Changes in the blood-brain transfer of L-phenylalanine elicited by arginine vasopressin.

Arginine vasopressin (AVP) coinjected into the carotid artery in physiological concentrations (0.1 nmol/l) with L-[3H]phenylalanine changed the kinetic parameters of blood-brain barrier (BBB) transport of this neutral amino acid. The half-saturation constant (Km) and the maximum velocity of transport (Vmax) were estimated in nine brain regions of male Wistar rats anaesthetized with ether. In the cerebral hemisphere Km decreased significantly from 0.107 to 0.061 mmol/l and Vmax from 37.6 to 27.4 mmol/min/g. Comparing the kinetic constants for the phenylalanine transfer of corresponding regions of control animals and those coinjected with the amino acid and AVP, 8 out of 9 (Km) and 7 out of 9 (Vmax) regions were found significantly lowered. The findings support the hypothesis that AVP binding to the endothelial cell layer of the brain capillaries induces alterations of the transfer of at least the large neutral amino acids (LNAAs) from blood to brain. The kinetic parameters estimated for L-phenylalanine favour the assumption that in vivo AVP induces a preferred transport of this amino acid across the BBB compared to L-leucine.