Studies have been performed in order to investigate the effects of catecholamines. histamine, peptide hormones and various other agents known to increase cellular adenosine 3':5'-cyclic monophosphate (cAMP) levels on the expression of Thy-1 and TL antigens on 14-day fetal thymic stem cells. It has been shown that the proportions of Thy-1 and TL positive cells, as detected by dye exclusion cytotoxicity tests, can be significantly increased by some of these agents and that these effects may be inhibited by beta adrenergic and H2 antagonists. The value of these tests as a means for studying normal thymic stem cell maturation and for investigating T cell deficiency is discussed.
Thymocytes have been studied a t various stages during ontogeny in relation to their responsiveness to f~ adrenoceptor agonists and ability to bind ~-[propyl-2,3-~H] dihydroalprenolol ( [3H] DHA), a specific radioligand of the fi adrenoceptor. Specific [3H] D H A binding and the activity of an isoproterenol-sensitive adenylate cyclase were compared and correlated between (14 to 20-day) fetal mouse thymocytes and young adult thymocytes. Preliminary experiments show that [3H] DHA binding to thymocytes demonstrated the kinetics, affinity and stereospecificity expected of binding to adenylate cyclase-coupled fi adrenoceptors. Similarly, isoproterenol was shown to selectively increase intracellular adenosine 3' : 5 '-cyclic monophosphate (CAMP) levels and was antagonized by propranolol (a fi adrenoceptor antagonist), but not by phentolamine (an CI adrenoceptor antagonist), nor by lignocaine (a local anesthetic). The rises in fetal thymocyte cAMP levels found after stimulation with isoproterenol were of greater magnitude than in adult thymocytes. The number of [3H] DHA-binding sites in both fetal and adult thymocytes was the same (Bmax = 50 fmoli2 X lo6 cells). However, the affinity of the binding sites for [3H]DHA was less for the adult cells than the fetal cells (with KD values of 8.0 and 2.2 nM, respectively); this was reflected in the amount of [3H] D H A bound being up to 8 times greater in the embryonic cells.
Experiments were carried out on 14-day fetal mouse thymic stem cells which were treated with agents known to increase cellular cAMP levels for a period of 90 min at 37 degrees C. When compared to controls, a high proportion of treated stem cells were found to develop Thy-1 (theta) antigen as indicated by dye exclusion tests using anti-Thy-1 serum and complement.
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