Changes in β-adrenoceptors and leucocyte subpopulations after physical exercise in normal subjects

Summary

Seven healthy volunteers were subjected to standardized bicycle ergometry. Before and at the end of exercise, leucocyte and lymphocyte subset distribution was assessed by immunofluorescence labelling with monoclonal antibodies and the cytofluorograf. Competition binding studies were performed on mononuclear leucocytes with a fixed amount of the radioligand ¹²⁵I-(−)-cyanopindolol (¹²⁵I-CYP) and increasing concentrations of the hydrophilic ligand (−)4-(3-tertiary-butylamino-2-hydroxypropoxy)-benzimidazol-2-one hydrochloride (CGP-12177). Total numbers of β-adrenoceptors per cell and the receptor-ligand affinities were then derived by computer analysis. In separate experiments with blood obtained from resting subjects, β-adrenoceptor numbers of lymphocyte subsets, which had been sorted by the fluorescence activated cell sorter, were determined by saturation binding of ¹²⁵I-CYP. During exercise there was a twofold increase in total leucocyte numbers. The ratio between monocytes (4%) and lymphocytes (55%) remained constant, but the composition of lymphocyte subsets had changed. A twofold increase was observed for the lymphocyte population carrying the Leu-7 and the CD8 antigens or only the Leu-7 antigen, whereas the number of B cells and CD4 positive T cells increased only slightly. The lymphocyte phenotype changes appeared after 3 min, reached a maximum at the end of ergometry and had disappeared 30 min after exercise. Exercise led to a doubling of the number of β-adrenoceptors in unseparated mononuclear leucocytes from 240 ± 46 to 535 ± 190 sites per cell (SD, n = 6). β-Adrenoceptor numbers were higher on sorted Leu-7 positive cells (K _D 14 ± 6 pmol/1, B _max 1174 ± 233 sites/cell) and on CD8 positive cells (K _D 145 ± 79 pmol/l, B _max 1577 ± 670 sites/cell) than on monocytes (K _D 39 ± 31 pmol/1, 647 ± 91 sites/cell). Very low specific ¹²⁵I-CYP binding was found on Leu-7 and CD8 negative cells (less than 0.2 pmol/1). Before exercise the displacement curves with CGP-12177 fitted best a single site model, whereas after exercise 12% of the receptors were in a low affinity state for CGP-12177 indicating internalized receptors. The extent of internalization, however, was rather small (12%) compared to that found in experiments in vitro (66% of receptors in a low affinity state). The present study shows for the first time that exercise induces an increase in β-adrenoceptor numbers per cell on mononuclear leucocytes and, at the same time, an alteration in the distribution of lymphocyte subsets in blood with a predominant enrichment of a population carrying many high affinity β-adrenoceptors.