Depolarization-induced changes in cellular energy production.

Abstract

Addition of high concentrations of KCI to preparations of rat brain synaptosomes incubated with either glucose or pyruvate caused a transient stimulation of oxygen uptake. This increased respiration was insensitive to 1 mM ouabain and 10 .IM ruthenium red but was dependent upon the presence of calcium. With 40 mM KCI in the incubation medium, the levels of high-energy phosphate compounds in the synaptosomes were unaltered, whereas pyridine nucleotides underwent a rapid, albeit small and temporary, oxidation. It is postulated that there is a calcium-dependent mechanism in synaptosomes through which the function of the mitochondrial respiratory chain or of oxidative phosphorylation is stimulated directly without the involvement of either adenine nucleotides or mitochondrial dehydrogenases.In the course of studies on the relationship between the activity of the neuronal Na/K pump and energy production (1), it was noticed that addition of high concentrations of KCI to a synaptosomal suspension containing physiological concentrations of potassium (3-5 mM) caused a transient increase in oxygen uptake. This rise in respiration rate was unusual in two respects: it was insensitive to 1 mM ouabain, an inhibitor of the Na/K-ATPase, and it showed an absolute requirement for external calcium.A rise in extracellular potassium concentration is an inevitable result of enhanced neuronal activity (2) and is accompanied by an augmentation of oxygen consumption, which is usually attributed to stimulation of the Na/K pump activity (3)(4)(5). However, the insensitivity to ouabain of the high K+ concentration-induced changes in respiration in isolated nerve endings (1) indicates that the Na/K-ATPase may not be involved and that there must be some other mechanism(s) that stimulates ATP synthesis. In view of the importance of maintaining a proper energy supply to actively firing neurons, we have investigated the effects of high concentrations of KCl on synaptosomes from rat brain in some detail. This preparation is well-suited for such studies because it contains a large number ofion channels and a high activity ofthe Na/K pump and because it is not susceptible to interference from secondary changes that arise in the intact brain from spreading depression. MATERIALS AND METHODSPreparation of Synaptosomes. Male Sprague-Dawley rats (200-250 g) were used throughout. Synaptosomes were isolated according to the method of Booth and Clark (6). In most experiments the synaptosomal pellet was washed and suspended in a modified Krebs-Henseleit buffer (140 mM NaCl/5 mM KCl/1.3 mM MgSO4/1 mM Na2HPO4) containing 10 mM Hepes neutralized to pH 7.4 with Tris base.Incubations. Synaptosomes were suspended at either 3-4 mg of protein per ml (measurement of oxygen uptake, pyridine nucleotide fluorescence, and metabolite levels) or 7-9 mg of protein per ml [measurement of 2-deoxy-D-glucose (2-DG) uptake] and incubated for 10 min at 30'C in a shaking water bath. In most experiments, the suspensions were supplemented with either 10 mM glucose or 5 mM ...

Contribution of Calcium Influx in Mediating Glucose-Stimulated Oxygen Consumption in Pancreatic Islets

“…These findings are similar to those found in brain (24) and muscle (25), and it appears that there is a calcium-activated process that increases energy turnover, which is ubiquitous in electrically excitable cells. We have used a perifusion system that simultaneously assesses OCR, cytochrome c reduction, and ISR in isolated rat islets (14,38) to further investigate the properties and implications of this process.…”

“…Moreover, oscillations in oxygen tension were reciprocally correlated with glucose-stimulated cytosolic calcium (23). Finally, similar to brain (24) and muscle (25), depolarization of the membrane by potassium decreased oxygen tension (26). Thus, it appears to be firmly established that calcium influx has an effect on oxygen consumption; however, questions remain regarding the mechanism mediating the interaction and regarding how much of the ATP generated in response to glucose is utilized by the calcium-dependent process(es).…”

Signalling Effect of Elevated Potassium Concentrations and Monoamines on Brain Energy Metabolism at the Cellular Level (Part 2 of 2)

“…The possibil ity that this response occurs in synapses is strengthened by the similarity with previous observations by Erecinska et al [69], who found a ouabain-resistant stimulation of both deoxyglucose utilization and oxygen consump tion in isolated synaptosomes exposed to an elevated extracellular K+ concentration (table 1). This stimulation is abolished in the absence of Ca2+ in the medium, but not in the presence of ruthenium red, the drug inhibiting mitochondrial Ca2+ uptake [69]. Taken together, the observations in the two types of cerebellar granule cell cultures and in synaptosomes strongly suggest that ele vation in the K+ concentration causes a ouabain-resistant stimulation, perhaps mainly of glycolytic energy metab olism.…”

“…Such a mechanism might explain the ouabain-resistant, relatively modest stimulation of oxidative metabolism and the more marked, but likewise quite ouabain-resistant stimulation of deoxyglucose uptake (table 1) in cerebellar granule cells with dendritic degeneration but normal morphology of presynaptic structures, including synaptic profiles [63]. The possibil ity that this response occurs in synapses is strengthened by the similarity with previous observations by Erecinska et al [69], who found a ouabain-resistant stimulation of both deoxyglucose utilization and oxygen consump tion in isolated synaptosomes exposed to an elevated extracellular K+ concentration (table 1). This stimulation is abolished in the absence of Ca2+ in the medium, but not in the presence of ruthenium red, the drug inhibiting mitochondrial Ca2+ uptake [69].…”

Use of CoA Biosynthesis Modulators and Selenoprotein Model Substance in Correction of Brain Ischemic and Reperfusion Injuries

“…Brain is very sensitive to disruptions of energy metabolism processes beacause brain tissue requires constant supply of energy substrates whereas sources for energy formation in brain are rather limited, turnover of metabolism is high, and metabolism is dependent on aerobic oxidation of glucose and constant supply of oxygen in a great extent [81,82] Maintenance for electric neuron activity and rate of impulse passage depend directly on presence and availability of energy substrates, too [83,84].…”