KINETIC CHARACTERIZATION OF SULFUR-CONTAINING EXCITATORY AMINO-ACID-UPTAKE IN PRIMARY CULTURES OF NEURONS AND ASTROCYTES

The uptake of the neuroactive sulphur amino acids L-cysteine sulphinate, L-cysteate, L-homocysteine sulphinate and L-homocysteate was investigated in astrocytes cultured from the prefrontal cortex; in neurons, cultured from cerebral cortex; and, in granule cells, cultured from cerebellum. It was shown that each amino acid acted as a substrate for a plasma membrane transporter in both neurons and astrocytes. Astrocytes and neurons exhibited a high-affinity uptake for L-cysteine sulphinate and L-cysteate with K(m) values ranging from 14-100-mu-M, and a low-affinity uptake for L-homocysteine sulphinate and L-homocysteate, with K(m) values ranging from 225-1210-mu-M. The uptake of all transmitter candidates studied was partially sodium-dependent. This sodium-dependency was most evident at low (< 100-mu-M) concentrations of each substrate. The apparent uptake measured in the absence of sodium was included as a component in corrections made for non-saturable influx. With the exception of L-cysteine sulphinate, uptake of each sulphur amino acid was greatest in astrocytes, with V(max) values ranging between 15-32 nmol min-1 mg-1 cell protein. Moreover, the uptake of each sulphur amino acid in cerebellar granule cells (V(max) values ranging between 10-25 nmol min-1 mg-1 cell protein) was consistently greater than that in cerebral cortex neurons (V(max) values ranging between 1.5-6 nmol min-1 mg-1 cell protein).