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 2 hr but returned to near baseline at 6 hr. 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 1 M glucose (30 ml/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.