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

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.