Adenine nucleotide pool size, adenine nucleotide translocase activity, and respiratory activity in newborn rabbit liver mitochondria

Mitochondrial morphology governs ATP production rate

Life is based on energy conversion. In particular, in the nervous system, significant amounts of energy are needed to maintain synaptic transmission and homeostasis. To a large extent, neurons depend on oxidative phosphorylation in mitochondria to meet their high energy demand. For a comprehensive understanding of the metabolic demands in neuronal signaling, accurate models of ATP production in mitochondria are required. Here, we present a thermodynamically consistent model of ATP production in mitochondria based on previous work. The significant improvement of the model is that the reaction rate constants are set such that detailed balance is satisfied. Moreover, using thermodynamic considerations, the dependence of the reaction rate constants on membrane potential, pH, and substrate concentrations are explicitly provided. These constraints assure that the model is physically plausible. Furthermore, we explore different parameter regimes to understand in which conditions ATP production or its export are the limiting steps in making ATP available in the cytosol. The outcomes reveal that, under the conditions used in our simulations, ATP production is the limiting step and not its export. Finally, we performed spatial simulations with nine 3-D realistic mitochondrial reconstructions and linked the ATP production rate in the cytosol with morphological features of the organelles.

Modeling of ATP-ADP steady-state exchange rate mediated by the adenine nucleotide translocase in isolated mitochondria

A computational model for the ATP-ADP steady-state exchange rate mediated by adenine nucleotide translocase (ANT) versus mitochondrial membrane potential dependence in isolated rat liver mitochondria is presented. The model represents the system of three ordinary differential equations, and the basic components included are ANT, F(0)/F(1)-ATPase, and the phosphate carrier. The model reproduces quantitatively the relationship between mitochondrial membrane potential and the ATP-ADP steady-state exchange rate mediated by the ANT operating in the forward mode, with the assumption that the phosphate carrier functions under rapid equilibrium. Furthermore, the model can simulate the kinetics of experimentally measured data on mitochondrial membrane potential titrated by an uncoupler. Verified predictions imply that the ADP influx rate is highly dependent on the mitochondrial membrane potential, and in the 0-100 mV range it is close to zero, owing to extremely low matrix ATP values. In addition to providing theoretical values of free matrix ATP and ADP, the model explains the diminished ADP-ATP exchange rate in the presence of nigericin, a condition in which there is hyperpolarization of the inner mitochondrial membrane at the expense of the mitochondrial Delta pH gradient; the latter parameter influences matrix inorganic phosphate and ATP concentrations in a manner also described.

Mitochondrial Transporters as Novel Targets for Intracellular Calcium Signaling

Ca2+signaling in mitochondria is important to tune mitochondrial function to a variety of extracellular stimuli. The main mechanism is Ca2+entry in mitochondria via the Ca2+uniporter followed by Ca2+activation of three dehydrogenases in the mitochondrial matrix. This results in increases in mitochondrial NADH/NAD ratios and ATP levels and increased substrate uptake by mitochondria. We review evidence gathered more than 20 years ago and recent work indicating that substrate uptake, mitochondrial NADH/NAD ratios, and ATP levels may be also activated in response to cytosolic Ca2+signals via a mechanism that does not require the entry of Ca2+in mitochondria, a mechanism depending on the activity of Ca2+-dependent mitochondrial carriers (CaMC). CaMCs fall into two groups, the aspartate-glutamate carriers (AGC) and the ATP-Mg/Picarriers, also named SCaMC (for short CaMC). The two mammalian AGCs, aralar and citrin, are members of the malate-aspartate NADH shuttle, and citrin, the liver AGC, is also a member of the urea cycle. Both types of CaMCs are activated by Ca2+in the intermembrane space and function together with the Ca2+uniporter in decoding the Ca2+signal into a mitochondrial response.

Effects of hypobaric hypoxia on adenine nucleotide pools, adenine nucleotide transporter activity and protein expression in rat liver

AIM: To explore the effect of hypobaric hypoxia on mitochondrial energy metabolism in rat liver.

METHODS: Adult male Wistar rats were exposed to a hypobaric chamber simulating 5000 m high altitude for 23 h every day for 0 (H0), 1 (H1), 5 (H5), 15 (H15) and 30 d (H30) respectively. Rats were sacrificed by decapitation and liver was removed. Liver mitochondria were isolated by differential centrifugation program. The size of adenine nucleotide pool (ATP, ADP, and AMP) in tissue and mitochondria was separated and measured by high performance liquid chromatography (HPLC). The adenine nucleotide transporter (ANT) activity was determined by isotopic technique. The ANT total protein level was determined by Western blot.

RESULTS: Compared with H0 group, intra-mitochondrial ATP content decreased in all hypoxia groups. However, the H5 group reached the lowest point (70.6%) (P < 0.01) when compared to the control group. Intra-mitochondrial ADP and AMP level showed similar change in all hypoxia groups and were significantly lower than that in H0 group. In addition, extra-mitochondrial ATP and ADP content decreased significantly in all hypoxia groups. Furthermore, extra-mitochondrial AMP in groups H5, H15 and H30 was significantly lower than that in H0 group, whereas H1 group had no marked change compared to the control situation. The activity of ANT in hypoxia groups decreased significantly, which was the lowest in H5 group (55.7%) (P < 0.01) when compared to H0 group. ANT activity in H30 group was higher than in H15 group, but still lower than that in H0 group. ANT protein level in H5, H15, H30 groups, compared with H0 group decreased significantly, which in H5 group was the lowest, being 27.1% of that in H0 group (P < 0.01). ANT protein level in H30 group was higher than in H15 group, but still lower than in H0 group.

CONCLUSION: Hypobaric hypoxia decreases the mitochondrial ATP content in rat liver, while mitochondrial ATP level recovers during long-term hypoxia exposure. The lower level of extra-mitochondrial ATP may be related to the decrease of ANT activity during hypoxia exposure.

Expression of the ADP/ATP carrier and expansion of the mitochondrial (ATP + ADP) pool contribute to postnatal maturation of the rat heart

The role of the ADP/ATP carrier (AAC), a key protein of the mitochondrial ATP-generating system, is not clear during postnatal rat heart development. To elucidate this role, the phosphorylating respiration (state 3), the activity and the content of AAC, the size of the exchangeable mitochondrial (ATP + ADP) pool and the control of AAC over respiration at state 3 were measured in mitochondria isolated from rat hearts at various postnatal ages. There was a 5-fold increase in the AAC activity from newborn to aged rat hearts, which was paralleled by a 1.5-fold increase in state 3 respiration. At birth, the AAC and the F0F1-ATP synthase exerted about 80% of the control over phosphorylating respiration (state 3: flux control coefficients 0.39 +/- 0.04 and 0.38 +/- 0.08). The strong increase in the AAC activity was partly caused by the doubling of the protein content. In addition, the turnover number of AAC increased by a factor of 2.5 due to the expansion of the (ATP + ADP) pool from 3.4 +/- 0.9 to 10.6 +/- 1.5 nmol.mg protein-1. The data strongly indicate that the increase in the AAC activity is an essential step in the postnatal maturation of rat heart mitochondria.

Developmental changes of the adenine nucleotide translocation in rat brain

The perinatal development of the adenine nucleotide translocation in isolated rat brain mitochondria was studied. For that purpose the content of the adenine nucleotide translocase (ANT), the activity of adenine nucleotide translocation and the control of the ANT protein over State 3 respiration were estimated. From the newborn to the adult state there was a 4-fold increase in State 3 respiration which was paralleled by a 3-fold increase in the respiratory control ratio. The capacity of uncoupled respiration exceeded that of State 3 respiration in all developmental stages indicating that the activity of oxidative phosphorylation is influenced by that of ANT and/or ATP synthase. The content of the ANT protein, measured as bound pmoles of [3H]atractyloside per mg mitochondrial protein, increased more than 2-fold from birth to adultness in the first three postnatal weeks. The size of the exchangeable matrix (ATP + ADP)-pool was only slightly expanded during the same period. The translocation activity increased 2-fold from the newborn to the adult state and was a linear function of the ANT protein. Control of the ANT protein over State 3 respiration (quantified as flux control coefficient, CJoANT), was remarkable in brain mitochondria from newborn rats (CJoANT = 0.45 +/- 0.15), but declined during further development (CJoANT = 0.11 +/- 0.03, at the 20th day). The obtained results suggest that the postnatal enrichment of the ANT protein in rat brain mitochondria is an essential factor for the development of oxidative phosphorylation capacity in the early postnatal period.

Increase in the adenine nucleotide translocase protein contributes to the perinatal maturation of respiration in rat liver mitochondria

An assay based on the high-affinity binding of tritium-labelled atractyloside to the adenine nucleotide translocase (ANT) was developed for estimation of its content in samples of mitochondria, cells and tissue homogenate. The assay was used to study the developmental change of the ANT protein concentration in perinatal rat liver. Within the last 3 days before birth the content of the ANT protein per mg tissue protein increased from 29 to 45% of the maximum value found 2 days after birth. A similar developmental change of the ANT protein was found in isolated mitochondria, demonstrating that the perinatal increase in the ANT protein content was due mainly to a mitochondrial differentiation process and not the result of an increase in the number of mitochondria per cell. A close proportionality between the ANT protein and the ADP-stimulated respiration of liver homogenate was found in the perinatal period from 3 days before to 2 days after birth. This finding suggests that the developmental change in the ANT protein content plays an important role in the onset of oxidative phosphorylation after birth.

Postischemic recovery of mitochondrial adenine nucleotides in the heart

BACKGROUND

Adenine nucleotides (AdNs) are lost from the mitochondrial fraction of the heart cell during ischemia. It is unknown whether this pool of AdNs can be replenished after reperfusion. The purpose of this study was to evaluate the postischemic recovery of the mitochondrial AdN pool.

METHODS AND RESULTS

The left anterior descending coronary artery (LAD) of the canine heart was occluded for 30 minutes followed by either no reflow, 30-minute reflow, 1-day reflow, or 7-day reflow. Systolic shortening in the LAD-supplied region was absent during occlusion but recovered to approximately 30% of preocclusion values during early reperfusion. Mitochondrial and tissue AdNs (ATP, ADP, and AMP) were determined in the LAD-supplied and left circumflex-supplied (control) regions of the heart. The AdN content (expressed as percent of control values) of mitochondria from the LAD region was 55 +/- 10% (p less than 0.002), 64 +/- 7% (p less than 0.001), 81 +/- 6% (p less than 0.03), and 94 +/- 8% for the no-reflow, 30-minute-reflow, 1-day-reflow, and 7-day-reflow groups, respectively. The AdN content (expressed as percent of control values) of tissue samples from the LAD region was 52 +/- 9% (p less than 0.002), 48 +/- 12% (p less than 0.02), 68 +/- 5% (p less than 0.002), and 70 +/- 9% for the no-reflow, 30-minute-reflow, 1-day-reflow, and 7-day-reflow groups, respectively. There was a good correlation between mitochondrial and tissue AdN (r = 0.95). Using initial exchange rates, adenine nucleotide translocase activities of mitochondria from the LAD and control regions were not significantly different. State 3 respiration of LAD mitochondria was depressed (approximately 25%, p less than 0.05) only in the no-reflow group. Acceptor control ratios of the LAD mitochondria were not significantly different from control values in any group.

CONCLUSIONS

After 30 minutes of regional ischemia, postischemic restoration of the mitochondrial AdN pool occurs between 1 and 7 days; this restoration is preceded by recovery of respiratory and adenine nucleotide translocase functions. Although the abnormally low levels of AdN persist in the mitochondrial compartment during the early reperfusion period, postischemic contractile dysfunction cannot be explained by depressed mitochondrial respiratory activity.

Regulation of fatty acid oxidation in isolated hepatocytes and liver mitochondria from newborn rabbits

The changes in long-chain fatty acid oxidation during the first 24 h after birth were studied in isolated rabbit hepatocytes and liver mitochondria. The eightfold increase in this oxidation which occurs in hepatocytes between birth and 24 h was not triggered by a concomitant decrease in long-chain fatty acid esterification. Indeed, in isolated hepatocytes from 24-h-old rabbits, the 75% inhibition of the oxidation by 2-tetradecylglycidic acid, resulted in a total redirection of oleate metabolized towards triacylglycerol synthesis. Polarographic measurements of mitochondrial respiration showed that oxidative phosphorylation and respiratory chain capacity were fully functional at birth. By contrast, in liver mitochondria isolated from newborn rabbits, the rate of oxygen consumption from palmitoyl-L-carnitine was 60% higher than from palmitoyl-CoA. Similarly palmitoyl-CoA oxidation was increased 1.5-fold in isolated mitochondria from 24-h-old rabbits. These results were in agreement with the twofold increase in the activity of hepatic carnitine palmitoyltransferase I between birth and 24 h. However it is unlikely that the twofold increase in this enzyme activity totally explained the eightfold increase in long-chain fatty acid oxidation in isolated newborn rabbit hepatocytes. It was shown that the rate of the oxidation in isolated hepatocytes was inversely related to the rate of lipogenesis. Nevertheless, malonyl-CoA concentration per se is probably not the factor involved in the regulation of the oxidation between birth and 24 h, since a 90% decrease in hepatic malonyl-CoA concentration was not associated with a stimulation of long-chain fatty acid oxidation. The more likely mechanism was the 30-fold decrease in the sensitivity of carnitine palmitoyltransferase I to malonyl-CoA inhibition.

Mitochondrial function after acute alteration of the endogenous insulin-to-glucagon ratio

Mannoheptulose (2g/kg i.p.) increases serum glucagon and decreases serum insulin via its effect on pancreatic islet cells. These changes in endogenous hormone status had effects on rat liver mitochondria that were comparable to the effects of injecting porcine glucagon (0.5 mg/kg i.p.). Mitochondrial adenine nucleotide content was increased 38 or 39% by mannoheptulose or glucagon respectively, citrulline synthesis by 165 or 193%, pyruvate carboxylation by 113 or 135%, coupled respiration by 34 or 42%, and uncoupled respiration by 40 or 54%. We conclude that the reciprocal changes in endogenous insulin and glucagon brought about by mannoheptulose offer a useful and interesting alternative to glucagon injection for studying the effects of these pancreatic hormones on liver mitochondria.

The stimulatory effect of alloxan diabetes on citrulline formation in rabbit liver mitochondria

The effect of alloxan diabetes on citrulline formation from NH4Cl and bicarbonate was studied in rabbit liver mitochondria incubated with glutamate or succinate as respiratory substrate, as well as with exogenous ATP in the presence of uncoupler and oligomycin. In contrast to ornithine transcarbamoylase, the activity of carbamoyl-phosphate synthetase (ammonia) was higher in mitochondria from diabetic animals than in those from normal ones. In diabetic rabbits the rates of citrulline synthesis were stimulated under all conditions studied. In contrast, levels of N-acetylglutamate, an activator of carbamoyl-phosphate synthetase (ammonia), were significantly increased only in the presence of glutamate, while the highest rates of citrulline formation occurred in uncoupled mitochondria incubated with exogenous ATP as energy source. Treatment of animals with alloxan resulted in an increase of both the intramitochondrial ATP level and the rate of adenine nucleotide translocation across the mitochondrial membrane. The results indicate that the stimulation of citrulline formation in liver mitochondria of diabetic rabbits is mainly due to an increase in carbamoyl-phosphate synthetase (ammonia) activity and an elevation of content of intramitochondrial ATP, a substrate of this enzyme.

Regulation of hepatic gluconeogenesis by rapid compartmentation of mitochondrial adenine nucleotides in the newborn rabbit

In newborn rabbit liver the mitochondrial adenine nucleotide pool (ATP + ADP + AMP) size increased from 6.4 +/- 0.4 to 14.5 +/- 0.7 nmol/mg mitochondrial protein within 2 hr after birth. Gluconeogenesis (from lactate) in isolated hepatocytes rose from 13.1 +/- 1.9 at birth to 42.3 +/- 2.4 nmol glucose/min/10(7) cells at 2 hr. Pyruvate carboxylation in isolated mitochondria increased in parallel from 42.8 +/- 4.9 at birth to 108.6 +/- 8.2 nmol H14CO-3/min/mg mitochondrial protein at 2 hr. The similar developmental time course for these three phenomena suggested that the rapid increase in gluconeogenesis might be a result of increased availability of adenine nucleotides to the ATP-requiring mitochondrial enzyme, pyruvate carboxylase. Manipulation of the mitochondrial adenine nucleotide pool size in vitro resulted in predictable changes in the rate of pyruvate carboxylation. We concluded that the postnatal increase in mitochondrial adenine nucleotide content stimulates pyruvate carboxylation, thereby causing a rapid increase in the rate of gluconeogenesis.