ELEVATION OF RAT-BRAIN AMINO-ACIDS, AMMONIA AND IDIOGENIC OSMOLES INDUCED BY HYPERSOMOLALITY

The effects of acute hyperosmolality obtained with NaCl, sucrose or glucose on rat brain and plasma osmolalities, electrolytes, lactate, amino acid, N2, NH3 and amino acids were studied. By 60 min after i.p. injection of each hyperosmolal medium, both brain and plasma osmolalities were increased and brain water decreased. Na+ and K+ in brain and plasma were increased by hyperosmolal Na+ or glucose injection, whereas hyperosmolal sucrose had an opposite effect. Acute hyperosmolality also increased brain amino acid N2, lactate and NH3. Hyperglycemia had the greatest effect on ammonia production. The increases of electrolytes, amino acid nitrogen, lactate, ammonia and glucose could only account for 90% of the total brain osmolality; thus, with acute hyperosmolality 10% of brain osmoles (40-50 mOsm[osmoles]/kg brain H2O) were unidentified or idiogenic osmoles. Brain amino acids including GABA, glutamine, glycine, lysine and threonine were increased by acute hyperosmolality. Increase of taurine, glutamate, aspartate and urea in brain was affected only by hyperosmolal NaCl, whereas the sugars had no effect. Acute hyperosmolality probably induces complex changes in brain amino acid metabolism, affecting especially the putative neurotransmitter amino acids associated with the formation of NH3 and of idiogenic osmoles.