PROLONGED EXERCISE ALTERS BETA-ADRENERGIC RESPONSIVENESS IN HEALTHY SEDENTARY HUMANS

To examine whether beta-adrenergic desensitization occurs after prolon ged exercise, echocardiograms, heart rate responses to isoproterenol, plasma catecholamines, and circulating lymphocyte beta-adrenergic rece ptors were examined in 10 sedentary normal subjects at rest and after brief (10 min) and exhaustive (mean duration 95 min) cycle exercise. R esting end-diastolic volume and ejection fraction were significantly r educed after exercise (from 120 +/- 34 to 100 +/- 26 ml and from 60 +/ - 0.4 to 54 +/- 0.6%, respectively; both P < 0.05). The amount of isop roterenol needed to increase heart rate 15 and 25 beats/min increased in a dose- (exercise duration) related fashion, and the increase in am ount of isoproterenol needed after prolonged exercise was closely rela ted to the decrease in ejection fraction (r(2) = 0.67, P = 0.004). Cir culating lymphocyte beta-receptor density and affinity, agonist bindin g, and adenylylcyclase levels were unchanged with prolonged exercise. In conclusion, prolonged exercise in sedentary normal subjects resulte d in reduced cardiac chronotropic responsiveness to isoproterenol that was not reflected in peripheral lymphocyte beta-adrenergic-receptor d ownregulation.