Characterization of a cytosolic triiodothyronine binding protein in atrium and ventricle of rat heart with different sensitivity toward thyroid hormone levels.

Cytosolic T3-binding protein (CTBP) has been identified in both the ventricle and atrium of adult rat hearts. Its biochemical characteristics and concentration have been determined in the two tissues as a function of thyroid hormone level. In both tissues association and dissociation constants were, respectively, k+1 = 1.3 x 10(8) M-1/min and k-1 = 0.025 min-1. Scatchard analysis of T3 equilibrium binding data revealed a single class of binding sites (Ka = 3.8 x 10(8) M-1). The maximal binding capacity (MBC) was 1400 fmol/mg protein in the ventricle and 730 fmol/mg protein in the atrium. The apparent mol wt of CTBP, determined by gel filtration, was 63.000. Among the thyroid hormone analogs tested in ventricular cytosol, D-T3 had the highest affinity, followed by L-T3, L-T4, 3,3',5-triiodothyroacetic acid, and rT3. These characteristics were very similar to those previously described for rat brain, and dog and rat liver and kidney CTBP. In hypothyroid rats MBC was only increased in the atrium (50-100%); after a single injection of T4 (2 micrograms/10 g BW 3 or 18 h before death) values returned to normal in the atrium and declined in the ventricle (-35%). During postnatal development, the highest MBC value (2000 fmol T3/mg protein) was observed in atria on day 10, i.e. when the serum T4 level was still low, and in the ventricle on day 30 (4000 fmol T3/mg protein) when the serum T4 level was at its highest. Binding affinities were similar in the two tissues at all ages studied. It was twice as high in both these tissues during the first week of development than in adulthood. These results favor a thyroid hormone down-regulation of the binding capacity of CTBP that would be more sensitive to the hormone in the atrium than in the ventricle.