Beta-adrenoceptor adaptation to endurance training.

Endurance exercise is associated with various cardiovascular adaptations, and these may include changes in sympathoadrenal activity and end-organ responsiveness to norepinephrine (NE). Because beta-adrenoceptor function is a major determinant of sympathetic responsiveness, we used the lymphocyte beta-adrenoceptor as a model to study the effects of endurance training in 19 male subjects before, during, and after preparation for a marathon race. Before the subjects trained, resting beta-adrenoceptor density was positively correlated to resting heart rate (r = 0.47, p less than 0.05) but this correlation was not evident after training. Eleven of the 19 subjects completed the 3 1/2-month training period, and mean (+/- 1 SD) resting beta-adrenoceptor density fell from 1,593 +/- 333 to 1,197 +/- 332 sites per cell (p less than 0.02) after the training period. Two weeks after cessation of training, receptor density was at pretraining values of 1,547 +/- 209 sites per cell. During training, the subjects ran two 21-km races. Each of these was associated with decreases in beta-adrenoceptor density. Our results demonstrate the episodes of endurance running result in reductions in lymphocyte beta-adrenoceptor density. We conclude that down-regulation of beta-adrenoceptors is an important component of the response to endurance training.