[Molecular mechanism of noradrenaline transport by rat auricles in vitro]

The transport of tritium-labelled noradrenaline (3H-NA) into bisected rat auricular appendages in vitro, has been studied, using 14C-inulin as an extracellular space tracer, thus allowing a precise measurement of the 3H-NA transported into the cells. The NA transport time course is relatively fast, and initial rates lasted 5 min. in 150 mM sodium, 3 min. in 26 mM sodium, and about 1 min. in 50 mM potassium. Intracellular accumulation of 3H-NA by synaptic vesicles, was found not to be important in the first minute of transport. In the presence of 150 mM sodium a transport Km for NA of 0.59 +/- 0.06 muM (mean +/- S.E.M.) and a maximal velocity (Vmax.) of 2.44 +/- 0.43 pmol/mg. protein/min. were estimated. When sodium was lowered to 26 mM, the Km increased to 2.26 +/- 0.7 muM (P less than 0.001), while Vmax. showed no change. With 0 mM sodium (choline substitution) active NA transport is completely suppressed, and only a diffusional component can be discerned. No binding of NA to beta adrenergic receptors was found, and a small but highly significant binding to the non-specific catechol receptors could be detected.