GABA transport in the rat thyroid

Abstract

The uptake of γ-aminobutyric acid (GABA) into rat thyroid slices was studied. Uptake of ¹⁴C-GABA was concentration-dependent: one unsaturable (diffusion) and two saturable components obeying Michaelis-Menten kinetics contributed to transport. The kinetic constants of saturable GABA transport systems were: K _m1 =1.5 μM, V ₁=4.0 nmol×(g wet weight)⁻¹×(20 min)⁻¹ (high-affinity uptake); K _m2 =800 μM, V ₂=260 nmol×(g wet weight)⁻¹×(20 min)⁻¹ (low-affinity uptake). Uptake mediated by each of the carrier systems was concentrative, entirely Na⁺-dependent, and required activation energies characteristic for active transport. High-affinity transport was structurally specific for GABA. The substrate specificity of low-affinity uptake resembled that of β-amino acid transport systems.