Synaptosomal non-mitochondrial ATPase activities: age-related alterations by chronic normobaric intermittent hypoxia

Hypometabolic Responses to Chronic Hypoxia: A Potential Role for Membrane Lipids

Metabolic suppression is an essential strategy to cope with chronic hypoxia. This review examines the physiological processes used to survive in low oxygen environments. It proposes a novel mechanism-the remodeling of membrane lipids-to suppress ATP use and production. Temperature (homeoviscous adaptation), diet (natural doping in migrant birds) and body mass (membrane pacemaker of metabolism) have an impact on the lipid composition of membranes, which, in turn, modulates metabolic capacity. Vertebrate champions of hypoxia tolerance show extensive changes in membrane lipids upon in vivo exposure to low oxygen. These changes and those observed in hibernating mammals can promote the downregulation of ion pumps (major ATP consumers), ion channels, mitochondrial respiration capacity (state 3, proton leak, cytochrome c oxidase), and energy metabolism (β-oxidation and glycolysis). A common membrane signal regulating the joint inhibition of ion pumps and channels could be an exquisite way to preserve the balance between ATP supply and demand in hypometabolic states. Membrane remodeling together with more traditional mechanisms could work in concert to cause metabolic suppression.

Hypoxia and brain aging: Neurodegeneration or neuroprotection?

The absolute reliance of the mammalian brain on oxygen to generate ATP renders it acutely vulnerable to hypoxia, whether at high altitude or in clinical settings of anemia or pulmonary disease. Hypoxia is pivotal to the pathogeneses of myriad neurological disorders, including Alzheimer's, Parkinson's and other age-related neurodegenerative diseases. Conversely, reduced environmental oxygen, e.g. sojourns or residing at high altitudes, may impart favorable effects on aging and mortality. Moreover, controlled hypoxia exposure may represent a treatment strategy for age-related neurological disorders. This review discusses evidence of hypoxia's beneficial vs. detrimental impacts on the aging brain and the molecular mechanisms that mediate these divergent effects. It draws upon an extensive literature search on the effects of hypoxia/altitude on brain aging, and detailed analysis of all identified studies directly comparing brain responses to hypoxia in young vs. aged humans or rodents. Special attention is directed toward the risks vs. benefits of hypoxia exposure to the elderly, and potential therapeutic applications of hypoxia for neurodegenerative diseases. Finally, important questions for future research are discussed.

Energy metabolism of cerebral mitochondria during aging, ischemia and post-ischemic recovery assessed by functional proteomics of enzymes

Stroke is a leading cause of death and disability, but most of the therapeutic approaches failed in clinical trials. The energy metabolism alterations, due to marked ATP decline, are strongly related to stroke and, at present, their physiopathological roles are not fully understood. Thus, the aim of this study was to evaluate the effects of aging on ischemia-induced changes in energy mitochondrial transduction and the consequences on overall brain energy metabolism in an in vivo experimental model of complete cerebral ischemia of 15min duration and during post-ischemic recirculation after 1, 24, 48, 72 and 96h, in 1year "adult" and 2year-old "aged" rats. The maximum rate (Vmax) of citrate synthase, malate dehydrogenase, succinate dehydrogenase for Krebs' cycle; NADH-cytochrome c reductase and cytochrome oxidase for electron transfer chain (ETC) were assayed in non-synaptic "free" mitochondria and in two populations of intra-synaptic mitochondria, i.e., "light" and "heavy" mitochondria. The catalytic activities of enzymes markedly differ according to: (a) mitochondrial type (non-synaptic, intra-synaptic), (b) age, (c) acute effects of ischemia and (d) post-ischemic recirculation at different times. Enzyme activities changes are injury maturation events and strictly reflect the bioenergetic state of the tissue in each specific experimental condition respect to the energy demand, as shown by the comparative evaluation of the energy-linked metabolites and substrates content. Remarkably, recovery of mitochondrial function was more difficult for intra-synaptic mitochondria in "aged" rats, but enzyme activities of energy metabolism tended to normalize in all mitochondrial populations after 96h of recirculation. This observation is relevant for Therapy, indicating that mitochondrial enzymes may be important metabolic factors for the responsiveness of ischemic penumbra towards the restore of cerebral functions.

Energy metabolism of rat cerebral cortex, hypothalamus and hypophysis during ageing

Ageing is one of the main risk factors for brain disorders. According to the neuroendocrine theory, ageing modifies the sensitivity of hypothalamus-pituitary-adrenal axis to homoeostatic signals coming from the cerebral cortex. The relationships between the energy metabolism of these areas have not been considered yet, in particular with respect to ageing. For these reasons, this study was undertaken to systematically investigate in female Sprague-Dawley rats aged 4, 6, 12, 18, 24, 28 months and in 4-month-old male ones, the catalytic properties of energy-linked enzymes of the Krebs' cycle, electron transport chain, glutamate and related amino acids on different mitochondrial subpopulations, i.e. non-synaptic perikaryal and intra-synaptic (two types) mitochondria. The biochemical enzymatic pattern of these mitochondria shows different expression of the above-mentioned enzymatic activities in the investigated brain areas, including frontal cerebral cortex, hippocampus, striatum, hypothalamus and hypophysis. The study shows that: (i) the energy metabolism of the frontal cerebral cortex is poorly affected by physiological ageing; (ii) the biochemical machinery of non-synaptic perikaryal mitochondria is differently expressed in the considered brain areas; (iii) at 4-6 months, hypothalamus and hypophysis possess lower oxidative metabolism with respect to the frontal cerebral cortex while (iv), during ageing, the opposite situation occurs. We hypothesised that these metabolic modifications likely try to grant HPA functionality in response to the incoming external stress stimuli increased during ageing. It is particularly notable that age-related changes in brain bioenergetics and in mitochondrial functionality may be considered as remarkable factors during physiological ageing and should play important roles in predisposing the brain to physiopathological events, tightly related to molecular mechanisms evoked for pharmacological treatments.

ATPases of synaptic plasma membranes from hippocampus after ischemia and recovery during ageing

Plasticity and relationships between individual ATPases linked to energy-utilizing systems of hippocampus, a very sensitive functional area to both age and ischemia, were studied during ageing on synaptic plasma membranes of 1-year-old "adult" and 2-year-old "aged" rats after 15 min of complete cerebral ischemia and different reperfusion times (01, 24, 48, 72, and 96 h). Activities of Na+, K+, Mg(2+)-ATPase, Mg(2+)-ATPase ouabain insensitive, Na+, K(+)-ATPase, "direct" or "basal" Mg(2+)-ATPase, and acetylcholinesterase (AChE) were evaluated in synaptic plasma membranes, where they play the major role in the regulation of presynaptic nerve ending homeostasis. This in vivo study of recovery time-course from 15 mins of cerebral ischemia indicated specific biochemical assessments of functional meaning: (a) Na+K(+)-ATPase of synaptic plasma membranes in adult and aged animals is stimulated by ischemia; (b) this "hyperactivity" is more markedly related to adult than to aged animals; (c) these abnormalities still persist after 72 and 96 h during the recirculation times, indicating the delayed postischemic suffering of the brain; (d) specific Mg(2+)-ATPase enzyme system possess a lower catalytic power in aged animals than in adult ones, but remained unaltered in adult animals by ischemia and reperfusion; (e) Mg(2+)-ATPase is stimulated in aged animals by ischemia, further increasing during reperfusion up to 72-96 h, indicating the delayed hyperactivity of hippocampus; (f) the increased metabolic activity of hippocampus is indicated by the increased activity of cholinergic system; (g) integrity of synaptic plasma membranes seems not to be altered by 15 min ischemia to a critical extent to compromise their catalytic functionality during reperfusion; (h) AChE activity increases in both adult and aged at some survival times. There are logical reasons for the hypothesis that the modifications in ATPase's catalytic activities in synaptic plasma membranes, which have been modified by ischemia in presynaptic terminals, may play important functional role during recovery time in cerebral tissue in vivo, especially as regards its responsiveness to noxious stimuli, particularly during the recirculation period from acute (or chronic) brain injury.

ATPases enzyme activities during ageing in different types of somatic and synaptic plasma membranes from rat frontal cerebral cortex

The catalytic properties of energy-utilizing ATPases enzyme systems related to ions homeostasis were evaluated in different types of synaptic plasma membranes (SPM) and in somatic plasma membranes (SM) from cerebral cortex of rats aged 5, 10, and 22 months. The following enzymes were evaluated: Na+, K+-ATPase, Ca2+, Mg2+-ATPase, Mg2+-ATPase and the activity of acetylcholine esterase (AChE) was also evaluated. The ATPases located on SM and SPM and synaptic vesicles are involved in the regulation of presynaptic nerve ending homeostasis and postsynaptic activities. Different types of SM and SPM (three types) were obtained by combinations of differential and density gradient ultracentrifugation techniques in sucrose-Ficoll media: the first was obtained by purification of the sediment of mitochondrial supernate and the second after synaptosomal lysis and purification on density gradient. In the cerebral cortex of 5-month-old rats, the catalytic properties of ATPases systems markedly differ according to the different types of SPM and SM, thus indicating that the metabolic role of each ATPase is determined by their subcellular in vivo localization. As regards ageing: (i) ATPase enzyme catalytic activities tend to decrease during ageing in a complex way; (ii) ageing induced specific modifications in individual ATPases according to their subsynaptic localization; and (iii) these effects are probably due to specific biochemical situations that take place at each age, reflecting the bioenergetic state of the cerebral tissue with respect to the energy demand. The cerebral concentration and content of SM proteins were increased by ageing suggesting that many defective noncatalytic proteins may be formed during ageing, as shown by immunoblotting techniques.

Effect of in vivo administration of naloxone on ATP-ase's enzyme systems of synaptic plasma membranes from rat cerebral cortex

Naloxone is a specific competitive antagonist of morphine, acting on opiate receptors, located on neuronal membranes. The effects of in vivo administration of naloxone on energy-consuming non-mitochondrial ATP-ases were studied in two different types of synaptic plasma membranes from rat cerebral cortex, known to contain a high density of opiate receptors. The enzyme activities of Na+, K(+)-ATP-ase, Ca(2+), Mg(2+)-ATP-ase and Mg(2+)-ATP-ase and of acetylcholinesterase (AChE) were evaluated on synaptic plasma membranes obtained from control and treated animals with effective dose of naloxone (12microg x kg(-1) i.m. 30 minutes). In control (vehicle-treated) animals specific enzyme activities assayed on these two types of synaptic plasma membranes are different, being higher on synaptic plasma membranes of II type than of I type, because the first fraction is more enriched in synaptic plasma membranes. The acute treatment with naloxone produced a significant decrease in Ca(2+),Mg(2+)-ATP-ase activity and an increase in AChE activity, only in synaptic plasma membranes of II type. The decrease of Ca(2+), Mg(2+)-ATP-ase enzymatic activity and the increased AChE activity are related to the interference of the drug on Ca(2+) homeostasis in synaptosoplasm, that leads to the activation of calcium-dependent processes, i.e. the extrusion of neurotransmitter. These findings give further evidence that pharmacodynamic characteristics of naloxone are also related to increase [Ca(2+)]i, interfering with enzyme systems (Ca(2+), Mg(2+)-ATP-ase) and that this drug increases acetylcholine catabolism in synaptic plasma membranes of cerebral cortex.

The lipid peroxidation product 4-hydroxynonenal potently and selectively inhibits synaptic plasma membrane ecto-ATPase activity, a putative regulator of synaptic ATP and adenosine

Synaptic plasma membrane (SPM)-bound, extracellular-facing (ecto) ATPases are Mg2+- or Ca2+-activated enzymes that regulate the synaptic levels of the excitatory neurotransmitter ATP and provide ADP for the further ecto-nucleotidase-mediated production of the inhibitory neuromodulator adenosine. The present results show that low concentrations (IC50 = 4 microM) of the lipid peroxidation product 4-hydroxynonenal (HNE) inhibited up to about 80% of the ecto-ATPase activity of SPM purified from rat brain cerebral cortex. In contrast, low concentrations of HNE did not inhibit the activity of the "intracellular"-facing Na+, K+, Mg2+-ATPase. In addition, the inhibition of SPM ecto-ATPase activity by HNE was largely irreversible and pH-dependent. Furthermore, structure-activity studies demonstrate that inhibition was dependent on the presence of the reactive functional groups of HNE. These findings suggest that HNE selectively inhibits SPM ecto-ATPase activity by a mechanism that may involve the covalent modification of functionally-critical nucleophilic amino acids. It is proposed that inhibition of SPM ecto-ATPase activity could contribute to the mechanisms by which lipid peroxidation and HNE formation promote excitotoxicity.

Aging impairs regulation of intracellular pH in rat hippocampal slices

We investigated whether aging changed H+ homeostasis in hippocampal slices bathed in HEPES buffer. Intracellular pH in hippocampal slices from rats aged 26-27 months (7.06 +/- 0.02) was significantly lower compared with that in slices from rats aged 6-7 months (7.16 +/- 0.04). Age did not influence extracellular ph. Age-related reductions in intracellular pH may reflect altered pH regulation that potentially affects brain function and could contribute to the increased vulnerability of the aged brain to metabolic stress.

Influence of age on the clearance of K+ from the extracellular space of rat hippocampal slices

We examined the hypotheses that aging alters the capacity of brain tissue to regulate extracellular K+ activity (K+o), and that age-related decreases in glucose metabolism may underlie these alterations. Hippocampal slices from young adult (6-9 months old), middle-aged (16-19 months old), and aged (26-29 months old) Fischer 344 rats were exposed to physiological solutions containing 5-20 mM glucose maintained at 36-37 degrees C. Schaffer collaterals in each slice were stimulated at 40 Hz for 2 s, and the resulting changes in K+o were recorded with K+o-sensitive microelectrodes placed in stratum pyramidale of hippocampal subfield CA1. We found that K+ clearance from the extracellular space of hippocampal slices was significantly slowed in the middle-aged group compared with the young adult group in physiological solutions containing 5 and 10 mM glucose. Age-related differences in K+o clearance disappeared in 20 mM glucose. Also, the rate of K+o clearance was modified by glucose concentration. These results suggest that K+ transport rates are modified by age, and that age-related alterations in glucose metabolism may be involved.