BETA-ADRENERGIC POTENTIATION OF THE INCREASED INVITRO ACCUMULATION OF CYCLOLEUCINE BY RAT THYMOCYTES INDUCED BY TRIIODOTHYRONINE

The authors 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 non-metabolized amino acids, alpha-aminoisobutyric acid, and cycloleucine (CLE) by thymocytes isolated from weanling rats. In these studies, they 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-hr periods of incubation. Cell water content, measured with [b3H]urea, was inaffected 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.