The accumulation of 3H-metaraminol in incubated lung tissue of the rat was examined in vitro. Lung tissue concentrated tritium after incubation with 3H-metaraminol. This uptake proceeds against a concentration gradient and shows saturation kinetics (Km 12.5 X 10(-7) M and Vmax 2.105 nM/g/15 min.) with great affinity for 3H-metaraminol. The accumulation of 3H-metaraminol is dependent of temperature and energy supply, and requires the presence of sodium ions. The loss of accumulated radioactivity showed the possibility that a small fraction of metaraminol is apparently firmly boune (e.g. neuronal sites), while a greater portion is taken up by extraneuronal structures (e.g. capillary endothelial cells). The first localization could represent uptake-1 and the second uptake-2. The results obtained also confirm the important role of the lungs as an organ which participates in the fate of endogenous substances and drugs.
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