Caffeine-induced changes in the composition of the free amino acid pool of the cerebral cortex

Caffeine and the central nervous system: mechanisms of action, biochemical, metabolic and psychostimulant effects

Caffeine is the most widely consumed central-nervous-system stimulant. Three main mechanisms of action of caffeine on the central nervous system have been described. Mobilization of intracellular calcium and inhibition of specific phosphodiesterases only occur at high non-physiological concentrations of caffeine. The only likely mechanism of action of the methylxanthine is the antagonism at the level of adenosine receptors. Caffeine increases energy metabolism throughout the brain but decreases at the same time cerebral blood flow, inducing a relative brain hypoperfusion. Caffeine activates noradrenaline neurons and seems to affect the local release of dopamine. Many of the alerting effects of caffeine may be related to the action of the methylxanthine on serotonin neurons. The methylxanthine induces dose-response increases in locomotor activity in animals. Its psychostimulant action on man is, however, often subtle and not very easy to detect. The effects of caffeine on learning, memory, performance and coordination are rather related to the methylxanthine action on arousal, vigilance and fatigue. Caffeine exerts obvious effects on anxiety and sleep which vary according to individual sensitivity to the methylxanthine. However, children in general do not appear more sensitive to methylxanthine effects than adults. The central nervous system does not seem to develop a great tolerance to the effects of caffeine although dependence and withdrawal symptoms are reported.

A simple animal model of hyperammonemia

Rats were fed a standard diet or the standard diet supplemented with ammonium acetate (20% w/w) for up to 100 days. The effect of the ingestion of the high-ammonium diet on some aspects of nitrogen metabolism in rats was studied. Ammonia levels in blood increased approximately 3-fold; in brain, liver and muscle the increases were 36, 34 and 50%, respectively. Urea levels in blood and urea excretion increased approximately 2-fold. There was no increase of carbamyl phosphate synthase. Liver glutamine synthase activity increased by 58% and glutamate dehydrogenase by 40%, whereas glutaminase was not affected. Glutamine content in brain was twice that of controls. This new animal model to study hyperammonemia offers several advantages over others: it is simpler, is bloodless, requires no animal manipulation and permits long-term studies.

Effect of caffeine on urea biosynthesis and some related processes, ketone bodies, ATP and liver amino acids

An increase in urea synthesis has been found in rats administered large doses of caffeine. A parallel increase in urea biosynthesis was also found in hepatocytes isolated from caffeine-treated rats, which confirms a greater capacity for urea synthesis induced by caffeine. This increase appeared only after some days of caffeine treatment; during the first days there was no increase in serum urea levels or in in vitro studies of urea synthesis in isolated hepatocytes. However, no appreciable changes were found in either cytosolic or mitochondrial redox states, or in ATP levels in in vivo and in vitro studies. A parallelism was observed between the decreased amino acid levels in caffeine-treated rats and in isolated hepatocytes incubated with different concentrations of caffeine. Several possible mechanisms to explain these findings are considered in the discussion.

In vitro effect of caffeine on some aspects of nitrogen metabolism in isolated rat hepatocytes

Long-term exposure to high doses of caffeine affects several aspects of nitrogen metabolism, such as purine, pyrimidine and urea synthesis. However, little is known about the mechanisms of these changes and if they occur at shorter term. We have studied in isolated hepatocytes: 1) the in vitro effect of high doses of caffeine on amino acid levels, 2) the main destination of ammonia and carbon chains from amino acid catabolism, and 3) the cytosolic and mitochondrial redox states. We have found that, whereas it has a small effect on urea synthesis and on the levels of the cofactors and intermediates, it decreases the levels of several amino acids, the gluconeogenesis and the redox state. Our results suggest that a longer exposure to caffeine is necessary to affect the normal functions of some metabolic pathways.

Long-term ingestion of ammonium increases acetylglutamate and urea levels without affecting the amount of carbamoyl-phosphate synthase

Rats were fed the following diets: standard (20% protein), high-protein (80%), protein-free, standard plus ammonium and protein-free plus ammonium for six weeks. The standard plus ammonium diet was prepared to contain ammonia equivalent to that supplied by the high-protein diet. Addition of ammonium acetate (20% by mass) to the 20% protein or protein-free diets results in 2.3- and 10-fold increases of urea excretion respectively, without increase of carbamoyl-phosphate synthase. Supplementation of the standard diet with ammonium increases the mitochondrial content of acetylglutamate from 830 to 1590 pmol/mg protein, and of the protein-free diet from 130 to 1040 pmol/mg. However, ingestion of ammonium did not increase the activity of acetylglutamate synthase. Therefore the efflux of acetylglutamate from mitochondria was determined. After 30 min at 37 degrees C liver mitochondria from rats on standard diet released 61% of the initial acetylglutamate while mitochondria from animals on standard plus ammonium diet released only 20%. These results indicate that ingestion of ammonium increases the content of acetylglutamate in rat liver by decreasing its efflux from mitochondria. This effect is similar to that produced in mice by a high protein diet [Morita et al. (1982) J. Biochem. (Tokyo) 91, 563-569]. However, while the high-protein diet increases carbamoylphosphate synthase content, the ammonium diet does not.

Long-term high-protein diet induces biochemical and ultrastructural changes in rat liver mitochondria

We have shown in previous work that long-term high-protein diet treatment (420 days) induces important biochemical and stereological changes in rat liver mitochondria. In this paper we have studied the time course for these changes in rats fed a high-protein diet for 30, 90, 180, and 420 days. The liver carbamoyl-phosphate synthetase I (ammonia), which represents 15-20% of the mitochondrial protein, increased ca. 2.5-fold in 30 days, with no further significant changes during the treatment. This increase was accompanied by an increment in the serum urea levels and a diminution in the half-life of blood urea, which could be interpreted as compensatory mechanisms for detoxification of blood and for maintaining osmotic pressure. The stereological study indicates that there is an enlargement of individual mitochondria in rats fed the high-protein diet, and that the maximum enlargement occurred at 90 days of treatment. The analysis of data shows, however, that the increase in mitochondrial volume density was due mainly to proliferation of normal mitochondria. These mitochondria were functionally normal as demonstrated by the unaltered P:O ratio during treatment. The total content of liver amino acids was increased, and the taurine/glycine ratio (which has been related to metabolic stress) was greatly increased. The possible correlation between the increases of both liver taurine levels and cell volume is discussed.

Effect of streptozotocin-diabetes on rat liver mitochondrial adenosine triphosphatase turnover

The apparent turnover rates of some mitochondrial enzymes can be modified in diabetes. We studied the effect of streptozotocin-diabetes on the half-life of a protein tightly bound to the inner membrane, ATPase. The half-life (t 1/2), measured by the double-isotope technique, decreased by approx. 20% in diabetes (from approximately equal to 2.56 days in controls to approximately equal to 2.06 days in diabetic rats). These results suggest that diabetes produces an increase in degradation of ATPase by a mechanism which is not yet clear, possibly influenced by alterations induced by diabetes in hepatic lysosomes that are associated with hepatic autophagy.

Rats fed prolonged high protein diets show an increase in nitrogen metabolism and liver megamitochondria

Rats were fed diets containing 20, 50 and 80% protein for 14 months. The urea excreted by the rats fed diets containing 50 and 80% protein when compared to rats fed diets containing 20% protein increased ca. 2- and 3-fold, respectively, in ca. 2 days; this increase was maintained essentially unchanged through the experimental period. The serum levels of urea increased 2.5- and 4-fold, respectively, in the first days and were also maintained during the experiment. Glutamate dehydrogenase activity of liver remained unchanged. The five urea cycle enzymes increased with respect to the control values. Orotic acid excretion increased as well as orotidylate decarboxylase and orotate phosphoribosyltransferase, but aspartate transcarbamylase did not. The key amino acids involved in the urea and pyrimidine pathways in liver were also measured; aspartic and glutamic acids and citrulline were increased, and ornithine and arginine did not change with the higher protein intake. In general, no differences were observed between animals fed 50 and 80% protein in their diets. Protein synthesis did not increase with the increase of protein content of the diet. Stereological analysis of ultrathin sections showed that the high protein diet induced a significant increment in the volumetric density, numerical density and size of hepatocyte mitochondria. Moreover, the presence of giant mitochondria, a hundred times larger than normal, was also observed in some periportal hepatocytes of rats fed the 80% protein diet.

Effect of L-carnitine on ketone bodies, redox state and free amino acids in the liver of hyperammonemic mice

L-Carnitine stimulates urea synthesis in mice given a LD100 of ammonium acetate. Unprotected mice show decreased levels of hepatic ketone bodies and lowered NADH/NAD+ ratio in both cytosol and mitochondria. L-Carnitine enhances markedly the production of beta-hydroxybutyrate and raises the NADH/NAD+ ratio in mitochondria. The alterations induced by ammonium acetate in the free amino acid pool are prevented by L-carnitine. The results shown in this paper indicate that L-carnitine stimulates fatty acid oxidation as well as flux through the Krebs cycle in hyperammonemic mice and that these effects may be responsible for the increase in urea synthesis in these animals.

Isolation and Characterization of an Fe(III)-Chelating Compound Produced by Pseudomonas syringae

The phytopathogenic bacterium Pseudomonas syringae produces a fluorescent pigment when it is grown in iron-deficient media. This pigment forms a very stable Fe(III) complex that was purified in this form by using a novel procedure based on ultrafiltration and column chromatography. The Fe(III) complex has a molecular weight of 1,100 and contains 1 mol of Fe(III). The pigment is composed of an amino acid moiety with three threonines, three serines, one lysine, δ- N -hydroxyornithine, and a quinoline-type fluorescent chromophore. These features and its stability constant (in the range of 10 32 ) suggest that the fluorescent pigment of P. syringae is related to the siderophores produced by another Pseudomonas species.

Rats that consume caffeine show decreased brain protein synthesis

The effect of caffeine on protein synthesis in brain and liver was studied. When caffeine was added to a post-mitochondrial supernatant from rat brain protein synthesis was inhibited, i.e. 1 mM caffeine about 20%. The effect on protein synthesis of two weeks administration of large doses of caffeine in the drinking fluid of rats was also measured. Caffeine decreased protein synthesis in rat brain by about 32% and 20% compared with "ad libitum" and pair-fed controls. Protein synthesis was calculated taking into account the levels of free leucine determined by HPLC:0.10 mumol/g brain of "ad libitum"; 0.11 for pair-fed and 0.07 for caffeine. The pattern of proteins synthesized was not significantly altered by caffeine as shown by gel-electrophoresis and fluorography. There was no effect on protein synthesis of liver. The possible significance of these results is briefly discussed.