Metabolism of normal and tumour tissue: The respiratory quotient of brain cortex

Versatility of microglial bioenergetic machinery under starving conditions

Microglia are highly dynamic cells in the brain. Their functional diversity and phenotypic versatility brought microglial energy metabolism into the focus of research. Although it is known that microenvironmental cues shape microglial phenotype, their bioenergetic response to local nutrient availability remains unclear. In the present study effects of energy substrates on the oxidative and glycolytic metabolism of primary - and BV-2 microglial cells were investigated. Cellular oxygen consumption, glycolytic activity, the levels of intracellular ATP/ADP, autophagy, mTOR phosphorylation, apoptosis and cell viability were measured in the absence of nutrients or in the presence of physiological energy substrates: glutamine, glucose, lactate, pyruvate or ketone bodies. All of the oxidative energy metabolites increased the rate of basal and maximal respiration. However, the addition of glucose decreased microglial oxidative metabolism and glycolytic activity was enhanced. Increased ATP/ADP ratio and cell viability, activation of the mTOR and reduction of autophagic activity were observed in glutamine-supplemented media. Moreover, moderate and transient oxidation of ketone bodies was highly enhanced by glutamine, suggesting that anaplerosis of the TCA-cycle could stimulate ketone body oxidation. It is concluded that microglia show high metabolic plasticity and utilize a wide range of substrates. Among them glutamine is the most efficient metabolite. To our knowledge these data provide the first account of microglial direct metabolic response to nutrients under short-term starvation and demonstrate that microglia exhibit versatile metabolic machinery. Our finding that microglia have a distinct bioenergetic profile provides a critical foundation for specifying microglial contributions to brain energy metabolism.

The Significance of Anoxæmia in Modern Psychiatric Treatment

Blood in its passage through the brain loses oxygen and glucose at relatively high rates, the amount of oxygen disappearing being approximately equivalent to the amount of glucose consumed, calculating on the basis that the sugar is completely oxidized. The respiratory quotient of brain in vivo is unity. These facts point to the dominance of carbohydrate oxidation in brain respiration in vivo and are similar to those found in studies of brain in vitro.

Various factors influence glucose oxidation in brain, e.g. changes in the ionic environment of the cells, vitamin B1, or the presence of narcotics. The latter bring about inhibitions of glucose oxidation in brain tissue which may in most cases be shown to be reversible in vitro. Glucose is not only important for the maintenance of respiration of brain but for enabling certain synthetic processes to occur. One of these is the formation of acetylcholine whose physiological significance is now well known and whose synthesis seems to be confined to the nervous system. This synthesis depends not only on the presence of glucose but on that of oxygen. The influence of glucose has been observed also in investigations on cortical potentials.

An important feature of the nerve cell is its vulnerability to the lack of oxygen. Reversibility depends on the degree and duration of the anoxæmia.

During insulin shock treatment studies of brain in vivo show lowered oxygen consumption and glucose utilization, these depending on the degree of hypoglycæmia. In cardiazol treatment, in vivo studies show that the oxygen content of the blood may fall to 42%. During the convulsion there is a greatly lowered arterial and venous blood-flow through the brain and cerebral anæmia becomes a marked feature. In narcosis treatment both in vitro and in vivo studies show a diminished ability of the brain to consume oxygen.

It is suggested that the most significant facts to be taken into account are (1) the importance of glucose and oxygen for the metabolism and function of the nervous system, (2) the vulnerability and varying sensitivities of nerve cells to lack of oxygen and glucose, (3) the occurrence of varying degrees of cerebral anoxæmia in narcosis, insulin and cardiazol treatments.

Inhibiting mitochondrial β-oxidation selectively reduces levels of nonenzymatic oxidative polyunsaturated fatty acid metabolites in the brain

Schönfeld and Reiser recently hypothesized that fatty acid β-oxidation is a source of oxidative stress in the brain. To test this hypothesis, we inhibited brain mitochondrial β-oxidation with methyl palmoxirate (MEP) and measured oxidative polyunsaturated fatty acid (PUFA) metabolites in the rat brain. Upon MEP treatment, levels of several nonenzymatic auto-oxidative PUFA metabolites were reduced with few effects on enzymatically derived metabolites. Our finding confirms the hypothesis that reduced fatty acid β-oxidation decreases oxidative stress in the brain and β-oxidation inhibitors may be a novel therapeutic approach for brain disorders associated with oxidative stress.