Characterization of 5-hydroxytryptamine uptake in sliced rat lung

Slices of rat lung were incubated with tritiated 5-hydroxytryptamine and the tissue uptake of tritium was studied after separating the free and bound radioactivity by filtration on glass fiber filters. At 37 degrees a rapid uptake occurred during the first 10 min. After that time the uptake was less marked but it was still present after 60 min. The uptake was moderately potentiated by the MAO inhibitor iproniazid (3 micrometers) after 30-60 min. incubation. The 5-hydroxytryptamine uptake was inhibited by some uptake inhibitors, their order of potency beeing: clomipramine greater than imipramine greater than or equal to nortriptyline greater than or equal to cocaine greater than or equal to desipramine greater than maprotiline. At 0 degrees the uptake of 5-hydroxytryptamine was negligible. Non-linear regression analysis of uptake data indicated that 5-hydroxytryptamine was taken up by two different mechanisms. One of the uptake processes was saturated by high concentrations of 5-hydroxytryptamine and showed an apparent Km of 8 X 10(-7) M. The other uptake was linearly related to the 5-hydroxytryptamine concentration and could not be saturated even by concentration up to 5 X 10(-5) M of the amine.