1. We studied 37 healthy men at rest in the supine position to examine the effect of ageing, smoking and physical training on β2-adrenoceptor function, plasma catecholamines and the proportions of various lymphocyte subsets.

2. In 14 young subjects the proportion of natural killer cells was correlated with cAMP production in lymphocytes and inversely correlated with plasma noradrenaline level.

3. In 16 elderly non-smokers plasma noradrenaline was negatively correlated with the natural killer cell subset CD3–CD16+. Lymphocyte cAMP responses did not differ between young and elderly non-smokers, whereas plasma noradrenaline increased slightly but significantly with age. Physical training did not influence either plasma noradrenaline or adrenaline at rest or cAMP in lymphocytes.

4. In seven elderly long-term smokers cAMP production and the viability of lymphocytes were reduced. Plasma noradrenaline attained its highest values in long-term smokers.

5. It is concluded that cAMP production and plasma noradrenaline are related to lymphocyte subset composition. The greater the proportion of natural killer cells and related subsets, the higher is cAMP production and the lower is plasma noradrenaline. Thus, the inverse correlation between lymphocyte cAMP and plasma noradrenaline is indirect and most likely mediated by variability in lymphocyte subset composition. In elderly subjects, reduced cAMP production was observed in long-term smokers, and this abnormality was probably due to a reduced viability of lymphocytes and especially of natural killer cells. The negative correlation between the proportion of natural killer cells and plasma noradrenaline at rest contrasts with a well-known mobilizing effect of adrenaline on natural killer cells.

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