Some evidence of the active uptake of noradrenaline in the guinea-pig isolated trachea.

1. Guinea-pig isolated trachea immersed in a low concentration (50 nM) of (-)-(3)H-noradrenaline accumulates radioactive material against an apparent concentration gradient.2. Compartmental analysis based on the decay curve of radioactive material content during washout and its comparison with that of (14)C-sorbitol shows that some extracellular noradrenaline is adsorbed. A considerable mass exists in a slowly exchanging compartment, i.e. is retained. This retention is inversely concentration dependent.3. Over a 25-fold range of concentration the entry of either total radioactive material or noradrenaline followed Michaelis-Menten kinetics.4. The uptake of radioactive material was sodium-dependent and ouabain-sensitive.5. The uptake was susceptible to metabolic poisons which deprive the tissue of all available energy but it was not exclusively associated with either oxidative or glycolytic energy supply.6. Although net uptake could not be distinguished from exchange with endogenous amine, other evidence has been obtained for an active noradrenaline transport system with properties similar to neuronal uptake.