Beta-Adrenergic Potentiation of the Increased In Vitro Accumulation of Cycloleucine by Rat Thymocytes Induced by Triiodothyronine

James Etzkorn; Patricia Hopkins; Janet Gray; Joseph Segal; Sidney H. Ingbar

doi:10.1172/JCI109411

Beta-Adrenergic Potentiation of the Increased In Vitro Accumulation of Cycloleucine by Rat Thymocytes Induced by Triiodothyronine

James Etzkorn, Patricia Hopkins, Janet Gray, Joseph Segal, and Sidney H. Ingbar

Published June 1, 1979 - More info

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Abstract

We have previously demonstrated that 3,5,3′-triiodothyronine (T₃), whether administered in vivo or added to suspending media in vitro, promptly stimulates the in vitro accumulation of the nonmetabolized amino acids, alpha-aminoisobutyric acid, and cycloleucine (CLE) by thymocytes isolated from weanling rats. In these studies, we have examined the in vitro interaction between catecholamines and T₃ with respect to this effect. The previously reported enhancement of CLE accumulation in thymocytes by T₃ in vitro (1 μM) was confirmed. When added alone in concentrations ranging between 10 nM and 0.1 mM, the adrenergic agonists, epinephrine and norepinephrine, had no effect on CLE accumulation. At a concentration of 1 μM, isoproterenol, terbutaline, and phenylephrine were also without effect. However, the effect of T₃ was clearly potentiated by the concomitant addition of epinephrine, norepinephrine, and possibly isoproterenol, whereas terbutaline and phenylephrine were without effect. Neither basal nor T₃-enhanced CLE accumulation was affected by the addition alone of the adrenergic blocking agents, propranolol (0.1 mM), phentolamine (10 μM), or practolol (0.1 mM). Nevertheless, the beta₁- and beta₂-antagonist, propranol, and the beta₁-antagonist, practolol, blocked the increment in CLE accumulation produced by epinephrine; the alpha-antagonist, phentolamine, was without effect.

The enhancement of CLE accumulation that occurred in the presence of T₃, with or without epinephrine, was seen to be a result of an inhibition of CLE efflux, because T₃ alone inhibited CLE efflux, and this effect was increased when epinephrine was also present. On the other hand, neither T₃ alone nor T₃ plus epinephrine appreciably altered the rate of inward transport of CLE. As judged from studies of the ability of thymocytes to exclude trypan blue, neither T₃ alone nor T₃ plus epinephrine either enhanced or impaired viability of cells during 3-h periods of incubation. Cell water content, measured with [³H]urea, was unaffected by T₃, either alone or in the presence of epinephrine. In confirmation of previous results, the stimulatory effect of T₃ on CLE accumulation was unaffected by concentrations of puromycin sufficient to inhibit protein synthesis by at least 95%, and the potentiating action of epinephrine on the response to T₃ was similarly unaffected.

From these findings, it is concluded that the effect of T₃ to increase CLE accumulation by thymocytes in vitro, though itself independent of adrenergic mediation, is potentiated by beta₁-adrenergic stimulation. This interaction appears distinctly different from other thyroid hormone-catecholamine interactions, in which thyroid hormones enhance physiological responses to catecholamines. Its mechanism remains unclear, but the properties of the T₃ effect, and possibly the interaction itself, suggest that T₃ enhances CLE accumulation by an action at the level of the cell membrane.