GABA transport in the rat thyroid.

1. The uptake of gamma-aminobutyric acid (GABA) into rat thyroid slices was studied. 2. Uptake of 14C-GABA was concentration-dependent: one unsaturable (diffusion) and two saturable components obeying Michaelis-Menten kinetics contributed to transport. 3. The kinetic constants of saturable GABA transport systems were: Km1 = 1.5 microM, V1 = 4.0 nmol x (g wet weight)-1 x (20 min)-1 (high-affinity uptake): Km2 = 800 microM, V2 = 260 nmol x (g wet weight)-1 x (20 min)-1 (low-affinity uptake). 4. Uptake mediated by each of the carrier systems was concentrative, entirely Na+-dependent, and required activation energies characteristic for active transport. 5. High-affinity transport was structurally specific for GABA. The substrate specificity of low-affinity uptake resembled that of beta-amino acid transport systems.