John A. Dirgo

Taurine: A role in osmotic regulation of mammalian brain and possible clinical significance

Abstract Chronic hypernatremic dehydration induced in developing mice by water deprivation and salt loading for 4 days increased 16 of the 19 amino acids measured in brain. Taurine accounted for over one-half of the total increase. It is well known that during adaptation to increased environmental salinity, levels of amino acids in invertebrate and amphibian tissues increase to maintain osmotic equilibrium and to limit the loss of cell water. The findings in young mice support a similar function for amino acids, taurine in particular, in mammalian brain and suggest that the phenomenon may be causally related to the cerebral edema that develops during overly rapid rehydration of infants and children with chronic hypernatremic dehydration.

Effects of a single therapeutic dose of glycerol on cerebral metabolism in the brains of young mice: possible increase in brain glucose transport and glucose utilization.

Abstract: This is a study of the effects of a single “therapeutic” dose of glycerol [2 g(22 mmol)/kg i.p.] on brain carbohydrate and energy metabolism in normal nursing weanling mice. Findings were correlated with brain water and electrolyte content and with metabolite changes in plasma, red blood cells, and liver.

Insulin and Brain Metabolism: Absence of Direct Action of Insulin on K+ and Na+ Transport in Mouse Brain

This is a study of the effect of insulin on the transport of K+ and Na+ from the blood into the brains of normal mice. Despite profound reductions in plasma and brain glucose levels, reduction of plasma K+ concentration and progressive deterioration of neurologic function 30–120 minutes after insulin injection, in 20–22-day-old animals there was no increase in brain K+ and Na+ concentrations. In fact, at 120 minutes, when the brain water content increased 0.7 per cent, brain K+ concentration was significantly reduced, not elevated. The effect of insulin on brain electrolyte and water content in adult mice was also studied. Although brain water increased 0.5 per cent at 120 minutes, there was no changes in brain Na+ or K+ concentrations at any time after insulin injection. The data from mice do not support a role of insulin in electrolyte transport in brain.

Insulin and brain metabolism. Absence of direct action of insulin on K+ and Na+ transport in normal rabbit brain.

In fed, unanesthetized rabbits, regular zinc insulin, 50 U./kg. intravenously, decreased plasma glucose levels 52 per cent, p = 0.002, 35 minutes after injection. In 15-hour-fasted, unanesthetized animals, the same dose of insulin decreased plasma glucose levels 68 per cent, p <0.001. Plasma K+ concentration was not affected by insulin injection in the fed animals; in fasted rabbits, plasma K+ levels fell 26 per cent, p = 0.006. Despite this unequivocal evidence of insulin action in both sets of animals, there was no change In the K+, Na+, or H2O content in the brains of the same animals 35 minutes after insulin injection. These results, which give no evidence of a direct effect of insulin on electrolyte transport in brain, are in sharp contrast with those found in anesthetized rabbits, which suggested that insulin affects brain potassium and water content before any change in plasma glucose occurs.

Effects of acute hyperosmolar NaCl or urea on brain H2O, Na+, K+, carbohydrate, and amino acid metabolism in weanling mice: NaCl induces insulin secretion and hypoglycemia

This study compares early and late effects of the injection of hyperosmolar NaCl and urea of equal osmolarity on selected aspects of brain water, electrolyte, carbohydrate, amino acid, urea, and energy metabolism in normal suckling-weanling mice. One hour after treatment, salt-treated mice were critically ill, while the behavior of urea-treated animals could not be distinguished from that of controls. This clinical difference could not be explained on the basis of differences in plasma osmolality, the brain water content, or the degree of hemorrhagic encephalopathy. The injection of NaCl induced a 14-fold increase in plasma insulin and a progressive fall in the plasma glucose concentration (a reduction of 66% at 1 hr). In contrast, plasma glucose levels in urea-injected mice were unchanged. Prior to the fall in plasma glucose levels, metabolite changes in the brains of NaCl-injected mice were compatible with facilitation of transfer of glucose from the blood to the brain, increased metabolic flux in the Embden-Meyerhof and Krebs citric acid cycle pathways, and increased energy production. With the exception of the glucose content (unchanged), similar metabolite changes were seen in brain soon after urea injection. In the brains of the hypoglycemic NaCl-treated mice, glucose levels were reduced 80%, and glycogen 41%. Other metabolite changes were compatible with decreased glycolysis and metabolic flux through the Krebs citric acid cycle. In contrast, with few exceptions, at a similar time after injection, metabolite levels had returned to normal in the urea-treated mice. Permeability of the brain to urea was also examined. Brain urea reached high levels at 2hr but returned to near baseline at 6hr. Both hyperosmolar solutions increased the brain content of aspartic and glutamic acids 1 hr after injection. The failure of hypoglycemic mice with hypernatremia and elevated plasma osmolality (range, 416–434 mOsm/kg H2O) to respond to 1M glucose (30ml/kg) may have been due to the ill effects of the additional hyperosmolar load. The possibility remains that the encephalopathy induced by hyperosmolar NaCl, but not by hyperosmolar urea, is in some way related to the sudden elevation of brain Na+ and/or Cl− ions.

Hyperglycemia, Hypoinsulinemia, and Hyperglucagonemia in Acute Water Intoxication

When acute (four-hour) hyponatremia with clinical signs of water intoxication was produced in normal weanling mice by the use of hypotonie glucose or deionized water, there was a two-to-fourfold increase in plasma glucose concentration. Concomitantly, concentrations of plasma insulin fell 63 to 68 per cent, whereas plasma glue agon increased to 262 per cent of control. The findings are compatible with stress-induced catecholamine release.

1032 INCREASED CEREBRAL METABOLIC RATE AND DECREASED ANOXIC SURVIVAL AFTER AMINOPHYLLINE IN YOUNG MICE

Last year we reported that in 17-21 d old mice, aminophylline (AP), 100 mg/kg i.p., increased brain levels of cyclic AMP 56% (p = 0.01), cyclic GMP 36% (p = 0.01), glucose 93% (p <0.001), ADP 12% (p = 0.04), and AMP 70% (p = 0.02). ATP, P-creatine, glycogen and lactate were unchanged. Increased ADP and AMP levels are sensitive indicators of ATP (∼P) breakdown and suggest an increased cerebral metabolic rate (CMR). To test this hypothesis, the effect of AP (100 mg/kg i.p.) on CMR was determined in 48 weanling mice (Lowry et al, J. Biol. Chem. 239:18, 1964). AP increased CMR 3-fold - 34 mmol/kg/min ∼P (1.03 mmol/kg/min glucose) vs 12 mmol/kg/min ∼P (0.36 mmol/kg/min glucose) in controls. Increased CMR is accompanied by increased extraction of glucose from blood and could explain the brain glucose elevation. Since increased CMR also reflects increased neuronal function, this may be the mechanism by which AP restores normal breathing in premature babies with apnea. However, with a decreased glucose and/or O2 supply to brain, increased CMR would be a distinct disadvantage. This suspicion was confirmed in 3-9 d old mice treated with a therapeutic dose of AP (7.5 mg/kg s.c.). Fifteen to 60 min after injection, one control and one AP-treated littermate were exposed to N2 gas for variable intervals. Survival rate of 32 mice was 62% for controls vs 0% in AP-treated animals. The finding suggests caution in the use of AP in hypoglycemic and/or anoxic newborns.

INCREASES IN BRAIN GLUCOSE AND PLASMA GLUCOSE, GLYCEROL AND |[beta]|-HYDROXYBUTYRATE AFTER AMINOPHYXLINE

Effects of aminophylline (100 mg/kg i.p.) on brain carbohydrate and energy metabolism were studied in 2 litters of normal 17-23-day-old mice (11 animals). Although no clinical effects were seen, 20 min after aminophylline plasma glucose increased 22% (11.79 ± 0.15 vs 8.90 ± 0.80 mM in controls, p = 0.025); plasma β-hydroxybutyrate was 242% of control (443 ± 75 μM vs 183 ± 28, p = 0.031) and plasma glycerol, 258% of control (570 ± 48 μM vs 221 ± 17, p <0.001). Since the methyl xanthines increase tissue cyclic AMP levels and/or catecholamine and glucagon release, these findings may reflect this action (increased hepatic glucose output and lipolysis).Brain cyclic AMP increased 56%, p = 0.014. Brain glucose increased dramatically, 1.47 ± 0.12 vs 0.76 ± 0.08 mmol/kg in controls, p = 0.001. Brain ATP, P-creatine, glycogen and lactate levels were unchanged. In the face of an apparently normal cerebral metabolic rate, the increased brain to plasma glucose concentration ratio, 0.13 ± 0.01 vs 0.09 ± 0.01 in controls, p = 0.048, suggests increased brain glucose transport.β-hydroxybutyrate is a major metabolic fuel of the brains of young animals and glycerol is an effective substrate for hepatic gluconeogenesis; furthermore, in anoxic brain, glucose is the only source of ATP. In view of these facts, the findings presented in this report may explain the mechanisms(s) of the beneficial action of methyl xanthines in apnea of prematurity.