Drug competition for intracellular triiodothyronine-binding sites.

A variety of substances, including frusemide, non-esterified fatty acids (NEFAs) and non-steroidal anti-inflammatory drugs (NSAIDs), can compete for triiodothyronine (T3)-binding sites in serum and at the cell surface. We examined the competitive potency of these agents at intracellular T3-binding sites in order to assess their potential to act as T3 antagonists. Competition for [125I]T3 binding was determined using hydroxyapatite separation in cytosols and nuclear extracts prepared from livers of Macaca fascicularis. The T3 affinities were 15.8 +/- 1.2 nmol/l in cytosol and 0.23 +/- 0.02 nmol/l in nuclear extract. Dose-response curves were analysed by a four-parameter sigmoid curve-fitting program to determine competitor potency. The nineteen agents tested included various NSAIDs, NEFAs, non-bile acid cholephils (NBACs), frusemide, amiodarone and the flavonoid EMD 21388. In nuclear extract the most active competitors were linoleic acid (8.5 mumol/l) and linolenic acid (7.8 mumol/l). Potencies of NSAIDs varied between 66 mumol/l (meclofenamic acid) and 525 mumol/l (diclofenac). In cytosol, NEFAs were less potent but NSAIDs were stronger competitors than in nuclear extract. Half-inhibitory potencies in cytosol were between 13.2 mumol/l (meclofenamic acid) and 63.1 mumol/l (flufenamic acid). The NBAC bromosulphthalein was one of the most potent inhibitors in both cytosol and nuclear extract. When expressed relative to T3, diclofenac was a more effective competitor in cytosol than it was in nuclear extract. Amiodarone and EMD 21388 were without effect both in cytosol and nuclear extract. Frusemide (759 mumol/l) was weakly active in cytosol only.(ABSTRACT TRUNCATED AT 250 WORDS)