Reproduction nutrition development vol:32 issue:3 pages:297-305
The characteristics of the nuclear T3 receptor in the liver of the chick embryo were studied from incubation day 18 until day 1 posthatching. Treatment of the nuclei with 3 mol.l-1 MgCl2, which removed the endogenously bound hormone. was used in order to determine the total amount of receptors. The affinity constant K(a) decreased between incubation day 18 (0.996 +/- 0.276.10(9) M-1) and day 19 (0.247 +/- 0.072.10(9) M-1), remained the same thereafter until hatching and increased again on day 1 posthatching (1.846 +/- 0.928.10(9) M-1). The total amount of receptors tended to increase from incubation day 18 to day 20 non-pipping (np) (from 4.40 to 11.55 fmol/mu-g DNA) and decreased thereafter to 2.38 fmol/mu-g DNA on day 1 posthatching. The amount of free binding sites reached a maximum on day 19 (6.91 fmol/mu-g DNA) and then decreased drastically until posthatching (0.19 fmol/mu-g DNA). The maximal specific binding was found on day 20 (np), just prior to penetration of the air chamber. During the time at which the level of T3 remains high in the plasma, a reduction in the amount of receptor was observed, which may be the consequence of a down-regulation by T3 itself.