Effects of catecholamines on thymocyte apoptosis and proliferation depend on thymocyte microenvironment.

Abstract The present study, through quantification of tyrosine hydroxylase (TH) expression and catecholamine (CA) content in the presence and in the absence of α-methyl- p -tyrosine (AMPT), a TH inhibitor, in adult thymic organ (ATOC) and thymocyte culture, demonstrated that thymic cells produce CAs. In addition, in ATOC an increase in β 2 -adrenoceptor (AR) mRNA expression and β 2 -AR thymocyte surface density was registered. Furthermore, AMPT (10 −4  M), as propranolol (10 −4  M), augmented thymocyte apoptosis and diminished thymocyte proliferation in ATOC. Propranolol exerted these effects acting on CD3 high thymocytes. However, in thymocyte cultures, propranolol (10 −6  M) acting on the same thymocyte subset exerted the opposing effect on thymocyte apoptosis and ConA-stimulated proliferation. This suggested that, depending on thymocyte microenvironment, differential effects can be induced through the same type of AR. Additionally, arterenol (10 −8 to 10 −6  M), similar to propranolol, diminished apoptosis, but increased ConA-stimulated thymocyte proliferation in thymocyte culture. However, differently from propranolol, arterenol affected manly CD3− thymocyte subset, which harbors majority of α 1 -AR+ thymocytes. Additionally, arterenol showed a dose-dependent decrease in efficiency of thymocyte apoptosis and proliferation modulation with the rise in its concentration. Considering greater affinity of arterenol for α 1 -ARs than for β 2 -ARs, the previous findings could be attributable to increased engagement of β 2 -ARs with the rise of arterenol concentration. Consistently, in the presence of propranolol (10 −6  M), a β-AR blocker, the arterenol (10 −8  M) effects on thymocytes were augmented. In conclusion, thymic endogenous CAs, acting through distinct AR types and, possible, the same AR type (but in different cell microenvironment) may exert the opposing effects on thymocyte apoptosis/proliferation.