INTRAVENOUS GAMMA-GLUTAMYL-TYROSINE ELEVATES BRAIN TYROSINE BUT NOT CATECHOLAMINE CONCENTRATIONS IN NORMAL RATS

A number of clinical situations may benefit from intravenous supplemen ts of tyrosine (Tyr). In total parenteral nutrition (TPN), the supply of Tyr is limited by its poor solubility. In both rats and infants mai ntained on pediatric TPN, plasma Tyr levels are approximately 30% of n ormal, and in rat brains Tyr concentrations are similarly reduced. We reported previously that supplementing a TPN solution with the soluble peptide, gamma-glutamyl-Tyr [Glu(Tyr)], normalizers plasma Tyr and do ubles brain Tyr in rats. To assess more fully the behavior of intraven ous Glu(Tyr) in vivo, 20 mmol/L Glu(Tyr) was infused into the inferior vena cava of rats at rates increased every 2 hours over an 8-hour per iod (300 to 450 mu mol Glu(Tyr)/kg body weight/h). The surgical proced ure for catheterization is described. At the maximum rate of infusion, plasma Tyr and Glu(Tyr) concentrations reached mean plateau vales of 326 and 252 mu mol/L, respectively. Brain Tyr concentrations were 71 a nd 264 nmol/g wet weight in control rats infused with heparinized sali ne (SAL, group) and rats infused with Glu(Tyr) (PEP group) respectivel y. No differences were found in concentrations of norepinephrine (NE), dopamine (DA), or homovanillic acid (HVA) in prefrontal cortex (PFC), striatum (STR), br remaining brain (Re) tissue in PEP and SAL rats. W e did not detect undegraded Glu(Tyr) in the brain, and less than 0.5% of infused Glu(Tyr) appeared in the urine. Copyright (C) 1996 by W.B. Saunders Company