Pharmacokinetics of arginine and aspartic acid administered simultaneously in the rat: II. Tissue distribution.

The distribution of arginine and aspartic acid in brain, testes and liver was studied in rats after the simultaneous oral and intravenous administration of 0.1 mmol of these two amino acids. Exogenous fractions were determined by incorporation of [U-14C]-arginine and [3H]-aspartic acid. A significant increase in the free forms of the two amino acids was observed in all the organs except the liver where aspartic acid decreased after intravenous administration. The oral route induced higher concentrations of arginine in the testes and the brain and of aspartic acid in the liver. The concentrations of aspartic acid were higher in the brain and the testes after intravenous administration. Up to 15% of the dose of arginine administered was found in the liver, most of it bound to protein. Free aspartic acid concentrations underwent two successive increases with return to baseline values between the two phases. The first phase seemed to be due to an accumulation of the amino acid in the organ, followed by binding of the amino acid to the proteins. The second increase seemed to be due to a displacement of the protein bound form towards the free form. The steep rise in cerebral arginine levels, peaking at 30 minutes, may be one of the determining factors governing GH secretion induced by the simultaneous oral administration of aspartic acid and arginine.