 hr starved mice; lactate or fructose was used as gluconeogenic precursor. 2. Increased ureogenesis caused about 40% inhibition both on aminopyrine oxidation and gluconeogenesis when lactate was added as gluconeogenic substrate. 3. On the other hand, only 10% inhibition of aminopyrine oxidation and about 15% inhibition of gluconeogenesis were observed when fructose was used as gluconeogenic precursor. 4. Aminopyrine has been reported to inhibit gluconeogenesis from fructose by 30% and from lactate by 85%. The inhibitory effect of the combined addition of aminopyrine, NH4Cl and ornithine on gluconeogenesis was also dependent on the applied gluconeogenic precursor. 5. The provoked ureogenesis by ammonia and ornithine was not inhibited by aminopyrine. N6, O2-dibutyryl cAMP known to cause an increase of gluconeogenesis a decrease of aminopyrine oxidation enhanced the inhibitory action of increased ureogenesis on aminopyrine oxidation and on gluconeogenesis further. 6. The role of NADPH in the regulation of drug oxidation and ureogenesis is underlined.

Permeability changes in hepatic mitochondria and altered glucose and urea metabolism in aroclor 1254-treated rats

The influence of Aroclor 1254 (ARO) treatment or pair-feeding (PF) on gluconeogenesis and urea synthesis and on isolated hepatic mitochondria was studied in rats of different ages. ARO (300 mg/kg, po on 4 consecutive days) induced variable weight loss in young (153 +/- 10 g (initial wt), -10.9%), intermediate-age (195 +/- 10 g, -17.0%), and old (232 +/- 23 g, -4.9%) rats. Isolated mitochondria contained equal amounts of cytochromes aa3, b, c1 and c with exception that c1 and c were lower in the young ARO rats than in the PF controls. Mitochondria from ARO rats, which lost more weight than ad libitum-fed (AF) rats, showed suppression of ADP-stimulated H+ and oxygen uptake and succinate plus valinomycin maximal swelling in a potassium acetate and sucrose medium. Mitochondria from young ARO rats absorbed less incident light than mitochondria from PF or AF rats. Maximally swollen mitochondria from intermediate-age ARO rats, contracted more rapidly with antimycin addition than those from PF or AF controls. These findings showed greater permeability of ARO mitochondria to impermeable and accumulated ions. In contrast, mitochondria from ARO rats without significant weight loss showed activation of ADP-stimulated H+ and oxygen uptake and maximal swelling in comparison to mitochondria from AF and PF rats, but contracted like these controls after the antimycin addition. Urea synthesis in ARO rats, which lost 9.9% of initial body wt (173 +/- 6 g) and experienced a nitrogen deficit (Ebner et al., 1986), was significantly increased 12 min postinjection of NH4 acetate, and was greater than the urea level in AF rats at this time point. In comparison, gluconeogenesis was significantly increased in AF rats 12 min postinjection of NH4 acetate and was greater than in ARO rats at this time point. These differences were also observed when the data were expressed as the rate of glucose or urea appearance in peripheral blood per 100 g body wt. In PF rats, blood glucose and urea concentrations were intermediate to and indistinguishable from the ARO and AF groups. These data demonstrate that hepatic mitochondria from ARO rats that experienced a significant loss of body weight were suppressed and more permeable to ions than AF and PF controls. These mitochondrial properties may have predisposed the ARO rats toward urea formation rather than glucose synthesis and nitrogen retention.

Mitochondrial pyruvate carrier--a possible link between gluconeogenesis and ketogenesis in the liver

Effects of various ketogenic substrates on gluconeogenesis from lactate or alanine were compared. The results suggest that, in intact liver cells, cytoplasmic pyruvate is transported into mitochondria in exchange for intramitochondrially generated acetoacetate. An interrelationship between gluconeogenesis and ketogenesis may thus exist in the liver at the level of mitochondrial pyruvate carrier.

Gluconeogenesis in goat hepatocytes is affected by calcium, ammonia and other key metabolites but not primarily through cytosolic redox state

1. Gluconeogenesis from propionate and lactate was studied in caprine hepatocytes. 2. Reducing cytosol with additions of ETOH, ammonium, or lactate decreased [2-14C]propionate conversion to glucose. 3. Calcium oxidized the cytosol and increased gluconeogenesis from propionate by 198% and from lactate by 220%. 4. Cells isolated from lactating does and wethers differed quantitatively in propionate conversion to glucose and response to calcium. 5. Acetoacetate decreased and 3-OH-butyrate slightly increased glucose production from propionate. 6. Neither ketone body had any significant effect on gluconeogenesis from lactate. 7. Results reported herein suggest gluconeogenesis from propionate is not limited by lack of cytosolic reducing equivalents.

Urea synthesis in freshly isolated and in cultured periportal and perivenous hepatocytes

Periportal hepatocytes isolated by digitonin/collagenase perfusion produced urea faster than did similarly prepared perivenous hepatocytes, in both the presence and the absence of amino acids and various urea precursors. There was no difference between the two cell types in rates of intracellular proteolysis. The initial difference in urea synthesis persisted for 5 days during primary culture, but then gradually disappeared. Our results demonstrate that the periportal dominance of urea formation is unrelated to the currently existing acinar microenvironment in the intact liver, but probably reflects differences in acinar key enzyme activities only slowly converging during culture.

Utilization of nitrogen from 15NH4Cl and [15N]alanine for urea synthesis in hepatocytes from fed and starved rats

Utilization of N from 15NH4Cl and [15N]alanine for urea synthesis in hepatocytes isolated from fed and 24 hr starved rats was investigated. In hepatocytes isolated from fed rats, 54 and 65% of the added [15N]ammonia was utilized for urea synthesis in the presence of 0.5 and 2.0 mM NH4Cl, respectively. This utilization of [15N]ammonia in hepatocytes from starved rats was 2-fold lower. The amount of urea synthetized from endogenous sources was, in the presence of 0.5 and 2.0 mM NH4Cl, about 44 and 60% higher than in the control conditions (without NH4Cl). The considerable amount of added ammonia (30-44%) was utilized in processes other than urea synthesis. Alanine markedly diminished the utilization of 15N from NH4Cl in hepatocytes from both fed and starved rats. In these conditions (NH4Cl present), alanine significantly increased the urea formation in hepatocytes from starved rats and failed to affect the urea production in hepatocytes from fed rats. On the basis of 15N determination, it was concluded that both NH4Cl and alanine caused an increase in the utilization of nitrogen from endogenous sources in rat hepatocytes. This conclusion is in contrast with the results based only on the changes in ammonia and urea concentrations.

Effects of glucose on alanine-derived urea synthesis

The effect of glucose on alanine-stimulated urea synthesis was studied in six healthy volunteers during 6 h of constant alanine infusion, 2.8 mmol h-1 kg-1 b. wht., and during 12 h of constant glucose infusion, 4.0 mmol h-1 kg-1 b. wht., with superimposed alanine infusion. The urea nitrogen synthesis rate (UNSR) was determined at intervals of 2 h as urinary excretion rate corrected for accumulation and intestinal hydrolysis. UNSR depended on the blood alanine and glucagon concentration, but was not correlated with glucose, lactate, or insulin concentrations. The slope of the linear relation between UNSR and alanine concentration (the 'Functional Hepatic Nitrogen Clearance') was on the average 24.4 1 h-1 and decreased to 12.8 1 h-1 by glucose (mean difference +/- SE of the difference 10.6 +/- 7.3, P less than 0.01). The relation between glucagon and alanine concentration was linear, and the slope was decreased to 40 per cent by glucose (P less than 0.05). The slope of the linear relation between UNSR and glucagon was not changed by glucose. Thus the catabolism of alanine nitrogen is decreased by glucose because of a reduction of the urea synthesis. Data suggest that this may be due to a depression of the glucagon response to alanine.

Effects of L-carnitine on urea synthesis following acute ammonia intoxication in mice

L-Carnitine protects mice against acute ammonia intoxication. The effect of L-carnitine on in vivo incorporation of [14C] bicarbonate into urea has been investigated in mice given large doses of ammonium acetate. The hepatic content of N-acetylglutamate has been measured. Following ammonia administration the animals given L-carnitine have much higher production of urea than the unprotected mice. The marked protective effect of L-carnitine on ammonium acetate-induced hyperammonemia and on the increase in urea synthesis is not due primarily to activation of N-acetylglutamate synthetase.

The elucidation of the effect of ammonium chloride on pyruvate distribution and pyruvate dehydrogenase interconversion in isolated rat hepatocytes

The distribution of pyruvate between cell compartments measured in isolated hepatocytes in the presence of lactate was in agreement with delta pH across plasma and mitochondrial membranes. In isolated liver mitochondria NH4Cl decreased the transmembrane potential (delta psi) by about 14 mV, whereas no change of delta pH was observed. In the presence of lactate or alanine NH4Cl increased the mitochondrial pyruvate concentration presumably due to the inhibition of the flux through pyruvate carboxylase. In the presence of lactate or alanine changes in the amount of the active form of pyruvate dehydrogenase (PDHa) were correlated with the mitochondrial pyruvate concentration, NH4Cl increased the amount of PDHa by lowering the mitochondrial ATP/ADP and NADH/NAD+ ratios.

Possible interrelationship between gluconeogenesis and ketogenesis in the liver

Effects of various ketogenic substrates on gluconeogenesis from lactate were examined. D,L-3-Hydroxybutyrate (5 mM) stimulated gluconeogenesis by 41%, the effect being the same as that of 5 mM acetate (49%). No stimulating effect of acetoacetate was observed; conversely, acetoacetate (up to 40 mM) partially or completely abolished the observed stimulating effects of acetate, oleate, and 3-hydroxybutyrate. The results suggest that, in intact liver cells, pyruvate is transported into mitochondria in exchange for acetoacetate and that an interrelationship between gluconeogenesis and ketogenesis at the level of mitochondrial pyruvate carrier may exist in the liver.

Turnover of N-acetylglutamate in isolated rat hepatocytes

Isolated hepatocytes from starved rats were loaded with N-[14C]acetylglutamate by preincubating them with [14C]bicarbonate, oleate, NH3, ornithine and lactate. Turnover of N-acetylglutamate in these cells was subsequently measured in an unlabelled medium under conditions of minimal flux (oleate alone present) and maximal flux (oleate, NH3, ornithine and lactate present) through the urea cycle. 1. Direct measurement of the distribution of N-[14C]acetylglutamate across the mitochondrial membrane in the hepatocytes showed that, under the conditions studied, the rate of degradation of total intracellular N-[14C]acetylglutamate was about equal to the rate of efflux of N-acetylglutamate from the mitochondria. 2. In the presence of oleate alone, intramitochondrial N-acetylglutamate decreased because mitochondrial N-acetylglutamate efflux predominated over the synthesis of N-acetylglutamate in the mitochondria. 3. In the presence of oleate, NH3, ornithine and lactate both the rate of synthesis of N-acetylglutamate and the rate of its transport out of the mitochondria were increased when compared with the condition with oleate alone. However, the intramitochondrial concentration of N-acetylglutamate increased because initially the rate of its synthesis exceeded that of its efflux from the mitochondria. Finally, a steady state was reached in which both rates were equal. 4. The data indicate that in hepatocytes from starved rats N-acetylglutamate transport out of the mitochondria takes place at a rate proportional to its intramitochondrial concentration. It is concluded that transport of N-acetylglutamate either occurs by diffusion or is mediated by a transport system with a high Km for intramitochondrial N-acetylglutamate.

Effect of palmitate on hepatic biosynthetic functions at hyperthermic temperatures

Livers of fasted rats were perfused for 80 min at 37 degrees-43 degrees C, supplemented with lactate, NH4Cl, and ornithine in the presence or absence of palmitate. Hepatic functional integrity was maintained from 37 degrees to 42 degrees C as assessed by gluconeogenesis, ureogenesis, and O2 consumption. Between 42 degrees and 43 degrees C a sharp decline in biosynthetic function occurred. The sharp decline in biosynthetic function occurred. The ratio of lactate disappearance to glucose formation increased progressively with increasing temperature when compared with the ratio obtained at 37 degrees C. Exogenous palmitate significantly decreased the ratio of lactate disappearance to glucose formation at 43 degrees C. Furthermore, palmitate attenuated the detrimental effects of hyperthermia on gluconeogenesis, ureogenesis, and O2 consumption found in the absence of palmitate. The 3-hydroxybutyrate/acetoacetate ratio progressively decreased as the liver temperature was increased in the presence or absence of palmitate, indicating a more oxidized mitochondrial redox state. Palmitate significantly increased the 3-hydroxybutyrate/acetoacetate ratio in the presence of gluconeogenic and ureogenic substrates at all temperatures examined. The data suggest that provision of fatty acid has a protective effect in thermally stressed liver. Moreover, palmitate may substitute for the increased energy requirements of the hyperthermic state.

Inhibition of urea-cycle activity by high concentrations of alanine

1. Conditions are described in which high intracellular alanine concentrations inhibit urea-cycle flux in isolated hepatocytes. 2. Inhibition of urea-cycle flux by added alanine is DL-cycloserine-insensitive and is accompanied by an increase in intracellular citrulline and a decrease in ornithine. 3. Argininosuccinate synthetase (EC 6.3.4.5) activity in rat liver cytosol is inhibited by alanine in a competitive manner with respect to citrulline. It is concluded that this effects is the primary cause of inhibition of urea-cycle flux by alanine.