Enhancement by glucocorticoid deficiency of the increase in cycloleucine accumulation induced in rat thymocytes by triiodothyronine.

In previous studies, we have demonstrated that 3,5,3'-triiodothyronine (T3), either added to suspended media in vitro or injected acutely in vivo, increases the in vitro accumulation of the non-metabolized amino acid cycloleucine (CLE) by thymocytes harvested from weanling rats. We now report that this reponse is greatly enhanced by prior adrenalectomy of the donor rat. In vitro, a significant increase in CLE accumulation in thymocytes from adrenalectomized rats was induced by T3 at a concentration of 1 x 10-10 M, while a concentration of 1 x 10-6 M was required to produce a similar and significant effect in thymocytes from intact animals. In adrenalectomized animals, a single iv dose of T3 (0.5 microgram/100 G bw) significantly increased the in vitro accumulation of CLE in thymocytes harvested two hours later. In contrast, ten-times that dose was ineffective in control animals. Increased sensitivity to T3 was abolished by physiological replacement doses of hydrocortisone. The data are consistent with the well-known opposing effects of physiological levels of thyroid and glucocorticoid hormones on the growth and function of lymphoid tissue in vivo and, together with other findings, suggest that thyroid hormones modulate the cellular accumulation of amino acids in the intact animal.