Diurnal rhythm in absolute and relative concentrations of large neutral amino acids in human plasma.

In order to obtain detailed information on the diurnal plasma rhythmicity in those large neutral amino acids (LNAA) which compete with each other for carrier-mediated transport from plasma into the brain, blood samples were collected every hour for 24h from six healthy men, aged 25-50 yr. The well-known diurnal rhythm in the concentration of plasma amino acids was confirmed and is described in greater detail in this paper. These data might be helpful in clinical situations where LNAAs (e.g. L-dopa, alpha-methyldopa and tryptophan) are used as therapeutic agents, since these amino acids compete with endogenous LNAAs for carrier-mediated transport from plasma into the brain. Furthermore, it was found that the amplitude of variation differed between the individual LNAAs. In addition the relationships between them, expressed as ratios of the concentrations of each single LNAA to total LNAA concentration also possess diurnal rhythms, different from those observed in absolute LNAA concentrations. This rhythmicity in relative LNAA concentrations might be of importance for brain function, since altered relationships between LNAAs in plasma might bring about altered concentrations of LNAAs in the brain and consequently, in monoamine synthesis.