Effect of insulin hypoglycemia upon cerebral energy metabolism and EEG activity in the rat

Regulation of extra- and intracellular pH in the brain in severe hypoglycemia

Severe hypoglycemia is associated with a marked curtailment of cerebral glucose supply and with consumption of endogenous carbohydrate metabolites and amino acids, many of which exist as anions of acids. Since metabolic control of intracellular pH in acute hypo- and hypercapnia seems to be dependent on the production and consumption of metabolic acids, it must be suspected that intracellular pH in the brain is poorly regulated in hypoglycemic animals. We induced hypocapnia (Paco2about 15 mm Hg) and hypercapnia (Paco2about 90 mm Hg) in insulin-injected animals in “precoma” (EEG pattern of slow waves, polyspikes) and “coma” (cessation of EEG activity) and measured CSF and intracellular acid-base changes using the CO2method.

The induced hypoglycemia did not measurably alter CSF acid-base changes from the normal during hypercapnia, but it did impair CSF pH regulation in hypocapnia. Animals in precoma showed an unchanged cerebral energy state during both hypo- and hypercapnia. Regulation of intracellular pH was not measurably affected in hypercapnia but was reduced in hypocapnia. These results could be accounted for by a reduced ability of the hypoglycemic animals to produce metabolic acids in response to the decrease in Pco2, while the capacity to “consume” acids was largely retained.

In comatose animals, cerebral energy state was held at normocapnic levels during hypercapnia but deteriorated during hypocapnia. In the latter condition, the reduction in adenylate energy charge correlated to a decrease in blood pressure. The capacity to alter metabolic acid levels was abolished. In spite of this, hypocapnia was associated with a marked rise in intracellular pH, in some animals to values of about 7.7 (control, 7.0), and hypercapnia caused only very moderate reduction in intracellular pH. It is proposed that the excessive increase in intracellular pH during hypocapnia was due to hypotension-induced energy failure with subsequent depolarization of cells and passive equilibration of HCO3−(or H+) across the cell membranes. In hypercapnia, the influx of HCO3−into cells was unrelated to further deterioration of cerebral energy state but could possibly have been caused by CO2-induced depolarization and/or increased cell membrane permeability to HCO3−/H+ions.

It is concluded that severe hypoglycemia disrupts intracellular pH regulation in the brain and that hypocapnia combined with moderate hypotension leads to an excessive intracellular alkalosis of potential importance for the development of cell damage.

Hypoglycemic brain injury: metabolic and structural findings in rat cerebellar cortex during profound insulin-induced hypoglycemia and in the recovery period following glucose administration

Previous results have shown that severe, prolonged hypoglycemia leads to neuronal cell damage in, among other structures, the cerebral cortex and the hippocampus but not the cerebellum. In order to study whether or not this sparing of cerebellar cells is due to preservation of cerebellar energy stores, hypoglycemia of sufficient severity to abolish spontaneous EEG activity was induced for 30 and 60 min. At the end of these periods of hypoglycemia, as well as after a 30 min recovery period, cerebellar tissue was sampled for biochemical analyses or for histopathological analyses or for histopathological analyses by means of light and electron microscopy. After 30 min of hypoglycemia. the cerebellar energy state, defined in terms of the phosphocreatine, ATP, ADP, and AMP concentrations, was better preserved than in the cerebral cortex. After 60 min, gross deterioration of cerebellar energy state was observed in the majority of animals, and analyses of carbohydrate metabolites and amino acids demonstrated extensive consumption of endogenous substrates. In spite of this metabolic disturbance, histopathologic alterations were surprisingly discrete. After 30 min, no clear structural changes were observed. After 60 min, only small neurons in the molecular layer (basket cells) were affected, while Purkinje cells and granule cells showed few signs of damage. The results support our previous conclusion that the pathogenesis of cell damage in hypoglycemia is different from that in hypoxia-ischemia and indicate that other mechanisms than energy failure must contribute to neuronal cell damage in the brain.

Studies on the linkage of energy metabolism and neuronal activity in the isolated perfused rat brain

An isolated rat brain preparation was perfused using glucose-free (= aglycemic) media. The high-energy phosphates, substrates of the glycolytic pathway, free amino acids, acetylcholine as well as the intracellular distribution of hexokinase activity were determined in brain tissues. The EEG was evaluated visually. The levels of glycolytic substrates, glutamate, and glutamine in cortical tissue decreased after aglycemic perfusion whereas the asparte level increased and the GABA level remained unchanged. The high-energy phosphate content seemed to be unaffected for about 15 min of aglycemic perfusion and fell significantly after 20 min. The EEG of the isolated brain changed rapidly after starting aglycemic perfusion and became isoelectric after 12--15 min. Hyperglycemic perfusion (35 mmol glucose per liter perfusion medium) did not alter the energy metabolism of the isolated brain. The breakdown of cerebral energy metabolism and of EEG activity was postponed when thiopental was added to the perfusion medium. The soluble hexokinase activity measured in cortical tissue was reduced after aglycemic perfusion and was enhanced after thiopental. Hyperglycemic perfusion did not influence the intracellular hexokinase distribution. The acetylcholine level in the striatum of the isolated rat brain was significantly decreased by aglycemia and was increased in hypothalamus by thiopental. It was suggested that hexokinase bound to the mitochondrial membrane may play an important role in the relationship of energy metabolism and neuronal activity.

Seizure propagation and ATP depletion in the rat stroke model

Cerebral infarction was produced in rats by internal carotid injection of 35 mu plastic microspheres. Electroencephalograms were recorded through the scalp and from the thalamus. Regional concentration of endogenous brain ATP was studied by the histochemical bioluminescent method. Following embolization, a specific pattern of seizure propagation is noted. Spike activity appears first in the contralateral hemisphere, then the contralateral thalamus, infarcted thalamus, and finally the cortex. The results of these present experiments suggest that seizure activity following an evolving focal ischemic injury in the brain appears in the face of lowered ATP content.

Protein synthesis in rat brain in hypoxia, anoxia and hypoglycemia

The effect of cerebral hypoxia on protein synthesis was investigated by exposing rats to 5% O2, and examining polypeptide synthesis and size distribution profiles of ribosomes. The findings were compared with the results from cerebral anoxia (decapitation) and hypoglycemia. In cerebral hypoxia there was suppression of polypeptide synthesis, though to a lesser extent than in cerebral anoxia, while no effect was detected in hypoglycemia. Among 4 different ribosomal fractions used for polypeptide synthesis, the microsome was the most sensitive for hypoxia and anoxia, and the polyribosome after short centrifugation was the least sensitive. The size distribution profiles of 3 different ribosomes revealed an increase in the size of the monomere-dimer complex and a decrease of the polysome peak both in cerebral hypoxia and anoxia. Comparison of the energy state and the extent of lactic acidosis in cerebral hypoxia, anoxia and hypoglycemia available in the literature and the functional and structural state of polyribosomes in the present investigation suggests that intracellular acidosis may be the main cause of the suppression of polypeptide synthesis and disaggregation of polyribosomes in hypoxia, and the depletion of energy reserve may be the main cause in anoxia-ischemia.

Experimental cerebral infarction. Part 2: Electroencephalographic changes produced by experimental thalamic infarction in dogs

Previously, one of the authors developed a reliable experimental model in dogs for producing cerebral infarction. The EEG increased detection of experimental cerebral infarction, and was useful in predicting ischemic regions or infarction.

Sequential cerebral biochemical and physiological events in controlled hypoxemia

Effects of controlled hypoxemia on cerebral functional activity were studied in rats using cyclic adenosine monophosphate (cAMP) and aminergic neurotransmitters in the brain tissue as special references. Evidence is presented that: (1) mild hypoxemic stress (PaO2 60 to 40 torr) may activate cerebral glycolysis with no evidence of anaerobic metabolism but that further reduction of PaO2 impairs cellular respiration, as evidenced by accumulation of glycolytic products; (2) glycogenolysis in the brain tissue, leakage of potassium ions from the brain cell, increase in brain water, and suppression of neural functional activity occur concomitant with accumulation of cAMP and prior to the fall of adenosine triphosphate; (3) the diminution of cerebral high-energy phosphates during hypoxia is associated with and may be caused by hypoxemia-induced neuroglycopenia and occurs at PaO2 15 torr; (4) induced hypoxemia per se does not affect the level or aminergic neurotransmitter substances in brain tissue.