Kinetic characterization of sulphur-containing excitatory amino acid uptake in primary cultures of neurons and astrocytes

The uptake of the neuroactive sulphur amino acids -cysteine sulphinate, -cysteate, -homocysteine sulphinate and -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 -cysteine sulphinate and -cysteate with K_m values ranging from 14–100 μM, and a low-affinity uptake for -homocysteine sulphinate and -homocysteate, with K_m values ranging from 225–1210 μM. The uptake of all transmitter candidates studied was partially sodium-dependent. This sodium-dependency was most evident at low (< 100 μM) 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 -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).