Putative Nuclear Triiodothyronine Receptors in Tadpole Erythrocytes during Metamorphic Climax*

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Abstract
The characteristics of the nuclear T3 [triiodothyronine] receptors present in red blood cells (RBCs) of R. catesbeiana tadpoles undergoing metamorphic climax were investigated with a T3 saturation technique. Because there were significant amounts of receptor-bound endogenous hormone, prolonged incubation in vitro (48 h at 21.degree. C) was necessary to achieve binding equilibrium. Receptor number, which averaged 783 .+-. 35 (.+-. SE) sites/nucleus during early prometamorphosis (stages XIV-XVI), increased rapidly during the subsequent stages of this phase. By stage XIX, receptor number had reached a maximum of 2464 .+-. 152. During climax, receptor number decreased steadily, and by stage XXV, it was lower than that observed during premetamorphosis (stages V-X). This decrease was attributed to the replacement of larval RBC, which have a high receptor content, by adult RBC, which possess significantly fewer sites/nucleus. At midclimax, adult and larval RBCs were separated on a Renografin continuous density gradient. Adult cells constituted more than 65% of the total cell population and were shown to contain only 126 .+-. 46 sites/nucleus. Although the striking increase in receptor number preceded the substantial increases in plasma T4 [thyroxine] and T3 levels that occurred during climax, it was associated with a measurable increase in plasma thyroid hormone levels. Receptor number in stage XIV tadpoles was increased markedly after immersion for 14 days in water containing sufficient T3 to raise plasma T3 only to levels normally observed in late prometamorphosis. Apparently, the increases in receptor number and plasma thyroid hormone levels that occur before climax in this species are causally related.