Effects of external Na⁺ and K⁺ on the initial rates of noradrenaline uptake by synaptosomes prepared from rat brain

1. 1. A technique was devised using Millipore filtration to quickly terminate the uptake of noradrenaline by rat brain synaptosomes, so that true initial rates of influx could be determined. 2. 2. The uptake of noradrenaline was linear for approximately 1 min but had significantly declined by 5 min. Uptake after 1 min incubation was therefore taken as an estimate of the initial rate of influx and was employed in all subsequent studies. 3. 3. Noradrenaline influx was inhibited by low concentrations of desmethylimipramine and cocaine. 4. 4. Reducing the concentration of Na⁺ in the medium reduced the V but did not alter the K_m for uptake. In the absence of external Na⁺, uptake was reduced but still obeyed Michaelis-Menten kinetics and had the same K_m as in the presence of 120 mM Na⁺. 5. 5. Removing K⁺ from the medium reduced the V but had no significant effect on the K_m for noradrenaline transport. 6. 6. Two models for the transport of noradrenaline across the pre-synaptic neuronal membrane are presented. In the first, Na⁺ increases the rate of movement of the carrier-noradrenaline complex across the membrane, whilst in the second, Na⁺ increases the total number of active sites for noradrenaline transport.