The neuronal noradrenaline transport system of PC-12 cells: Kinetic analysis of the interaction between noradrenaline, Na+ and Cl? in transport

The uptake of ³H-noradrenaline into reserpine-pretreated PC-12 cells (a clonal cell line which possesses “uptake₁”) was abolished when at high extracellular Cl⁻¹ all the extracellular Na⁺ was replaced by Tris⁺ and when at high extracellular Na⁺ all the extracellular Cl⁻ was replaced by isethionate. Increases in the external Cl⁻ concentration (at a fixed high Na⁺ concentration) progressively increased the uptake of ³H-noradrenaline. The same was found with increase in the external Na⁺ concentration (at a fixed high Cl⁻ concentration). From the anions tested only Br⁻ and SCN⁻ were able to partially mimic the transport-stimulating effect of Cl⁻ (with about 40% and 20% effectiveness, respectively). When chloride was replaced by nitrate or larger anions such as sulphate, methylsulphate or isethionate, virtually no transport of ³H-noradrenaline was observed. The initial rate of uptake of ³H-noradrenaline showed saturation with increasing concentrations of noradrenaline when determined at several fixed concentrations of either Na⁺ or Cl⁻. The apparent K_m for noradrenaline transport (\(K_{m_{NA} } \)) progressively decreased and the \(V_{max_{NA} } \) increased with increases in the concentration of Na⁺ (at a high concentration of Cl⁻) or Cl⁻ (at a high concentration of Na⁺). The stimulation of the initial rate of uptake of ³H-noradrenaline by increasing concentrations of either Na⁺ or Cl⁻ obeyed saturation kinetics when determined at several concentrations of noradrenaline. The concentration of Na⁺ (or Cl⁻) which caused half-maximal stimulation of uptake (i.e., the apparent \(K_{m_{Na + } } \) and the apparent \(K_{m_{Cl - } } \)) decreased with increases in the concentration of noradrenaline. These results strongly suggest that Na⁺ and Cl⁻ are co-transported with noradrenaline.