TRANSPORT OF LYSINE FROM CEREBROSPINAL FLUID OF THE CAT

—The clearance from cerebrospinal fluid of l-[¹⁴C]lysine and l-[³H]arginine was measured during ventriculo-cisternal perfusions of anaesthetized cats. Increasing in the perfusate the concentration of unlabelled l-lysine produced a gradual reduction in clearance of the labelled amino acids without altering the uptake of l-[¹⁴C]lysine by the choroid plexus. Net transport of l-lysine out of cerebrospinal fluid occurred by saturable and non-saturable components. The saturable component satisfied Michaelis-Menton kinetics, while the behaviour of the non-saturable component was consistent with diffusion. A V_max of 0·017 μmol/min and an affinity constant (k_t) of 0·83 mm were estimated. The clearance of l-lysine was unaffected by the addition to the perfusate of high concentrations of selected neutral amino acids, but was stimulated by the presence of l-cystine. Conversely, a high concentration of l-lysine did not affect the clearance of glycine or cycloleucine. The dibasic amino acids appear to be removed from cerebrospinal fluid by a relatively specific, mediated transport system which may serve to regulate their concentrations in the cerebrospinal fluid.