Potassium modulation of methionine uptake in astrocytes in vitro.

Methionine participates in a large variety of metabolic pathways in brain, and its transport may play an important regulatory role. The properties of methionine uptake were examined in a preparation of neonatal rat brain astrocytes. Uptake is linear for 15 minutes, up to 2.5 microM. At steady state conditions, methionine is concentrated 30-50-fold. Measured methionine homoexchange accounts for a significant fraction of uptake at concentrations greater than 10 microM. We recently reported that methionine uptake is decreased by elevations in extracellular K+. Potassium induced efflux cannot account for this apparent effect; and thus for concentrations less than 2.5 microM, and for short times of incubation, measured rates of methionine uptake represent unidirectional flux. At extracellular concentrations of K+ equal to 6.9 mM, the apparent Vmax of methionine transport is 182 pmol/min/mg protein, and the Km is 1.3 microM. Where K+ is shifted to 11.9 mM, the Km remains unchanged, and the Vmax is reduced by half.