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

Abstract

A B S T R A C T 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 ,uM) 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 ,uM, 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 ,uM), or practolol (0.1 mM). Nevertheless, the beta1-and beta2-antagonist, propranol, and the beta,-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 This study was presented in part at The Eighth Annual Meeting of the European Thyroid Association, Lyon, France, October 1977.Received for publication 21 July 1978 and in revised form 6 December 1978. 1172 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 ofthymocytes 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 ofT3 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 itselfindependent ofadrenergic 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 inte...

Direct and Synergistic Interactions of 3,5,3′-Triiodothyronine and the Adrenergic System in Stimulating Sugar Transport by Rat Thymocytes

“…These interactions differ, however, in several respects from the ones that we describe here. First, in the two foregoing systems, the action of epinephrine alone, like that we describe in thymocytes (6), is clearly /8-mediated, but the thyroid hormones themselves are said to have no independent stimulatory effect. Second, with respect to these systems, there is no evidence that the participation of an a-component in the synergistic interaction between thyroid hormones and epinephrine is required.…”

“…We have previously presented evidence that various in vitro responses to T3 are mediated by extranuclear mechanisms, possibly at the level of the cell membrane (6)(7)(8). Evidence in support of this view was the ability of T3 in vitro to promptly stimulate, in isolated rat thymnocytes, the accumulation of the amino acid analogues, a-amino isobutyric acid, and cycloleucine, as well as the inward transport of the glucose analogues, 2-DG and 3-0-MG.…”

“…Evidence in support of this view was the ability of T3 in vitro to promptly stimulate, in isolated rat thymnocytes, the accumulation of the amino acid analogues, a-amino isobutyric acid, and cycloleucine, as well as the inward transport of the glucose analogues, 2-DG and 3-0-MG. In the case of cycloleucine accumulation, f8-adrenergic agonists, though themselves without effect, synergistically increased the stimulatory response to T3 (6). In these studies, we have examined the interaction between T3 and the adrenergic system with respect to sugar transport.…”

“…In previous studies we have presented evidence suggesting that, in addition to the actions on the nucleus and mitochondrion postulated by others (1)(2)(3)(4)(5), a primary action of the hormones is also exerted at the level of the plasma membrane. Part of this evidence is the finding that in the isolated rat thymocyte in vitro, 3,5,3'-triiodothyronine (T3)' promptly stimulates both the accumulation of certain amino acids (6,7) and the inward transport of glucose analogues (8), responses that are independent of new protein synthesis. An unusual feature of the effect of T3 on amino acid accumulation is the fact that 18-adrenergic agonists, which themselves have no effect on amino acid accumulation in this system, promptly synergize the stimulatory effect of T3 (6).…”

“…Part of this evidence is the finding that in the isolated rat thymocyte in vitro, 3,5,3'-triiodothyronine (T3)' promptly stimulates both the accumulation of certain amino acids (6,7) and the inward transport of glucose analogues (8), responses that are independent of new protein synthesis. An unusual feature of the effect of T3 on amino acid accumulation is the fact that 18-adrenergic agonists, which themselves have no effect on amino acid accumulation in this system, promptly synergize the stimulatory effect of T3 (6). This finding seemed to I Abbreviations used in this paper: 2 point further toward an action mediated at the plasma membrane level.…”