In previous studies of the uptake of catechol amines in the rat isolated heart it was shown that adrenaline and noradrenaline were accumulated by a common mechanism (Iversen, 1963, 1965). In these experiments the uptake saturated at an external amine concentration of approximately 0.2 μg/ml. noradrenaline or 0.5 μg/ml. adrenaline. However, in subsequent experiments in which hearts were perfused with (±)-adrenaline at a concentration of 5.0 μg/ml. an unexpectedly large uptake of adrenaline was observed. This fortuitous observation led to an examination of the uptake of adrenaline and noradrenaline during perfusions at higher concentrations of each amine than had previously been studied.
The results of these experiments led to the conclusion that a second type of uptake operates at high perfusion concentrations. The properties of this second process are sufficiently different from those described previously to justify a clear-cut distinction between the two processes.