BackgroundAcute mental stress evokes responses in the cardiovascular and the immune systems. In particular, the subset of natural killer (NK) cells is found to be responsive to mental stress. The role of β-adrenergic mechanisms in these processes is the subject of this investigation.Methods and ResultsHealthy male volunteers (n=31) were subjected to two consecutive mental tasks. Subjects were randomly assigned to a β-blocker (propranolol 40 mg) or a placebo group. The capsules were ingested 1 hour before the tasks. The tasks evoked sympathetic responses, as indicated by an increase in heart rate and a decrease in the preejection period. These effects were abolished under β-blockade, indicating that effective β-blockade was achieved. In the immune system, significant increases were found for the number of NK cells and NK cell activity in the placebo group; these increases were absent in the propranolol group. In addition, an increase in all lymphocyte subsets was observed in subjects who had ingested propranolol. This increase, however, was also observed in subjects who had received propranolol but had not performed the tasks, indicating that these non-subset-specific increases in lymphocytes were a side effect of the βblocker.ConclusionsMental stress induces activation of the sympathetic nervous system, with concomitant increases in the number of NK cells in the circulation. These changes were inhibited by propranolol, indicating that stress-induced increases in the number and activity of NK cells in the circulation are controlled by a β-adrenergic mechanism.