β-Adrenergic receptor desensitization in man:: insight into post-exercise attenuation of cardiac function

Desensitization of the beta-adrenoreceptors (beta-AR) may contribute to a post-exercise reduction in left ventricular (LV) function. However, attenuation of the chronotropic and inotropic responses to a beta-AR agonist may depend upon alterations in parasympathetic tone. Furthermore, changes in cardiac output (Q) and LV diastolic function in response to a beta-AR agonist, pre- to post-prolonged exercise, remain unclear. Seven trained males (mean +/- S.D., age 27 +/- 6 years) performed 4 h of ergometer rowing. Peak heart rate (HR) and LV systolic and diastolic functional responses to incremental isoproterenol (isoprenaline) infusion (2, 4 and 6 mu g kg min(-1)) were assessed after vagal blockade (glycopyrrolate, 1.2 mg). LV systolic function was assessed by the pressure/volume ratio (systolic blood pressure/end systolic volume) and Q, whilst diastolic function was evaluated as peak early and late transmitral filling velocities. Following exercise, the pressure/volume ratio decreased by 25% (P < 0.05), whereas Q was unchanged (P > 0.05). The early/late filling ratio was reduced by 36% after exercise, due to an elevation in late LV filling (P < 0.01). The increase in HR response to isoproterenol infusion was blunted post-exercise at both 4 and 6 mu g kg min(-1) (127 +/- 7 and 132 +/- 6 beats min(-1)) compared with pre-exercise (138 +/- 8 and 141 +/- 12 beats min(-1), P < 0.05). Additionally, the pressure/volume ratio and Q were blunted post-exercise in response to isoproterenol (P < 0.05). In contrast, diastolic function was similar before and after exercise during isoproterenol infusion (P > 0.05). Desensitization of the beta-AR contributes to an attenuated left ventricular systolic but not diastolic function following prolonged exercise.