The role of internal and external Na⁺ and K⁺ on the uptake of [³H]noradrenaline by synaptosomes prepared from rat brain

1. 1. Rat-brain synaptosomes prepared on a Ficoll density gradient took up [³H]noradrenaline. This uptake was depressed by poisons (2,4-dinitrophenol and NaCN) and virtually abolished in the absence of K⁺. 2. 2. Raising the external Na⁺ concentration increased the uptake of [³H]-noradrenaline into non-poisoned synaptosomes but not into poisoned synaptosomes, Raising the external K⁺ concentration decreased the uptake of [³H]noradrenaline by non-poisoned synaptosomes. 3. 3. Synaptosomes prepared in a high-Na⁺ medium had a high internal Na⁺/K⁺ ratio whereas those prepared in a high-K⁺ medium had a lower internal Na⁺/K⁺ ration. These differences in internal ionic composition had no effect on the uptake of [³H]noradrenaline by these synaptosomes. 4. 4. Poisoning reduced the uptake of [³H]noradrenaline by synaptosomes to the same extent whether they initially contained a high Na⁺/K⁺ ratio or a low Na⁺/K⁺ ratio. 5. 5. Synaptosomes having a high internal Na⁺/K⁺ ratio throughout incubation took up [³H]noradrenaline to a much greater extent than did poisoned synaptosomes having a lower internal Na⁺/K⁺ ratio. 6. 6. These findings suggest that an inward-directed Na⁺ concentration gradient and/or an outward-directed K⁺ concentration gradient cannot be the only sources of energy for the uptake of [³H]noradrenaline by synaptosomes.