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

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Abstract
Desensitization of the β‐adrenoreceptors (β‐AR) may contribute to a post‐exercise reduction in left ventricular (LV) function. However, attenuation of the chronotropic and inotropic responses to a β‐AR agonist may depend upon alterations in parasympathetic tone. Furthermore, changes in cardiac outputand LV diastolic function in response to a β‐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 μg 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, 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), whereaswas 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 μg 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 andwere 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 β‐AR contributes to an attenuated left ventricular systolic but not diastolic function following prolonged exercise.