Energy state of bovine cerebral microvessels: comparison of isolation methods

Isolation procedures employed by various laboratories to obtain cerebral microvessels generally utilize meshes to sieve and collect the microvessels from homogenized brain. This is followed in some cases by further purification using density gradients of Percoll or sucrose, or albumin flotation. We have evaluated microvessels prepared by these methods in terms of ATP content and ATP/ADP ratio, which reflect the cellular energy state, and enrichment of the marker enzymes, alkaline phosphatase and gamma-glutamyltransferase. Albumin flotation generally increased the enrichment of marker enzymes; however, preparations using albumin flotation or a Percoll gradient exhibited considerable variability in ATP content and ATP/ADP ratio with the mean ATP/ADP ratio significantly lower than that observed in microvessels isolated by sieving through meshes. More uniformly high values for both ATP (approximately 1.6 nmole ATP/mg protein) and the ATP/ADP ratio (approximately 2.3) were obtained with meshes alone. Use of a sucrose gradient consistently resulted in preparations with a much lower ATP content and ATP/ADP ratio, compared with preparations obtained with the other methods. Values using the other methods were higher than those previously reported, yet were still lower than the ATP content of about 23 and ATP/ADP ratios of 18 and 7 we found in cultured microvascular endothelium and pericyte, respectively. These low values were not improved by supplying additional fuel to the microvessels during isolation, suggesting they were not the result of fuel deprivations during isolation. Despite the probable damage incurred during isolation, microvessel preparations are a useful in vitro model in which fuel metabolism appears to reflect the prior hormonal/nutritional state of donor animals. However, our data indicate the advisability of measurements of ATP content and ATP/ADP ratio for quality control of preparations used for metabolic studies, especially after Percoll density gradient or albumin flotation steps.

Respiratory control in the glucose perfused heart. A 31P NMR and NADH fluorescence study

The phosphate metabolites, adenosine diphosphate (ADP), inorganic phosphate (Pi), and adenosine triphosphate (ATP), are potentially important regulators of mitochondrial respiration in vivo. However, previous studies on the heart in vivo and in vitro have not consistently demonstrated an appropriate correlation between the concentration of these phosphate metabolites and moderate changes in work and respiration. Recently, mitochondrial NAD(P)H levels have been proposed as a potential regulator of cardiac respiration during alterations in work output. In order to understand better the mechanism of respiratory control under these conditions, we investigated the relationship between the phosphate metabolites, the NAD(P)H levels, and oxygen consumption (Q02) in the isovolumic perfused rat heart during alterations in work output with pacing. ATP, creatine phosphate (CrP), Pi and intracellular pH were measured using 31P NMR. Mitochondrial NAD(P)H levels were monitored using spectrofluorometric techniques. Utilizing glucose as the sole substrate, an increase in paced heart rate led to an increase in Q02 from 1.73 +/- 0.09 to 2.29 +/- 0.12 mmol Q2/h per g dry wt. No significant changes in the levels of Pi, PCr, ATP, or the calculated ADP levels were detected. Under identical conditions, an increase in heart rate was associated with a 23 + 3% increase in NAD(P)H fluorescence. Thus, under the conditions of these studies, an increase in Q02 was not associated with an increase in ADP or Pi. In contrast, increases in Q02 were associated with an increase in NAD(P)H. These data are consistent with the notion that increases in the mitochondrial NADH redox state regulate steady-state levels of respiration when myocardial work is increased.

A surgical preparative technique for coronary bypass grafts of human saphenous vein which preserves medial and endothelial functional integrity

Measurements of adenosine triphosphate concentration and adenosine triphosphate/diphosphate ratio provided quantitative markers for medial integrity and stimulated (vortex-mixing) prostacyclin production for endothelial function. Freshly isolated vein had an adenosine triphosphate concentration of 470 +/- 60 nmol X gm-1 wet weight and an adenosine triphosphate/diphosphate ratio of 2.50 +/- 0.13; it produced prostacyclin at a rate of 9.3 +/- 1.0 pg X min-1 X mg-1 wet weight. Vein subjected to dissection, proximal anastomosis, and distention with the patient's own arterial pressure had an adenosine triphosphate concentration of 490 +/- 70 nmol X gm-1 wet weight and an adenosine triphosphate/diphosphate ratio of 2.29 +/- 0.13; it produced prostacyclin at a rate of 10.4 +/- 2.2 pg X min-1 X mg-1 wet weight. All values were indistinguishable from those in freshly isolated vein. In vein subjected to dissection, distention at less than 300 mm Hg with patient's heparinized blood, and distal anastomosis, adenosine triphosphate concentration, adenosine triphosphate/diphosphate ratio, and prostacyclin production (5.5 +/- 0.6 pg X min-1 X mg-1 wet weight) were all significantly (p less than 0.001) reduced. These results demonstrated that surgical preparation by first proximal anastomosis preserved both medial and endothelial function.

Compartmented coupling of chicken heart mitochondrial creatine kinase to the nucleotide translocase requires the outer mitochondrial membrane

The kinetic coupling of mitochondrial creatine kinase (MiMi-CK) to ADP/ATP translocase in chicken heart mitochondrial preparations is demonstrated. Measuring the MiMi-CK apparent Km value for MgATP2- (at saturating creatine) gives a value of 36 microM when MiMi-CK is coupled to oxidative phosphorylation. This Km value is threefold lower than the Km for enzyme bound to mitoplasts or free in solution. The nucleotide translocase Km value for ADP decreases from 20 to 10 microM in the presence of 50 mM creatine only with intact mitochondria. Similar experiments with mitoplasts do not give decreased Km values. The observed Km differences can be used to calculate the concentration of ATP and ADP under steady-state conditions showing that the observed differences in the kinetic constants accurately reflect the enzyme activities of MiMi-CK under the different conditions. The behavior of the Km values provides evidence for what we term compartmented coupling. Therefore, like the rabbit heart system (S. Erickson-Viitanen, P. Viitanen, P. J. Geiger, W. C. T. Yang, and S. P. Bessman (1982) J. Biol. Chem. 257, 14395-14404) compartmented coupling requires an intact outer mitochondrial membrane. The apparent Km values for normal or compartmentally coupled systems can be used to calculate steady-state values of ATP and ADP by coupling enzyme theory. Hence, the overall kinetic parameters accurately reflect the behavior of the enzymes whether free in solution or in the intermembrane space.

Aberrant ribonucleotide pattern in lymphoid cells from patients with chronic lymphocytic leukaemia or non-Hodgkin lymphoma

The intracellular purine and pyrimidine ribonucleotide concentrations were determined in lymphoid cells from peripheral blood of 16 patients with chronic lymphocytic leukaemia (CLL) and from peripheral blood and/or lymphoid tissue of 18 patients with non-Hodgkin lymphoma (NHL). Compared to normal peripheral lymphocytes, the lymphoid cells from CLL patients contained lower, and those from NHL patients higher amounts of nucleotides. The lymphoid cells of NHL patients showed an imbalance in the nucleotide pool compared to either normal resting peripheral or proliferating tonsillar lymphocytes. The lymphoid cells of patients with CLL showed an imbalance only when compared to normal, resting peripheral lymphocytes. The abnormalities in the nucleotide pools involved decreased ratios of purine:pyrimidine, adenine:guanine and uracil:cytosine nucleotides. Lymphocytes from CLL and NHL patients contained increased amounts (relative and/or absolute) of UDP sugars, and NHL lymphocytes also showed a changed composition of the UDP sugars. Analysis of the ribonucleotides in the lymphoid cells provides useful information for the differential diagnosis of patients suspected of having CLL or NHL, and may be valuable for the design of new chemotherapeutic regimens.

On the origin of the limited control of mitochondrial respiration by the adenine nucleotide translocator

A thermodynamic control theory previously developed has been applied to mitochondrial oxidative phosphorylation with emphasis on the role of delta microH and coupling and within the paradigm of delocalized chemiosmotic coupling. The basis for the observed distribution of flux control over the participating enzymes is shown to lie in the relative magnitudes of so-called delta microH elasticity coefficients, i.e., the delta microH dependencies of the different mitochondrial processes. In particular the relatively strong delta microH dependence of mitochondrial respiration is responsible for the significant role of the adenine nucleotide translocator in the control of oxidative phosphorylation. Uncoupling decreases the control exerted by this translocator on respiration but increases that exerted on phosphorylation.

Extracellular adenosine, inosine, hypoxanthine, and xanthine in relation to tissue nucleotides and purines in rat striatum during transient ischemia

Extracellular (EC) adenosine, hypoxanthine, xanthine, and inosine concentrations were monitored in vivo in the striatum during steady state, 15 min of complete brain ischemia, and 4 h of reflow and compared with purine and nucleotide levels in the tissue. Ischemia was induced by three-vessel occlusion combined with hypotension (50 mm Hg) in male Sprague-Dawley rats. EC purines were sampled by microdialysis, and tissue adenine nucleotides and purine catabolites were extracted from the in situ frozen brain at the end of the experiment. ATP, ADP, and AMP were analyzed with enzymatic fluorometric techniques, and adenosine, hypoxanthine, xanthine, and inosine with a modified HPLC system. Ischemia depleted tissue ATP, whereas AMP, adenosine, hypoxanthine, and inosine accumulated. In parallel, adenosine, hypoxanthine, and inosine levels increased in the EC compartment. Adenosine reached an EC concentration of 40 microM after 15 min of ischemia. Levels of tissue nucleotides and purines normalized on reflow. However, xanthine levels increased transiently (sevenfold). In the EC compartment, adenosine, inosine, and hypoxanthine contents normalized slowly on reflow, whereas the xanthine content increased. The high EC levels of adenosine during ischemia may turn off spontaneous neuronal firing, counteract excitotoxicity, and inhibit ischemic calcium uptake, thereby exerting neuroprotective effects.

Effects of Sarcocystis miescheriana infection on carcass weight and meat quality of halothane-tested fattening pigs

Ten halothane-positive pigs (Group A) and ten halothane-negative pigs (Group C) were each infected per os by 50,000 sporocysts of Sarcocystis miescheriana when they had reached a mean body weight of 36 kg. Twelve halothane-positive pigs (Group B) and ten halothane-negative pigs (Group D) served as non-infected controls. Thirteen weeks p.i. (post infection), the pigs were slaughtered and the carcass weights and the number of cystozoites in several muscles were determined. In addition, 11 parameters of meat quality were measured in Musculus longissimus dorsi (M.l.d.) and Musculus semimembranosus (M.s.): pH and temperature 30 min and 24 h p.m. (post mortem), electrical conductance 45 min, 60 min and 24 h p.m., colour brightness (FOP) and rigor values 30 min and 24 h p.m. Additionally, the AMP, ADP, ATP and lactate contents were determined in samples from the M.l.d. The mean carcass weights of the infected pigs (A: 79.9 +/- 6.9 kg; C: 76.3 +/- 11.6 kg) were lower than those of the pigs of the non-infected groups (B: 85.7 +/- 8.0 kg; D: 87.5 +/- 7.0 kg). The pH values at 24 h p.m. were significantly higher in the M.l.d. of infected pigs than in M.l.d. of non-infected pigs. Electrical conductance in the M.s. at 45 min p.m. was significantly higher in the halothane-negative infected pigs, but at 24 h p.m., the mean values of electrical conductance in the M.s. of the pigs of both infected groups were significantly lower than in the control groups. FOP values at 30 min p.m. were raised in the M.l.d. of the halothane-negative infected pigs, but at 24 h p.m., differences between infected and non-infected pigs could no longer be found. Mean rigor values were higher in the pigs of the infected groups at 24 h p.m., but the difference from the non-infected groups was not statistically significant. AMP was reduced only in the meat of halothane-positive infected pigs, while lactate was reduced in both infected groups. Using conventional methods for the determination of meat quality, the hypothesis of impaired meat quality in Sarcocystis-infected pigs could not be confirmed. The meat quality parameters investigated were, in part, better in infected pigs than in non-infected controls.

A rapid assay method for ammonia using glutamine synthetase from glutamate-producing bacteria

A rapid enzymatic assay method for ammonia was developed by using glutamine synthetase from glutamate-producing bacteria together with pyruvate kinase, lactate dehydrogenase, and NADH. The time required for determination of 25 nmol of ammonia was 5 min with 1 unit of glutamine synthetase, as opposed to 14-30 min with 1 unit of glutamate dehydrogenases from various sources. The present method was used to determine ammonia in serum, microbiol-culture broth, and waste water. The method can be modified for spectrophotometry in the visible region by substituting pyruvate oxidase, peroxidase, and appropriate chromogens for lactate dehydrogenase and NADH. With 4-aminoantipyrine (4AA) and phenol, and with 4AA and N-ethyl-N-2-hydroxyethyl-m-toluidine as chromogens, the sensitivity of ammonia determination was 0.65 and 1.7 times that with glutamate dehydrogenase, respectively. The present method was also applicable to the continuous detection of the activity of some ammonia-forming enzymes such as guanase, adenosine deaminase, and urease and to the determination of 0.5-30 microM ATP-ADP after some modification of the mixture.

Platelet aggregating activity mediated by thrombin generation in the NCG human neuroblastoma cell line

Platelet aggregating activity of the NCG human neuroblastoma cell line was compared with that of the HL-60 human promyelocytic leukemia cell line. NCG, in intact cell suspensions and ultracentrifuged pellets, induced platelet aggregation most significantly in heparinized platelet rich plasma (PRP) containing 2.5 units/ml of heparin, but not in the presence of higher concentrations of heparin or 5 mM ethylenediamine-tetraacetate or in citrated PRP. NCG induced platelet aggregation was also inhibited by hirudin or (2R,4R)-4-methyl-1-[N2-(3-methyl-1,2,3,4-tetrahydro-8-quinolinesulfon yl)-L- arginyl]-2-piperidinecarboxylic acid (MD 805) in the same manner as that of tissue thromboplastin induced platelet aggregation. HL-60 cells did not induce platelet aggregation in our heparinized PRP assay systems; however, after treatment with neuraminidase HL-60 cells became active in aggregating platelets in either heparinized or citrated PRP. NCG demonstrated high procoagulant activity by either intact cell suspensions or ultracentrifuged pellets. The procoagulant activity of NCG was reduced in Factor VII deficient human plasma as it was in the results obtained by tissue thromboplastin. These results suggest that NCG induces platelet aggregation via thrombin generated through procoagulant activity which is shed in association with microvesicles demonstrated in the ultracentrifuged pellets. This type of platelet aggregating activity found in NCG is significantly different from that of HL-60.

Allowance for "analyte valence" in the retention model of ion-exchange chromatography. Studies of adenosine 5'-phosphates on DEAE cellulose

Quantitative expressions are derived for various retention models of the ion-exchange behaviour of multivalent analytes. Their effectiveness as descriptions of experimental results is then tested by application to partition equilibrium studies of the effect of phosphate concentration on the interaction between the three adenosine 5'-phosphates and diethylaminoethylcellulose at pH 4.4. A completely general multi-state model has pointed to quantitative deficiencies in the currently accepted cooperative two-state model, which is, however, superior to a multi-state model devoid of cooperative effects. A similar situation pertains to the ion-exchange behaviour of cytochrome c on carboxymethylcellulose at pH 7.0.

Effect of ozone on ATP, cytosolic enzymes and permeability of Saccharomyces cerevisiae

Treatment of a yeast suspension with ozone inactivates a number of cytosolic enzymes. Among 15 studied, the most drastic inactivation was found for glyceraldehyde-3-phosphate dehydrogenase and to lesser extents: NAD-glutamate dehydrogenase, pyruvate decarboxylase, phosphofructokinase-1 and NAD-alcohol dehydrogenase. Ozone treatment also effects the quantity of ATP and of other nucleoside triphosphates, reducing to about 50% of the initial value. The ATP missing in the cells appears in the medium. NAD and protein also accumulate in the medium suggesting that the yeast cells have been permeabilized. Permeabilization of the yeast cells by treatment with ozone precedes the inactivation of glyceraldehyde-3-phosphate dehydrogenase and other cytosolic enzymes.

Glucose-induced changes in cytosolic ATP content in pancreatic islets

The cytosolic and mitochondrial contents in ATP, ADP and AMP were measured in islets incubated for 45 min at increasing concentrations of D-glucose and then exposed for 20 s to digitonin. The latter treatment failed to affect the total islet ATP/ADP ratio and adenylate charge. D-Glucose caused a much greater increase in cytosolic than mitochondrial ATP/ADP ratio. In the cytosol, a sigmoidal pattern characterized the changes in ATP/ADP ratio at increasing concentrations of D-glucose. These findings are compatible with the view that cytosolic ATP participates in the coupling of metabolic to ionic events in the process of nutrient-induced insulin release.

Localization of the phalloidin and nucleotide-binding sites on actin

Phalloidin was found to block nucleotide exchange in F-actin, without interfering with nucleotide hydrolysis. This inhibition of nucleotide exchange occurs under conditions in which monomers are able to exchange. The distance separating a fluorescent chromophore attached to phalloidin from the nucleotide on actin was determined using fluorescence resonance energy-transfer spectroscopy. They are separated by less than 1.0 nm. Added confirmation of the close proximity of phalloidin to nucleotide was obtained by extracting a small peptide-ATP complex from an actin digest. The peptide comprises residues 114-118, which are from the same region as the residues that others have shown to crosslink to phalloidin [Vandekerckhove et al. (1985) EMBO J. 4, 2815-2818]. The results suggest that phalloidin has two major effects. It traps actin monomers in a conformation which appears to be distinct from G-actin and it stabilizes the structure of F-actin, an event accompanied by the trapping of ADP.

Role of cellular energy state and adenosine in the regulation of coronary flow during variation in contraction frequency in an isolated perfused heart

Regulation of coronary flow as a function of myocardial energy expenditure was investigated in isolated perfused rat hearts electrically paced at the desired frequencies. The sinoatrial node was excised to lower the endogenous heart rate. The main covariants measured were phosphagen, inorganic phosphate, adenosine, inosine and hypoxanthine concentrations in the tissue, washout of nucleosides and hypoxanthine into the perfusate, oxygen consumption and coronary flow. Oxygen consumption was linearly correlated with heart rate and coronary flow, while the correlation between coronary flow and perfusate adenosine was nonlinear. The adenosine concentrations in the tissue and perfusate showed a mirror image curvilinearity reminiscent of a threshold pattern for adenosine washout. The tissue adenosine content had a negative linear correlation with the adenylate phosphorylation potential (long(ATP/ADP X Pi)). Adenosine output was linearly correlated with free AMP concentration in the tissue, the latter being calculated from the equilibrium of the adenylate kinase reaction. The results confirm the correlation between cellular energy state and coronary flow and support the notion that the mediators between the former and the vascular smooth muscle involve the concentration of free AMP in the tissue, suggesting that the formation of adenosine may be limited by the availability of AMP. The results are in agreement with the hypothesis that adenosine is the diffusible extracellular mediator in the energy-linked regulation of coronary flow.

Effects of halothane and decreased PO2 on high energy phosphate levels maintained by isolated rat liver mitochondria

Steady states of oxidative phosphorylation were achieved in mitochondrial suspensions continuously equilibrated with constant gas mixtures, simulating the conditions under which mitochondria contribute to the cellular energy status in vivo. The dependence of the mitochondria-maintained adenosine triphosphate/adenosine diphosphate (ATP/ADP) ratio on oxygen and halothane levels was examined at predetermined, clinically relevant concentrations of both gases. Inclusion of 1% halothane in the gas mixture decreased ATP/ADP by about half when mitochondrial respiration was supported by NAD-linked substrate (glutamate); succinate-supported ATP/ADP was not inhibited. With either substrate, and whether or not 1% halothane was present. ATP/ADP was unaffected by decreases in PO2 to values as low as 1.6 mm Hg. Under a range of typical in vivo conditions, therefore, 1% halothane significantly inhibited the mitochondrial contribution to steady state energy balance, whereas decreases in PO2 did not. Combined effects of 1% halothane and reduced PO2 on ATP/ADP were not seen, i.e., halothane did not increase the critical PO2 level (hypoxic threshold) for inhibition of mitochondrial ATP production.

Consequences of cadmium toxicity in rat hepatocytes: mitochondrial dysfunction and lipid peroxidation

Cadmium (Cd) (10-100 microM) decreased the ATP/ADP ratio and enhanced lipid peroxidation (LPO) (measured as thiobarbituric acid reactants) in incubated rat hepatocytes. Analysis of the subcellular distribution of Cd indicated its preferential attachment to the inner membranes of mitochondria. Incubation of isolated mitochondria with 0.005-0.05 microM Cd resulted in increased formation of formazans from nitroblue tetrazolium salts, indicating enhanced membrane permeability to succinate. These Cd-concentrations also diminished mitochondrial ATP. LPO in mitochondria strongly increased only after Cd-exposures above 1 microM Cd. Similarly, in Cd-treated hepatocytes decreases in ATP/ADP ratios corresponded to increases in LPO stimulation only at 30 and 60 min but not at 15 min of incubation when ATP/ADP ratios were already affected. Moreover, neither hepatocellular ATP/ADP decrease nor mitochondrial formazan formation due to Cd were prevented by (+)-cyanidanol-3, an effective inhibitor of Cd-induced LPO. These data suggest that even low Cd-concentrations in the hepatocyte disturb the integrity of its mitochondrial membranes concomitantly impairing the hepatocellular energy supply. LPO, only observed at higher Cd-concentrations, is not responsible for these adverse Cd-effects.

A metabolite-regulated potassium channel in rat pancreatic B cells

In B cells from dispersed rat islet of Langerhans we have identified an inward rectifying voltage-independent K+ channel whose behavior parallels the metabolically regulated potassium permeability (PK) found in tracer flux and microelectrode recording studies. In cell-attached patches of membrane, the channel is closed when any one of several substrates (glucose, mannose, leucine, or glyceraldehyde) is added to the cell's bathing solution but is reopened on addition of an appropriate metabolic inhibitor, which prevents utilization of that substrate. In inside-out excised patches, a K+ channel with nearly identical kinetic features is closed by addition of micromolar concentrations of ATP to the "cytoplasmic" solution. The ATP sensitivity of channel activity is modified by addition of ADP, suggesting competition at a nucleotide binding site. These results suggest the presence of a metabolically regulated K+ channel gated by intracellular concentrations of ATP or the ratio of ATP/ADP concentrations.

Myocardial high-energy phosphate levels in cardiomyopathic turkeys

A congestive cardiomyopathy (CCM) model occurs in inbred broad-breasted turkeys and is manifested by reduced hatchability and a high mortality within a week of hatching. In the survivors, cardiac dilation begins by 3-4 weeks of age and further mortality occurs from chronic congestive heart failure. The mechanisms behind these changes is unknown, and, therefore, we investigated what role, if any, myocardial energy metabolism might play in these events. Ventricular myocardial samples were obtained for analysis of adenine nucleotides (ATP, ADP, AMP) and creatine phosphate (CP) in control and CCM turkeys, 1-31 days old. The adenine nucleotide energy charge (EC) was calculated using the formula EC = ATP + 1/2ADP/(ATP + ADP + AMP). We found the myocardial ATP levels and EC in CCM hearts at 1-2 days were reduced. In control turkeys, no significant age-related differences were found in myocardial high-energy phosphate compounds or in the EC. This depression in the energy metabolism of CCM turkeys may also be reflected in their poor hatchability. By 6-10 days, however, ATP levels had recovered and remained normal despite the onset of cardiac dilation and failure at 3-4 weeks of age in CCM turkeys. Because CP levels in control and CCM turkey hearts were similar in all age groups, significant ischemia did not appear to be present after hatching in CCM turkeys. Our results suggest, therefore, that an insult probably prior to hatching produced depressed myocardial energy levels in CCM turkeys and led to reduced hatchability. This early insult appears to be significant, in that late cardiac dysfunction resulted despite the recovery of myocardial ATP levels.

Fast estimation of ATP/ADP ratio as a special step in pharmacological and toxicological studies using the cell-free translation systems

We have developed a simple and effective reversed-phase HPLC procedure for rapid estimation of the ATP/ADP ratio in a cell-free translation system containing creatine kinase. Analysis of the acetone-extractable pool derived from a reticulocyte lysate cell-free system was carried out by automatic chromatography on S5CN-ODS stationary phase using a linear 10-65% pyridine elution gradient formed on the basis of methanol/water (9:1, v/v) mobile phase. This method was used to detect and characterize the inhibition of translation induced by considerable suppression of ATP resynthesis in vitro. It was shown that methyl mercury, unlike cycloheximide, pactamycin, CCl4 and barbituric acid, exerts inhibitory effect on the ATP regeneration in a cell-free translation system.

Synaptosomal bioenergetics. The role of glycolysis, pyruvate oxidation and responses to hypoglycaemia

The bioenergetic interaction between glycolysis and oxidative phosphorylation in isolated nerve terminals (synaptosomes) from guinea-pig cerebral cortex is characterized. Essentially all synaptosomes contain functioning mitochondria. There is a tight coupling between glycolytic rate and respiration: uncoupler causes a tenfold increase in glycolysis and a sixfold increase in respiration. Synaptosomes contain little endogenous glycolytic substrate and glycolysis is dependent on external glucose. In glucose-free media, or following addition of iodoacetate, synaptosomes continue to respire and to maintain high ATP/ADP ratios. In contrast to glucose, the endogenous substrate can neither maintain high respiration in the presence of uncoupler nor generate ATP in the presence of cyanide. Pyruvate, but not succinate, is an excellent substrate for intact synaptosomes. The in-situ mitochondrial membrane potential (delta psi m) is highly dependent upon the availability of glycolytic or exogenous pyruvate; glucose deprivation causes a 20-mV depolarization, while added pyruvate causes a 6-mV hyperpolarization even in the presence of glucose. Inhibition of pyruvate dehydrogenase by arsenite or pyruvate transport by alpha-cyano-4-hydroxycinnamate has little effect on ATP/ADP ratios; however respiratory capacity is severely restricted. It is concluded that synaptosomes are valuable models for studying the control of mitochondrial substrate supply in situ.

Magnetic resonance spectroscopy for the determination of renal metabolic rate in vivo

The method of magnetic resonance spectroscopy has been validated and applied to the determination of renal metabolic rate in vivo. Using an indwelling detector coil, 31P NMR spectra from one kidney of anesthetized rats were quantified. The concentration of ATP was the same as that determined enzymatically, but both ADP and Pi were substantially lower. Only 25% of renal Pi and virtually none of the ADP were detected by NMR. The remainder is assumed to be bound to proteins. These concentrations of metabolites contributed to a significantly increased phosphorylation potential, which in turn should increase the free energy of hydrolysis of ATP. Saturation transfer, a non-destructive magnetic technique for the measurement of chemical exchange, was readily able to detect synthesis of ATP from ADP and inorganic phosphate. The rate of ATP synthesis determined was comparable to that determined in parallel studies of renal oxygen consumption. An ATP:O ratio of approximately 2 was found, indicating that fatty acid is the preferred fuel of respiration of the rat kidney in vivo.