Density and functioning of human lymphocytic beta-adrenergic receptors during prolonged physical exercise

In order to study the regulation of beta-adrenergic receptor number and function in response to prolonged physical effort, lymphocytic beta-adrenoceptor density (determined by (-)[125I]iodocyanopindolol binding), lymphocytic basal and isoproterenol-stimulated cyclic AMP (cAMP) production and concentrations of plasma catecholamines were measured before and during 3 h running exercise in eight healthy volunteers. A significant (P less than 0.01) increase of the lymphocytic beta-adrenoceptor density from 45 +/- 4 to 81 +/- 9 fmol mg-1 protein (mean +/- SEM) took place during the first hour of exercise. As the exercise was continued for up to 2.1-3 h, the receptor densities did not change significantly any more and remained elevated (72 +/- 9 fmol mg-1 protein) in comparison to the resting levels (P less than 0.02). The isoproterenol-stimulated cAMP production of the lymphocytes increased during the first hour of running from 190 +/- 36 to 269 +/- 56 pmol mg-1 protein (P less than 0.01) and returned to the resting level at the end of the exercise (182 +/- 38 pmol mg-1 protein). The mean levels of plasma catecholamines increased approximately sixfold during the first hour of exercise and remained elevated until the end of the running. This study demonstrates that the beta-adrenergic receptor system is activated in lymphocytes during prolonged aerobic physical exercise. This activated state becomes, however, attenuated within 2-3 h of exercise as indicated by a diminishing ability of beta-adrenoceptors to mediate catecholamine-induced cAMP production.