Differential effects of hypertonic mannitol and glycerol on rat brain metabolism and amino acids.

Intraperitoneal injections in rats of two different dosages of hypertonic solutions containing mannitol or glycerol caused complex and differential changes in brain amino acids. When plasma osmolalities were elevated to toxic levels of 397--432 mOsm/kg H2O, brain sodium was increased, whereas plasma sodium was decreased. Brain potassium was not affected. Brain water decreased significantly, concomitant with elevation of plasma osmolality. Both brain lactic acid and [125I]albumin space rose significantly. Brain amino acids (mostly aliphatic and basic amino acids) as well as Gaba and glycine (putative inhibitory neurotransmitters) increased after both mannitol and glycerol. Ammonia was stimulated by mannitol but was unaffected by glycerol. Plasma amino acids, which generally increased after mannitol, were decreased by glycerol. When the plasma osmolalities were elevated only to moderate levels (about 350 mOsm/kg H2O), only glycerol induced a significant increase in brain taurine, aspartic acid, alanine, leucine and lysine. Thus, with moderate hyperosmolality, glycerol has striking effects on brain amino acid metabolism that are not observed with mannitol.