In vivo protein synthesis of circulating human T lymphocytes does not respond to a cortisol challenge within 24 h

Background: Although immunocompetence is often measured by assessing respon siveness of lymphocytes to mitogenic stimulation in vitro, this approach ma y not reflect the in vivo situation. The aim of this investigation was to d etermine in vivo the protein synthesis rate (FSR) in isolated T lymphocytes and to study the effect of a short-term cortisol infusion on FSR. Methods: Healthy volunteers (n=24) were randomised into 4 groups. A continu ous cortisol infusion (6 mu g kg(-1) min(-1)) during 6 h was given to group s 1 and 2, whereas groups 3 and 4 served as control groups and received sal ine infusion. Protein synthesis was studied before and after 6 h of the cor tisol/saline infusion (groups 1 and 3) or 24 h after the start of the infus ion (groups 2 and 4). FSR was determined in vivo by the flooding method. Th e isotopic enrichment of phenylalanine in plasma and lymphocyte protein was determined with gas chromatography-mass spectrometry. Results: The FSR in T lymphocytes was 13.6+/-0.9%/24 h as a mean value (+/- SD) of the first determination in 4 groups. There was no significant differ ence in FSR from the baseline value immediately after the cortisol infusion (group 1: 13.3+/-1.4%/24 h vs 13.5+/-28%/24 h) or 24 h after the start of the infusion (group 2: 13.6+/-0.7%/24 h vs 12.3+/-24%/24 h). Conclusion: The metabolic activity of circulating T lymphocytes, as reflect ed by a quantitative measurement of in vivo protein synthesis of human T ly mphocytes, was not affected by the increased level of cortisol.