Inhibition of the Na+/I- symporter by harmaline and 3-amino-1-methyl-5H-pyrido(4,3-b)indole acetate in thyroid cells and membrane vesicles

Abstract

Novel inhibitors of the Na+/I- symporter were identified using rat-thyroid-derived FRTL-5 cells and sealed vesicles from calf thyroid as model systems. Na+-dependent 125I- uptake was inhibited by the hallucinogenic drug harmaline and by a chemically related convulsive agent, 3-amino-1-methyl-5H-pyrido(4,3-b)indole acetate (TRP-P-2). TRP-P-2 (K(i) = 0.25 mM) was tenfold more effective as an inhibitor than harmaline (K(i) = 4.0 mM). Inhibition by TRP-P-2 was competitive with respect to Na+ and was fully reversible. Although TRP-P-2 is a relatively low-affinity inhibitor, its affinity for the Na+ site of the Na+/I- symporter is over 100 times higher than that of Na+ (K(m) = 50 mM). 45Ca2+-efflux rates in calf thyroid membrane vesicles were not affected by TRP-P-2, indicating that membrane integrity is not disrupted by the drug. These findings show that TRP-P-2 may be a potentially useful tool for the identification and characterization of the Na+/I- symporter.