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Quantitation of 3,5,3'-triiodothyronine (T3) receptors by a microcentrifuge exchange assay: evidence for the existence of a nucleoplasmic pool of rat liver T3 receptors.

The total T3 receptor content of small aliquots of rat liver nuclear extracts was determined by incubation at a single, near-saturating concentration of [125I]T:) (3 nM) at 25 C for 2 h (exchange assay). Under these assay conditions the detectability of receptors occupied by endogenous T3 was only minimally reduced. Approximately 85% of the maximal binding capacity established by Scatchard analysis was determined by this technique. Compared to Scatchard analysis a 5-fold increase in the number of samples which could be assayed was realized without unacceptably high levels of background radioactivity. A nucleoplasmic pool of T3 receptors was obtained by three successive extractions of purified rat liver nuclei with a 0.15 M KC1 medium; a chromatin-bound pool was obtained by successive extractions of the residual chromatin with a 0.4 M KC1 medium. Approximately 41% (118 fmol/mgeq DNA) of the total extractable T3 receptor was recovered in the nucleoplasmic pool and 59% (170 fmol/mgeq DNA) in the chromatin-bo...